FIREBall-2: The Faint Intergalactic Medium Redshifted Emission Balloon Telescope
Erika Hamden, D. Christopher Martin, Bruno Milliard, David Schiminovich, Shouleh Nikzad, Jean Evrard, Gillian Kyne, Robert Grange, Johan Montel, Etienne Pirot, Keri Hoadley, Donal O'Sullivan, Nicole Melso, Vincent Picouet, Didier Vibert, Philippe Balard, Patrick Blanchard, Marty Crabill, Sandrine Pascal, Frederi Mirc, Nicolas Bray, April Jewell, Julia Blue Bird, Jose Zorilla, Hwei Ru Ong, Mateusz Matuszewski, Nicole Lingner, Ramona Augustin, Michele Limon, Albert Gomes, Pierre Tapie, Xavier Soors, Isabelle Zenone, Muriel Saccoccio
DDraft version July 20, 2020
Typeset using L A TEX default style in AASTeX63
FIREBall-2: The Faint Intergalactic Medium Redshifted Emission Balloon Telescope
Erika Hamden, D. Christopher Martin, Bruno Milliard, David Schiminovich, Shouleh Nikzad, Jean Evrard, Gillian Kyne, Robert Grange, Johan Montel, Etienne Pirot, Keri Hoadley, Donal O’Sullivan, Nicole Melso, Vincent Picouet, Didier Vibert, Philippe Balard, Patrick Blanchard, Marty Crabill, Sandrine Pascal, Frederi Mirc, Nicolas Bray, April Jewell, Julia Blue Bird, Jose Zorilla, Hwei Ru Ong, Mateusz Matuszewski, Nicole Lingner, Ramona Augustin, Michele Limon, Albert Gomes, Pierre Tapie, Xavier Soors, Isabelle Zenone, and Muriel Saccoccio University of Arizona, Steward Observatory, 933 N Cherry Ave, Tucson, AZ 85721, USA California Institute of Technology, Division of Physics, Math, and Astronomy, 1200 E California Blvd, MC 278-17, Pasadena, CA91105, USA Laboratoire d’Astrophysique de Marseille, 38 Rue Frdric Joliot Curie, 13013 Marseille, France Columbia University, 550 W 120th St, New York, NY 10027, USA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Centre national d’tudes spatiales, 18 Avenue Edouard Belin, 31400 Toulouse, France French Alternative Energies and Atomic Energy Commission, France Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218 Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104 (Received February 26, 2020; Revised June 12, 2020; Accepted June 30, 2020)
Submitted to ApJABSTRACTThe Faint Intergalactic Medium Redshifted Emission Balloon (FIREBall) is a mission designed toobserve faint emission from the circumgalactic medium of moderate redshift (z ∼ Keywords:
Circumgalactic Medium, UV Spectroscopy, Multi-Object Spectroscopy, High Altitude Bal-looning, UV Telescope INTRODUCTIONWe have built and successfully flown the Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2),a joint mission funded by NASA and CNES. FIREBall-2 is designed to discover and map faint emission from thecircumgalactic medium of moderate redshift galaxies, in particular via Ly- α (1216˚A), OVI (1033˚A) and CIV (1549˚A) Corresponding author: Erika [email protected] a r X i v : . [ a s t r o - ph . I M ] J u l Hamden et al. redshifted into the 1950-2250 ˚A stratospheric balloon window at redshifts of z(Ly- α ) 0.7, z(OVI) 1.0, and z(CIV) 0.3.The FIREBall-2 balloon payload is a modification of FIREBall (FB-1), a path-finding mission built by our teamwith two successful flights (2007 Engineering, 2009 Science Tuttle et al. 2008; Milliard et al. 2010). FB-1 providedthe strongest constraints on intergalactic and circumgalactic (IGM, CGM) emission available from any instrumentat the time (Milliard et al. 2010). In contrast, FIREBall-2 was launched in a time of great strides in CGM science,with many detections of large Ly- α emitting structures surrounding high redshift quasars (QSOs, Martin et al. 2015;Cantalupo et al. 2014; Borisova et al. 2016). Despite these new, exciting discoveries, the lower redshift (z <
2) CGMuniverse remains unexplored due to the inaccessibility of Ly- α from the ground below a redshift of 1.9 ( ∼
350 nm).The FIREBall-2 mission is currently the only telescope and instrument designed to observe this crucial component ata lower redshift.FIREBall-2 consists of:1. A 1-meter primary parabolic mirror2. A sophisticated 6 axis attitude and pointing control system, yielding less than 1 arcsec RMS over long integrationtimes.3. A delta-doped, AR-coated UV optimized electron multiplying CCD (EMCCD), which provided >
50% efficiencyin the 1980-2130 ˚A bandpass.4. The first balloon flight of a multi-object spectrograph, using pre-selected targets and custom spherical slit masks.5. A flight test of an aspheric anamorphic grating developed at HORIBA Jobin Yvon. The grating acted as a fieldcorrector.6. Partnership between NASA and CNES.7. 4 completed PhDs (including 3 women), with an additional 7 students receiving significant mission training aspart of their thesis work.The 2018 flight of FIREBall-2 occurred on September 22nd from Fort Sumner, NM, with launch support providedby NASA’s Columbia Scientific Ballooning Facility (CSBF). During the flight, all systems of this sophisticated payloadperformed as expected. The balloon and payload reached an altitude of 39 km several hours after launch and thenbegan a slow descent as a result of a hole in the 40 million cubic foot (MCF) balloon. The balloon flight was terminatedafter 4 hrs of dark time, with less than one hour spent above the minimum science altitude of 32 km. Upon landing,the payload suffered some structural damage that the team is working to repair. We expect a second launch of therefurbished payload in 2021, due to delays from COVID-19.This paper presents an overview of the FIREBall-2 mission. In particular, we describe changes from the previousversion of the mission, FIREBall-1, and both the motivation and results of those changes. We provide performancedetails about all subsystems, and references to more specific papers about those subsystems in the case of the detectorand CNES guidance systems. We give an overview of the analysis of the data collected during flight, which was severelylimited by the low altitude and high background.The paper is arranged as follows. We briefly describe the current state of the art of CGM science in Section 1.1.Previous flights of the FIREBall payload are described in Section 1.2, with changes from the earlier spectrograph designdetailed in Section 1.3. Major instrument components and their performance are described in Section 2, including thetelescope (Section 2.1), spectrograph optical design (Section 2.2), delta-doped UV detector (Section 2.3), cooling andvacuum system (Section 2.4), thermal control system (Section 2.5), coarse and fine guidance systems (Sections 2.6and 2.7), and the communications system (Section 2.8). The flight itself is described in Section 3, including anomalies(Section 3.1), target fields observed (Section 3.2), and overall in flight performance (Section 3.3). Some preliminarydata analysis is shown in Section 4, covering noise from smearing, cosmic rays, scattered light, and overall sensitivity.We discuss the future of the FIREBall mission in Section 5 and the importance of continuing UV CGM science infuture, more ambitious telescopes in Section 6.1.1.
State of the art in CGM science
Studies of the circumgalactic medium (CGM) are now entering their second decade. Since the installation ofthe Cosmic Origins Spectrograph (COS) on HST (Green et al. 2012), observations of the CGM were conducted via
IREBall-2 α is observable at visible wavelengths.After the first exciting discoveries of the Slug and other similar nebulae (Cantalupo et al. 2014; Martin et al. 2015),there has been an increasing body of work examining this new population of CGM structures. Borisova et al. (2016)have found a 100% incidence of large Ly- α nebula around z > α halos around 21 out of 26 galaxies with a redshift range of 3 < z <
6. Cai et al.(2018) found one of many extremely large Ly- α nebula as part of a close QSO pair (around z ∼ < z < < z < α structures to be less frequent and with a lower covering fraction than those athigher redshift, likely indicating an evolutionary change between redshifts.The realm of CGM direct imaging is still in its infancy and there have been few attempts to link these individualobservations to the larger surveys conducted via absorption lines. In addition, for the lowest redshift galaxies wherestar formation rates are declining throughout the universe, CGM direct imaging is not possible on the ground due tothe limits of atmospheric transmission.The typical blue cutoff on the ground is around 350nm, where extinction is primarily driven by Rayleigh scatteringButon et al. (2013). Below 320nm, the primary absorber is Ozone, which remains significant through the ultraviolet.At sufficient altitudes (above 30 km), there is a transmission window between 195 and 220 nm where ozone andmolecular oxygen both have troughs in their absorption cross sections (Matuszewski 2012; Ackerman 1971). Thetransmission increases with altitude. Figure 1 (Figure 3.2 from (Matuszewski 2012)) shows atmospheric transmissionat 34 km.In this rich landscape, FIREBall-2 provides crucial detections of the CGM around several hundred low redshiftgalaxies, a unique capability at a perfect time.1.2. Previous flights of FIREBall
The FIREBall telescope has flown two previous times, from Palestine, TX in 2007 (Tuttle et al. 2008) and from FortSumner, NM in 2009 (Milliard et al. 2010). The original FIREBall instrument (FB-1) consisted of a fiber-fed integralfield unit (IFU), which used an Offner spectrograph design and a GALEX NUV spare microchannel plate (MCP) asthe detector.The FB-1 spectrograph had a resolution of R=5000, a plate scale of 12 µ m arcsec − , an angular resolution of 10arcsec, and a circular field of view with a diameter of 160 arcsec. The fiber bundle consisted of 300 fibers with a corediameter of around 8 arcsec per fiber. The overall system throughput was 0.5% (Tuttle et al. 2008). The telescope (1m parabolic primary mirror and 1.2 m flat siderostat mirror) and gondola structure remained unchanged between allflights of FIREBall, including FB-1 in 2007 and 2009 and the most recent FIREBall-2 flight in 2018.1.2.1. Hamden et al.
Figure 1.
Figure 3.2 and caption from Matuszewski (2012) . A computed atmospheric transmission curve for observationsfrom an altitude of 34 km (112 kft, 7 mbar), roughly that for the second FIREBall flight in 2009. The calculation includedabsorption by O2, N2 and N2O, and Rayleigh scattering effects. The solid curve shows the transmission for a target at themaximal elevation of the FIREBall telescope, 70 ◦ , the dot-dashed curve for the minimum, 40 ◦ . The dashed and dotted curvesshow the oxygen (O2) and ozone contributions to the transmission losses. The light gray areas lie outside of the FIREBallbandpass; the narrow bands near the center correspond to three nitric oxide airglow bands. The accessible redshift ranges forthe three principal FIREBall emission lines are overplotted on the axes near the top of the image. The 2007 flight of FIREBall-1 was an engineering flight. The telescope was launched on July 22nd, 2007 from theCSBF location in Palestine, TX. This flight achieved 3 hours of dark time while the total time of flight was 6 hours.The instrument was only able to maintain pointing for up to 1 minute at a time due to a pivot failure that occurredduring the launch and severely limited pointing control.1.2.2.
The 2009 flight was launched from Fort Sumner, NM on June 7th, 2009. This payload used the same spectrographas the 2007 flight, with an improved fiber bundle and a reinforced pivot. The balloon reached an average altitude of113 kft (yielding 25% atmospheric transmission) due to an unusual monsoon weather pattern, rather than the desired120 kft (up to 80% transmission). The payload performed flawlessly and obtained a full night of observations on threescience targets.The science targets consisted of a section of the GROTH strip (Davis et al. 2007), QSO PG1718+481 (Crightonet al. 2003), and the DEEP2 ZLE field (Simard et al. 2002). Due in part to low instrument throughput, no CGMemission was detected from these targets down to a sensitivity limit of ∼ − s − sr − ). This lack of detection motivated the complete redesign of theFIREBall-2 spectrograph as described in Section 1.3.1.3. Changes between FIREBall-1 and FIREBall-2
The spectrograph was redesigned to increase the chances of detecting emission from the CGM of z=0.7 galaxies viaLy- α . The changes increased the field of view, number of targets per observation, and overall instrument throughput.In addition, galaxies were pre-selected based in part on likelihood of expected emission. IREBall-2
5A two-mirror field corrector was designed and added to the optical path after the prime focus to increase the usablefield of view to a ∼
30 arcmin diameter circle. The field of view was a 28x12 arcmin rectangle set by the size of thedetector. The fiber bundle was replaced with a multi-object slit mask, which could target up to 70 galaxies per field,although this density of targets causes some spectral overlap. A rotating carousel allows for selection between ninedifferent masks with 4 designed for particular galaxy fields. Four other masks are used for calibration and one slotis left empty. Using a slit mask instead of a fiber bundle increased the sensitivity by eliminating the potential forUV absorption in the fibers. The angular resolution was also improved, from 10 arcsec to 4.5 arcsec through a newspectrograph design, in order to better separate the galaxy emission from the CGM signal. The GALEX spare NUVMCP detector was replaced with a high efficiency delta-doped EMCCD, which increased throughput by a factor of 8.The change in detector from an MCP to an EMCCD drove additional technical changes from FB-1. This changeadded a requirement that the spectrograph track the sky for long ( ∼
100 s) exposures. A MCP is able to time tagphotons and therefore the instrument does not need sidereal tracking ability, while an EMCCD would be read out atlonger than 100 second intervals, necessitating better pointing. The use of the slit mask also demanded an improvedpointing system to keep all targets in their slits without excess jitter. The gondola pointing system was improved toprovide significantly finer pointing stability (requirement went from 6 to < < -100 ◦ C to reduce thermal noise, and sorequired a cryocooler and associated cold chain, charcoal getter, and vacuum system. The CCD controller, CMOSguider, and cryocooler reject heat also required significant cooling and the addition of a thermal control system.1.3.1.
Overall Sensitivity Improvements
As described in the proceeding sections, FIREBall-2 is designed to increase the sensitivity over FB-1. The expectedsensitivity calculation is conducted in detail in Picouet (2019), and is briefly summarized here. The calculation forexpected performance is based on throughput measurements of the optical system, noise performance of the detector,expected sky background, and nominal altitude transmission at 40k or 130 kft. The anticipated sensitivity is 8,000LU, an improvement of a factor of 8 over FB-1. MAJOR INSTRUMENT COMPONENTSThe major instrument components of FIREBall-2 are described below. The telescope and instrument are shownin Figure 2. More detail is provided in additional papers that focus on the fine guidance system (Montel 2019), theoverall attitude control, the calibration strategy (Picouet 2019), and the detector performance (Kyne 2019). Whereappropriate, the performance in flight for each subsystem is also described. The overall instrument performance isdescribed in Section 3.3. 2.1.
Telescope Assembly and Gondola
The FIREBall-2 telescope and gondola are the same structures described in Tuttle et al. (2008) and Milliard et al.(2010). The telescope assembly consists first of a flat 1.2 m siderostat mirror, which provides elevation control between40-70 ◦ altitude and coarse x and y pointing via a tip/tilt frame. the elevation limits are due to a mechanical hardstop at 40 ◦ and the balloon and top of gondola at 70 ◦ . The flight train is about 90 m in this case. The siderostatfeeds the primary mirror, an f/2.5 1 m parabolic mirror. Both optics had survived two previous descents and landings.The parabola debonded during the 2007 landing, but was otherwise unharmed. The mirror coatings were stripped byOptical Mechanics, Inc., which originally fabricated them, and were then re-coated at Goddard Space Flight Centerwith an Al/MgF coating optimized for 205 nm.The gondola structure consists of carbon-fiber rods with a sparse set of connecting nodes, forming a stiff (firstresonance above 20 Hz), thermally-stable kinematic structure. Optical mounts to the gondola have been athermalizedto compensate for residual expansion effects in the rods and the aluminum couplings. The gondola has also flown twoprevious times, sustaining damage typically only to the carbon fiber rods, which are replaced between flights.2.2. Instrument Optical Design
The choice to increase the field of view and change from a fiber-fed IFU to a multi-object mask spectrographnecessitated a complete redesign of the spectrograph optics. A comprehensive discussion of the optical design andspecifications is provided in Grange et al. (2016). Briefly, there is a two mirror field corrector to increase the qualityof the field over a 30 arcmin field of view. The slit mask is a spherical surface to match the focal plane from the field
Hamden et al.
Figure 2.
Left:
The FIREBall-2 light path through the entire instrument assembly.
Right:
FIREBall-2 as-built, awaitinglaunch. corrector. The spectrograph consists of two Schmidt mirrors (one as collimator and one as camera) with folding flats forcompactness and an aspherized reflective Schmidt grating. The grating was manufactured using a double replicationprocess at HORIBA Jobin Yvon. It is a novel high throughput cost-effective holographic grating with a groove densityof 2400 l mm − over a 110x130 mm aspherized reflective surface (Grange et al. 2014). The shape of the grating correctsfor the spherical aberrations of the rest of the optical system. The grating consists of an aluminum substrate withnative oxide. Back up gratings were capped with a 70nm thick layer of MgF to optimize in the FIREBall-2 bandpass and a 28 ◦ angle of incidence, but were not used in the 2018 flight. The flight grating reflectance exceeded 50% inthe band pass, a significant improvement over the FIREBall-1 grating performance of 17% (Quiret et al. 2014). Thegrating provides a slit-limited resolution of R ∼ Delta-doped AR-coated EMCCD
The electron multiplying CCD used on this flight was a Teledyne-e2v CCD201-20 architecture with 13 µ m squarepixels. Nominally the CCD201s are frame transfer devices and have a 1k ×
1k image area and storage region. However,FB-2 requires use of the entire pixel array for a larger FOV and so the detector is read out in line transfer mode asa 2k ×
1k device. These detectors also have an additional 1k pixel extension to the serial register. Out of these, 604act as multiplication pixels. When these pixels are clocked with a voltage above ∼
39 V, their wells are deep enoughto allow impact ionization of electrons as they are moved through the serial register. The net result of this is thatsingle electron events in the image area will be multiplied to many times above the read noise, significantly increasingthe signal-to-noise ratio (SNR). A more detailed description of the operation of these devices can be found in (Daigleet al. 2008, 2010; Tulloch 2010; Tulloch & Dhillon 2011; Kyne 2019).The flight device was the end result of several years of technology development undertaken by JPL, Caltech, andColumbia University for use in the UV, funded by NASA. This development builds on JPL’s pioneering work ondelta-doping and UV detector development. The maturation of these devices is detailed in a series of papers (Jewellet al. 2015; Hamden et al. 2016; Nikzad et al. 2017; Kyne 2019).The detector performance was verified via testing at JPL and Caltech, with additional validation of performance atTeledyne-e2v. JPL testing included QE verification (measurement shown in Figure 3) following the method describedin Jacquot et al. (2010), while Caltech measured QE at limited wavelengths using N¨uv¨u v2 and v3 CCCP controllersand a custom flight printed circuit board (PCB). Caltech also independently tested the detectors on sky at Palomarusing the Cosmic Web Imager instrument (Matuszewski et al. 2010). These controllers provide 10 and 5 ns granularity,respectively, to optimize the pixel clocking strategy, readout speed, and wave form shape/height to minimize clock-induced-charge (CIC), read noise, and deferred charge (Hamden et al. 2015; Kyne et al. 2016). The PCB was designedusing the suggested configuration from N¨uv¨u to reduce additional read noise from the readout process. Total readnoise in the lab camera system was 100 e − . Measured CIC from lab data was 4.2 × − e- pix − frame − in the IREBall-2 Figure 3.
Left: Model transmittance/performance for 2D-doped silicon detectors with multilayer AR coatings tailored for theFIREBall bandpass. Adding complexity increases peak QE, but results in a narrower peak. Theoretical response for a bare2D-doped silicon detector is also shown. Left: Experimental results for 2D-doped silicon detector with the 3-layer FIREBall ARcoating. As measured QE is shown alongside QE corrected for quantum yield (Hamden et al. 2016; Kuschnerus et al. 1998). serial register and 7 × − e- pix − frame − in the parallel clocking, for a total of 4.9 × − e- pix − frame − . Thedark current level was 8 × − e- pix − hr − at a temperature of -115 ◦ C.The detector was installed into the spectrograph in the spring of 2016, and was extensively tested in the spectrographsystem. Both the detector and N¨uv¨u v2 controller performed reliably and with stability throughout integrationand testing showing no change in behavior between installation and flight. A brief discussion of detector noise andperformance is in Section 4 and is discussed in detail in Kyne (2019).2.4.
Cooling and vacuum system
The choice to change the detector from a MCP to a CCD required the addition of a cooling system to reduce thenoise contribution from dark current. To avoid the build up of ice or other contaminants on the detector surface,and maintain an effective vacuum during the flight, a cryopumping system was developed. A schematic of the coolingsystem is shown in Figure 4.A Sunpower CryoTel CT cryocooler was used to cool both the detector and charcoal getter. The CT provides upto 120 W of cooling power, providing a lift of greater than 10 watts at cryogenic temperatures. This was sufficient tocool an 0.75 liter charcoal getter and maintain a pressure of less than a few 10 − Torr for the expected ∼
24 hour flighttime. In order to achieve this vacuum, a careful strategy of tank and component bakeouts was implemented. Thespectrograph tank interior (black anodized aluminum) itself was a significant source of water in the vacuum systemand needed to be baked out at > ◦ C for several weeks to achieve and maintain a high vacuum.The flight charcoal getter interfaces with a gold plated solid copper coldfinger, which has a one-inch diameter circularcross section and connects the cold head of the cryocooler to the rest of the system. There is a single joint connectionin the cold finger to enable installation in a tightly packed spectrograph tank. The getter is the first thermal loadon the cryocooler and can be kept at a much lower temperature than the EMCCD (typically ∼ ◦ C colder than thedetector). The EMCCD is held by a cold clamp coupled to the far end of the coldfinger via a flexible cold ribbon. Allcold surface interfaces have a layer of indium for better contact and thermal conductivity.In flight, the cryocooler was operated at 120 W to maintain the charcoal at a temperature of -180 ◦ C and the detectorat -115 ◦ C. 2.5.
Thermal control system
An additional requirement for FIREBall-2 was removing excess heat generated by a number of systems: The cry-ocooler, the N¨uv¨u EMCCD controller, and the pco.edge 5.5 guider camera (discussed in Section 2.7) all generatedwaste heat that needed to be managed in flight. The expected pressure at float altitude of 3 mbar presents a difficultthermal environment in which normal convective cooling is not effective.The cryocooler in particular was most sensitive to lack of convective cooling, as the temperature of the heat rejectionpoint is directly coupled to the temperature of the cold head. An inability to cool the reject point would result ininefficient cooling, a higher detector operating temperature, and higher dark current. While the cooler can operatewith a reject of up to 80 ◦ C, this is a significantly less efficient operating mode than at lower reject temperatures. Athermal vacuum test conducted in the winter of 2017 at CNES in Toulouse indicated the need to actively cool thecryocooler heat rejection point.
Hamden et al.
Cryocooler Cold Finger
Charcoal Getter H e a t E x c h a n g e r C oo li n g C i r c u i t C o l d C l a m p E M CC D Flexible Copper Ribbon
Figure 4.
Schematic of detector and getter cooling system. Both are cooled by the cryocooler, and connected to the cryocoolercold head by a gold plated copper cold finger. A flexible copper ribbon connects the end of the cold finger to the cold clampwhich cools the back of the EMCCD detector. The charocoal getter is directly connected to the cold finger.
To address this we used a water circulation system. It consisted of a dewar filled with 0 ◦ C ice and water with anarrow outlet, allowing for thermal evaporation to the 3 mbar atmosphere to maintain a low temperature in the waterdewar. A circulating water circuit passed through the dewar before reaching water blocks connected to the crycoolerrejection point and body, the N¨uv¨u pressure vessel, and thermal blocks connected via copper straps to the guidercamera pressure vessel inside the spectrograph tank. The dewar volume of 20 liters had sufficient cooling power for 24hours of operation. Both the N¨uv¨u and guider camera were turned off during the ascent and daytime float phases ofthe flight to save cooling power for night time operations. A schematic of the cooling approach is shown in Figure 6.We saw no evidence of any impact of the dewar system on the guidance due to, for example, sloshing of the coolingliquid. The mass of the cooling liquid was marginal compared to the rest of the payload (0.7% of the total mass).During the flight, the cooling system maintained a cryocooler reject temperature of 19 ◦ C at 100 W of cooling power.Because of the short duration of the flight, less than one third of the water volume was consumed. However, the loweraltitude experienced during the flight meant the evaporative cooling scheme was less efficient than anticipated. Towardsthe end of the flight the temperature of the cooling system, including cryocooler reject, cold head, charcoal getter, anddetector, started to trend upwards, which can be seen in Figure 5. By this time, the payload altitude was so low thatthis did not significantly impact data collection and the flight was terminated shortly afterward.2.6.
Coarse guidance system
The attitude control system for FIREBall-2 was designed primarily by CNES. It is an update from the generic CNESsystem and the one used on the previous two flights of FIREBall. A detailed discussion of the basis for the systemcan be found in Montel (2019), and more detailed results from the flight will be published in an upcoming paper.The pointing requirements are extremely constrained for a balloon payload ( <
1” pointing stability in three axes overseveral hours), as the system is frequently disturbed and in motion. The 3 degrees of freedom of the instrument arecontrolled through a 4-axis control system which uses:1. Azimuth control of the gondola from the pivot connection to the balloon flight train2. 2-axis control of the flat siderostat mirror (tip/tilt in elevation and cross elevation)3. A rotation stage that also serves as the spectrograph tank mount.The CNES attitude control system (ACS) uses multiple sensors to determine the position and actuate each of thefour axes of control. The azimuth fine pointing system consists of an IMU-gyrocompass and anologic gyro (IXBLUE,France) that are used to measure azimuth. This system controls the azimuth via the pivot, which links the telescopepayload to the balloon. The boresight fine pointing system uses both the IMU and fine guider (described in Section2.7). This system controls elevation and cross elevation via two fine pointing actuators, encoders, and two reactionwheels. The field rotation pointing system uses the rotation error signal from the fine guider. This system controlsfield rotation via the spectrograph tank rotation stage and encoder.
IREBall-2
100 200 300 400 500 600 700 800Mins after launch- 2 0 0- 1 5 0- 1 0 0- 5 00 T e m pe r a t u r e EMCCD TemperatureCryocooler ColdheadCryocooler Reject
Figure 5.
Figure of temperatures for three critical parts of the cooling chain: The EMCCD (red), the cyrocooler coldhead(blue), and the cryocooler reject (green). The three temperatures are correlated, with increases in the reject temperatureresulting in increases in the cyrocooler cold head and therefore eventually EMCCD temperature. A heater on the EMCCDmaintains a constant temperature, so the EMCCD measurement will not be immediately impacted by the reject temperatureincrease. In the last 200 minutes of the flight, the reject temperature started to increase due to the loss of cooling capacity atlower altitudes as described in Section 2.5.
A large field of view attitude sensor (ASC from DTU, Denmark) was also on board as a back-up sensor in case ofguider failure and for coarse positioning information. The DTU was mounted on the siderostat frame and observed aregion of the sky adjacent, but not overlapping, with the science FOV. The DTU sensor is a well known star trackerthat can deliver a 10” 3-axis attitude measurement. Both the DTU and IMU systems from CNES were the same asthe ones used on FB-1.The flight train of FIREBall-2 can be modeled as a multiple torsion double pendulum. The CNES system is able topredict and account for many of the expected modes of the gondola and provide damping of the dynamic modes. Thereare both pendulum motion modes and wobbling modes. The pendulum modes are well understood, corresponding toroll and pitch, and behaved as expected during the flight. The primary low frequency pendulum modes have periodsof 23s and 9s, while the high frequency wobbling modes have periods of 1.8s and 2.2s. The high frequency modes aredamped with an active damping system using 2 reaction wheels. The wobbling frequencies in particular are sensitiveto the ladder length below the parachute and the mass of the payload. The amplitude of the wobbling modes is directlyproportional to the quality of the pointing. Changes in balloon altitude or changes in the rotator torque excited thewobbling modes and were difficult to dampen. This resulted in operating with reduced gains on the azimuth pointingcontrol loop and adjustment of the roll reaction wheel filter frequency.This reduced pointing control was particularly an issue during the observations of Field 2 (described below in Section3.2), where the altitude was quickly decreasing and the balloon was rotating through large angles (from 200 ◦ to nearly300 ◦ , then back down to 200 ◦ over a 1.5 hr period), requiring a lot of torque for azimuth control. The ballast drops,which also excited a natural vertical (buoyancy) oscillation mode of the gondola, contributed to the large disturbancesto position and azimuth during science operations. Figure 7, taken from Montel (2019), shows GPS measurements ofthe altitude, horizontal speed, and heading of the payload during the flight. Blue arrows indicate ballast drops, andsubsequent oscillations in heading can be seen. Later in the flight, the heading becomes even more unstable, rotatingthrough a full 360 ◦ before the end of the flight.0 Hamden et al.
Vapor exhaust& pressure valve +4 mB(20°C)NUVU 80W G u i d e r c a m W r a d i a t e d Heat exchangerThermal sensorCoolerbody 40WCoolerreject80W Circulation Pump Y or T connectionSpectrograph tank 100 W Expansion vase & purge Coolant circuitValve or 3-way valve Fill pointCopper spiral exchangerAgitation pump PumpDewar water tankCopper Strapsto Guider cam Guider Coolingpoint 15W
Figure 6.
Block diagram of cooling scheme. Spectrograph tank and items requiring active cooling are shown as blue blocks,with blue and red lines indicating the cooling circuit. There are two identical cooling circuits, splitting and re-joining at the topof the tank. For clarity, only one is shown. The configuration shown is the same as during the flight, with a water ice solutionin the dewar. During ground testing, a cooling circuit is used instead of water ice.
Despite these significant challenges and the extremely degraded observing conditions, the full guidance systemperformed within the 1” stability requirement throughout the flight. A detailed description of the fine guider andinput to the CNES guidance control system is described in Section 2.7.2.7.
Fine guidance system
The scientific objectives of FIREBall-2 and the multi-object nature of the spectrograph impose stringent requirementson the instrument pointing. Each science target must be centered in a 80-90 µ m wide slit, corresponding to an accuracyof 6-8 arcseconds. In addition, FIREBall-2 aims to detect very faint emission, requiring long integration times (up to100 s). During each integration, the instrument must maintain stability to within a pixel on the detector ( < . (cid:48) × . (cid:48) patch of sky adjacent to each IREBall-2 Figure 7.
Figure 3 from Montel (2019). Figure shows the FIREBall-2 gondola altitude, horizontal speed, and heading fromthe on-board GPS system. science field. Masks are selectable and pre-designed for particular target fields. Optical light reflected off the guidermask is fed through a double petzvel lens system into a dedicated guider camera (pco.edge 5.5) with a 2560 × Hamden et al.
Figure 8.
Figure showing pointing performance while observing Field 2 in closed loop fine guiding. The 1D distributions alongthe top and right panels are probability density histograms fit with a 1D Gaussian. The solid vertical lines are the 1-sigmavalues for the 1D distribution. The 2D probability density distribution in the center is interpolated with Gaussian kernel densityestimation and displayed as shaded contours (where 1-sigma level corresponds to 0.39 arcsec − ). guide star positioning in the guider field, guider image reduction pipeline, and spectrograph tip-tilt. The same errorsignals used to refine the guide star positions in fine pointing mode are used to set the initial positions of the guidestars on the guider image. These positions are carefully calibrated pre-flight to ensure that each science target passesthrough its designated slit on the multi-object slit mask (this calibration is discussed in Picouet 2019).The optical image that is re-directed to the guider camera is approximately con-focal with the UV image on thedetector and the guider camera is used to perform a through focus on each science field in-flight.The overall performance in fine guiding is shown in Figure 8, with excellent RMS over the course of nearly two hours.The balloon descended through nearly 3 vertical km during this observing period, which increases the perturbationson the guidance system.A more careful analysis of the guidance control system, the target and guide star selection, and performance can befound in Montel (2019), Montel et al. (2016), and in a forthcoming paper, Melso et al. (in prep).2.8. Communications system
The FIREBall-2 communications system consists of three primary transmitters/receivers, all operating via a line-of-sight link: A Consolidated Instrument Package (CIP) uplink/downlink (1200/38,400 baud respectively), a 1 Mbit s − downlink, and a video downlink. A schematic of the telemetry/telecommand (TM/TC) sub-system of the FIREBall-2 IREBall-2 Figure 9.
Block Diagram of the communications set-up in 2018. White boxes show devices. Yellow boxes show the baud rates.Orange circle indicates a critical software component. Dashed lines indicate enclosures, not necessarily vacuum sealed. communications system is shown in Figure 9. This was by far the most complex part of the communications system,and required both interleaving of commands from four different inputs, and a downgrade from 38,400 baud to 1200baud for uplinking commands. Due to overheads on the uplink per dataword the speed was effectively 32 baud.The CIP uplink/downlink provided by CSBF took an input from a multiplexer (DBC, Inc., model SR-04) whichslowed down and interleaved commands from the detector ground computer, fine guider ground computer, and gondolacontrol ground computer to match the uplink speed. A complementary multiplexer on board de-multiplexed thetransmitted signals and relayed them to the appropriate on board computer. Because of the slow uplink speeds,commanding from the ground was laborious and nearly all typical flight actions were scripted ahead of time. Thedownlink followed a similar path, where the on board computers (detector, guider, and gondola OBCs) sent signals tothe multiplexer which sent them down through the CIP. Because the downlink was significantly faster than the uplinkhousekeeping data for the Gondola OBC and the guider computer were sent via the CIP. Intermittent housekeepingdata from the detector computer was also transmitted during flight but the images generated by the detector andassociated housekeeping used the faster 1 Mbit s − downlink.The 1 Mbit s − downlink was used to send cropped images from the science detector. This was necessary both toverify that the targets were falling into the slits on the mask and to allow for nearly real time image processing. Asimages were generated by the detector, the 1 Mbit s − downlink would grab the most recent one, strip the overscan andprescan regions, compress it with the most recent housekeeping file, and relay the package to the CSBF transmitter.The time to send a full frame image through was roughly 80 seconds, which necessitated not downlinking every image.Roughly 50% percent of images were transmitted during flight, with the remaining images collected after the flightincluding over-scan and pre-scan regions of the downlinked images. The downlink was stable during the flight. Onlyone image out of several hundred was corrupted during the downlink process. This stability was a result of CSBF’svery reliable communication equipment and significant ground testing to eliminate dropped bits as much as possible.One unexpected issue during flight was a strain on the detector computer, which also managed the downlink. Theprocessing power required for encoding images and sending to the 1 Mbit s − downlink while also operating the N¨uv¨ucontroller would freeze the flight computer and require a restart. This freezing was correlated with the rate of imageacquisition. A simple re-start would correct the issue, but image acquisition was stopped while this happened. Forfuture flights, the detector computer has a faster processor which should eliminate this problem.The video downlink consisted of a VGA to video adapter for the guider on board computer that would process thevideo generated by the guider camera. This was sent to a transmitter and the downlink was then relayed to a televisionscreen on the ground and was used as described in Section 2.7. FLIGHT ON SEPTEMBER 22ND, 2018FIREBAll-2 was launched from a 40 MCF balloon from Fort Sumner, NM on the morning of September 22nd, 2018 at10:20 AM MDT. At launch, all flight systems were nominal and the payload experienced minimal jerks or accelerationduring the launch. Throughout the ascent phase (from the ground to float altitude of 128,000 ft) the payload systems4
Hamden et al.
Figure 10.
CSBF generated map of trajectory for FIREBall-2 flight on September 22nd, 2018. The flight duration wasapproximately 13 hours at float after a 3 hr ascent. and communication continued to behave normally. The payload reached float altitude at approximately 1 PM localtime, northeast of the launch site.The stratospheric winds were very low during the flight and so the balloon traveled slightly north towards SantaRosa, NM, before eventually drifting south west and ended up over Vaughn, NM. The GPS map of the flight is shownin Figure 10. The altitude of the payload over the course of the flight is shown in Figure 11 including ballast dropsand other events. The balloon is expected to lose altitude as the sun sets due to changing thermal conditions andthen stabilize at a lower altitude after sunset. Ballast is dropped during sunset to minimize the altitude loss, and theexpected nighttime altitude was 118 kft to 108 kft, depending on the particulars of the atmosphere and weather. Inpart due to anomalies discussed below (Section 3.1), the balloon lost altitude starting mid-afternoon and continuedto descend through the night. The most likely cause of this was a hole allowing helium gas to escape. Ballast dropsdid not help to slow the descent and additionally created some extra pointing challenges since the timing excited a5 minute resonant frequency in the balloon altitude. The balloon altitude was below the required science minimumaltitude of 32 km at 660 minutes after launch (roughly 9:20 PM MDT). The team continued to collect data throughoutthe remainder of the flight until it was required to power down all systems by CSBF prior to termination. The balloonand payload were separated at roughly 1 AM MST, taking about an hour to land.The flight was terminated 50 miles west of Fort Sumner, near the municipality of Vaugn, NM. The payload wasrecovered on September 23rd. The landing, due in part to higher winds at night, was rough and both large opticssustained edge fractures. The siderostat fracture was minimal, while the primary mirror lost roughly 5% of the areaincluding the location of two of the six bond pads, which hold the optic to its mount. The damage for both mirrors wason the same side so was likely a result of a hard landing. In addition, the large mirror of the field corrector was crackedat the edge of the mirror along one of the bond locations. Additional inspection of the remaining mirrors had beenconducted. A measurement of the surface figure of both large optics indicated that despite the damage the surfaceshapes were not changed significantly and there are no additional cracks or fractures that could propagate throughthe optic. Both optics have since had the broken edges sanded down and were re-aluminized at Goddard Space FlightCenter in 2019. 3.1.
Flight anomalies
During the flight, there were three anomalies, two related to the balloon. During the launch, the spool that keepsthe balloon in position during inflation flew off the inflation vehicle. A subsequent investigation by CSBF into theorigins of this is ongoing. Both CSBF and NASA believe this was unrelated to the later anomaly.
IREBall-2 Figure 11.
CSBF generated plot showing balloon and payload altitude vs. time since launch. Indicated are sunset on theballoon and several ballast drops.
After 3 hours at float altitude the balloon began to lose altitude, likely due to a hole that was either present fromthe launch or developed while at float. As a result, the payload was above the science minimum altitude of 32 km foronly ∼ Field Selection and Observation Hamden et al.
Four fields were selected for the 2018 Fort Sumner campaign. Observations with FIREBall-2 target dense galaxy fieldsat z=0.7 tracing known large scale structure. With the initial launch planned in mid-September from Ft. Sumner NM,several survey fields were visible during the launch window and were also within the 40-60 deg elevation limitation dueto balloon and gondola obstruction. Several surveys were examined in detail including BOSS, WIGGLEZ, CANDLES,UKIDSS, CFHTLS, VIPERS, and DEEP2. DEEP2 is a galaxy redshift survey of ∼ α ,OVI and CIV respectively. For Ly- α targets, redshifts z = 0.682 - 0.696 were eliminated since the emission line wouldcoincide with a bright sky line.Aside from scientific considerations, several observational constraints needed to be considered when selecting thefields that FIREBall-2 targeted. The potential dates for the Fort Sumner launch window, the single night of observation,and the restriction on elevation range due to the balloon and gondola provided limits on potential fields. In addition,selected fields needed an adequate number of stars at the desired bands and magnitudes that fall in the field of viewof the guidance system and act as guide stars.3.2.1. Targets within fields
After fields were selected, the slits needed to be filled with desired targets. The criteria for selection includedmaximizing the number of Ly- α galaxies at z=0.7, spectral lengths of up to a few arcmin per galaxy, providingsufficient space between spectra to avoid overlap on the detector and avoiding redshifts where important galaxyemission lines would overlap on atmospheric features (NO in the upper atmosphere in particular at 2030 and 2047 ˚A).When additional space was available that couldn’t be filled by a Ly- α target, OVI and CIV galaxies were also used,in that order. The slit length in the cut masks was slightly larger than planned due to the strength of the laser, so inpractice there was some slit overlap.For the mid-September launch from Ft. Sumner, NM, the DEEP2 Fields 2, 3, and 4 were chosen and several maskalternatives were produced and reviewed. Field 2 contains 190 Ly- α galaxies, 372 OVI galaxies, and 2 CIV galaxies.The mask areas in Field 2 were chosen by eye to contain a maximized number of Ly- α galaxies at z = 0.7.The star fields were chosen from SDSS Sky Server D12 with magnitudes between 12.0 and 18.0 in the g and r bands.The stars and exact mask coverage areas are chosen to ensure an adequate number of guide stars exist for each targetfield. FIREBall-2 has resolution on the scale of µ m; therefore, distortions from coordinate conversion and telescopeproperties have to be accounted for and corrected.Due to the low altitude, only FB Field 2, a sub-region from the DEEP2 survey field (Newman et al. 2013), wasobserved for a significant length of time. Field 2 was acquired at 7:30 PM local time, prior to sunset on the balloon,which is 30 minutes after sunset on the ground. The field was lined up using a standardized acquisition procedure basedon the known positions of guide stars until science operations began after the sunset ballast drops. FB-2 observedField 2 until 9:37 PM MDT for a total of about 45 minutes of exposure time post twilight.After leaving Field 2 we moved to the center of M31 and then a QSO field (center at RA, Dec: 17.87698, 34.563395)near QSO SDSS J011133.38+343028.5. These target changes were an attempt to observe brighter objects as thepayload lost altitude. While data was acquired at each of these locations the altitude was less than 32 km andatmospheric transmission in the balloon window was significantly degraded.3.3. In flight performance
The instrument in-flight performance is described in more detail in Picouet (2019) but briefly summarized here.Post flight processing of the data from Field 2 was used to determine the in-flight performance of the spectrograph.Due to the degraded conditions described in Sections 3.1 and 3.2 the performance of some systems were sub-optimal.Several objects are detected in the co-added data from Field 2. The brightest was continuum emission froma UV bright horizontal branch star with M F UV =17.8, which was detected by FB-2 with SNR >
15 (ID: 2MASSJ16515894+3459322, known as Bright Star 1 hereafter). Two fainter UV bright stars were also detected. BrightStar 1 was used to measure the system efficiency, finding 30% degradation compared to the ground calibrations withatmospheric losses. This is likely due to a combination of uncertainties including target centering, true atmospherictransmission, focus, etc.In flight spatial resolution is calculated as full width at half max (FWHM) 7”, 40% greater than the 5” expected basedon the optical design after convolution with the slit widths and performance on the ground. Part of this degradation
IREBall-2 µ m de-focus in flight could lead to a 1 arcsec deterioration in spatial resolution alongthe slit. Temperature variations in components of the spectrograph and tip/tilt stage can also change the focus.In flight spectral resolution could not be measured because the UV bright stars that we observed have no spectralfeatures in this narrow bandpass. There are some features from atmospheric transmission on the UV bright star thatcan be used, but otherwise we rely on ground calibration. In earlier tests we found a spectral resolution of R ∼ ∼ ∼
1” degradation because of lowered surface quality of FC2 during polishing.The PSF degradation beyond that may be due to spherical aberrations introduced by the grating but this is pendingverification. PRELIMINARY DATA ANALYSISWhile the flight provided validation of a number of technology advances, the science data was significantly degradeddue to the flight anomalies. Here we discuss the various noise sources and their impact on the data. A detaileddiscussion of the detector performance can be found in Kyne (2019), which also includes a significant discussion onthe performance of Teledyne-e2v’s CCD201-20s in a lab setting.We are taking steps for the next flight to reduce the effect of the noise sources described below. This includesexploring slowing down the clocking speed to 1 MHz, making adjustments to minimize other sources of noise such ascable length, serial clocking scheme, and increasing the operating temperature of the CCD for better charge transferefficiency (which involves a trade off with increased dark current). Reducing the impact of smearing will also reducethe impact of cosmic rays on the images as their tails will inevitably be shorter.4.1.
Detector in-flight performance and measured noise
In typical visible wavelength observations the largest source of noise is sky background. In the UV the sky backgroundis orders of magnitude lower than in the visible (Leinert et al. 1998), typically leaving detector noise as the limitingnoise contribution. For an EMCCD multiple sources of detector noise need to be considered. These include clock-induced-charge (CIC), read noise (RN), dark current, multiplication gain noise, and any background light sources(scattered light, sky noise, etc). In this case, the primary contribution of noise in the data was the excess scatteredlight from the balloon onto the detector surface via a parasitic optical path that was not baffled. This contributedto a varying background on the order of 0.7 e- pix − frame − , which is well above the count rate limit of 0.1 e-pix − frame − for photon counting (Picouet 2019). Our analysis of the resulting data is focused on understandingand reducing the effect of this scattered light as much as possible. Here we discuss the scattered light and additionalsources of noise in the detector system. 4.1.1. Cosmic Ray Rate
One significant consideration in operating an EMCCD, especially at balloon altitudes, is the effect and impact ofcosmic rays on the data. Pixels with cosmic ray hits in an EMCCD will be amplified in the gain register just like anyother pixel and have a tendency to then spill over into the trailing pixels (due in part to the event smearing describedin Section 4.1.4). While this issue has been largely mitigated in more modern EMCCD architectures (those beingtested for WFIRST), the FIREBall-2 devices do not have the overspill register implemented. During the flight wemeasure 5-10 cosmic rays per second, contaminating 300-600 pixels (10-15% of the image on average and up to 25%in the most extreme cases) depending on the exposure time with longer exposures more affected. Figure 12 showsexample images with the full extent of cosmic ray smearing.4.1.2.
Dark current and other noise
Detector noise performance was difficult to disentangle from the effect of excess scattered moonlight from the balloon.With the doors of the gondola closed and tank shutter closed we measured a noise of 1.7 × − e- pix − s − . This islikely a combination of dark current, light leaks due to an imperfect shutter/tank cap design, and Cherenkov radiation,and was 10 times higher in-flight than during ground testing. The read noise of the entire system was around 90e-with a pre-amp gain of 0.53 ADU e- − . We operated in a range of gain modes with amplified gain of between 400 and2000 e-/e- depending on the circumstances. These gain values are typically lower than the normal high values used inphoton counting mode because of the excess scattered light.8 Hamden et al.
Figure 12.
Figure from Kyne (2019): Detector cosmic ray rate measured during the 2018 flight in Fort Sumner, NM.
Left:
30 second exposure.
Right : 50 second exposure with a particularly large cosmic ray hit with significant smearing into theoverscan region. Overall, we measure a cosmic ray rate of 5 - 7 cosmic rays per second at 128 kft.
Scattered light and residuals
The scattered light was primarily moonlight being scattered and re-directed from the deflated balloon. During theflight the moon was 93% full. Most of our target fields were more than 45 ◦ away from the moon and we anticipatedthat moonlight wouldn’t directly enter the optical system. Previous tests observing Vega indicated that the expectedscattered light within the optical path would be on the order of ≤ − of the in-band signal. The deflated balloonshape, however, acted as a lens and caused illumination that bypassed the optical path entirely and instead had anun-baffled view of the detector board and nearby optics.The bulk pattern of scattered light (shown in the central panel of Figure 13) fluctuated on 5 minute intervals, insync with the balloon vertical oscillations. We were able to re-create much of the signature of the scattered light afterthe fact using pseudo-dome flats and other methods to fully illuminate the entrance aperture to the spectrograph tank,beyond just the pupil.In addition to large scale scattered light there were occasional flashes that are visible in the flight data, shown inFigure 14. These flashes are typically much brighter ( ∼ Event smearing
The smearing of events in the multiplication register is a common problem in EMCCDs. It derives from a combinationof low temperature (which can reduce CTE) and the fast pixel clocking speed (10 MHz in this case). The FIREBall-2spectrograph set-up required the detector inside the vacuum tank pressure vessel while the N¨uv¨u controller box was
IREBall-2 Figure 13.
Field 2 summed images.
Left : The summed image before cosmic ray removal.
Center:
The summed image afterde-smearing and cosmic ray removal.
Right:
The summed image after de-smearing, cosmic ray removal, and subtracting thebackground scattered light. The sharp drop-off in scattered light on the bottom fifth of the image makes the subtraction difficult,yielding a residual along the edge of the scattered region. The dark region at the top of the detector was not delta-doped dueto the placement of a mask during the MBE process. These pixels are not responsive to UV light. The bright slash of pixels inthe middle right of each image is a cluster of hot pixels. Additional hot pixels are clustered in the bottom right of the images.This detector had multiple defects but the highest QE out of the available devices, which is why it was selected. All imageshave been smoothed with a Gaussian kernel with sigma of 1.5 and a radius of 3 pixels. The emission from the brightest star,Bright Star 1, is visible in all three images as a horizontal line roughly in the middle. Emission from other sources can be seenfaintly in the right most image. Hamden et al.
Figure 14.
Two sequential images showing the brightness of the ‘flashes’ visible in the data. The scale and color bars arethe same for both images. White dots are uncorrected cosmic rays.
Left: a typical frame with normal scattered light pattern.
Right: the frame immediately proceeding which contains a ‘flash’. located outside, connected by an 11 inch cable. This is at the very limit of what N¨uv¨u recommends for the 10 MHzpixel clocking speed, and was just barely manageable given the spectrograph configuration.The net effect of event smearing is to reduce the measured gain value (since high count pixels will be spread out over3-4 trailing pixels, lowering the high end of the histogram and increasing the low end). In addition, it makes it difficultto distinguish between events in adjacent pixels vs. a single event that has been smeared. The smearing signature isan exponential and is fairly easy to characterize, but like the amplification process itself, does not necessarily followthe same pattern every time. The impact of smearing is to reduce the number of pixels that are above the standardEMCCD 5 σ threshold, and a reduction in the apparent gain. A de-smearing algorithm using median absolute deviationis able to reduce the impact of the smear and is detailed in Kyne (2019).4.2. Co-added data analysis
Here we present the co-added data from Field 2, from the DEEP2 survey. This image was made by co-adding 55minutes worth of data, taken under sub-optimal conditions as described in Section 3.1. Figure 13 shows three differentversions of the same field and data. The left most shows the summed image before cosmic ray removal. The centerimage shows the summed image after de-smearing and cosmic ray removal, with individual images cosmic ray removedprior to summing. The pixels with cosmic rays are filled with data interpolated from the surrounding pixels. Theright image shows the summed image after de-smearing, cosmic ray removal, and subtracting the background scatteredlight.The process of co-adding is typical for that used for EMCCDs as described in Kyne (2019). The multiplication gainis calculated for each individual image in both the image area, pre-scan, and over-scan regions. Smear correction isdone using the median absolute deviation (MAD) method and a 6-sigma correction. After the smear correction, themultiplication gain is re-measured. Typically the post-smear correction gain is 2-3 times higher than the pre-smearcorrection gain, which is expected. Individual images are converted from counts to electrons using a conversion gainmeasured from a photon transfer curve (1.77 e- ADU − in this case). Because of the high background, these imageswere not taken in a pure photon counting mode and instead the analysis uses a gain mode. The EM gain calculatedfor each image is used to convert the image back to photons, which results is a √ α was detected aroundindividual galaxies. Summing spectra from many galaxies also did not yield any significant detections. IREBall-2
In-flight sensitivity
Due to the low altitude, scattering from the balloon de-shape, and degraded performance, the overall sensitivity wasworse than the FB-1 flight. Picouet (2019) calculates a sensitivity of 80,000 LU (vs. 74,000 LU for FB-1) using themeasured emission from the detected UV bright stars. This was significantly higher than the expected sensitivity of8,000 LU and was driven entirely by increased noise. FUTURE FLIGHT POSSIBILITIESWhile the payload sustained significant damage upon landing, the resulting analysis has shown that the majorcomponents (the two large optics, the gondola structure, the pivot, the spectrograph) can all be re-flown as they areor with limited refurbishment. Additionally, nearly every aspect of the payload and spectrograph system performedas expected and within specifications. We do not need to redesign any subsystems given the excellent performance.Thus, we are working towards a 2021 re-flight of the FIREBall-2 payload. We will be able to mitigate the effects ofscattered light and are investigating some upgrades to the cooling/detector system to reduce the cooling power andheat load required as well as upgrade to the N¨uv¨u v3 CCCP controller to reduce the read noise in the detector. Weare currently on track for a 2021 flight attempt from Fort Sumner using similar fields and targets as for the 2018 flight.We are adding an additional field with several QSOs and QSO pairs, in response to the more recent observations ofQSO pairs described earlier.Future upgrades beyond 2021 (i.e. for a FIREBall-3) may include a re-design of the spectrograph using microshutterarrays and potentially upgraded detector architecture, but are currently in a very preliminary phase. FIREBALL-2 AS A PATHFINDER FOR FUTURE UV TELESCOPESThe FIREBall sub-orbital program has always been carried out with three goals in mind. The first is to train andpromote early career instrument scientists as future mission PIs. In this sense, it has already succeeded, generatingover 10 PhDs, and giving many students both flight hardware experience and the opportunity to experience a fieldcampaign. Additionally, it provides an opportunity for postdocs to take on important, leadership roles that wouldotherwise not be accessible to them, giving them training as future PIs.The second goal has been technology demonstrations. The flight of a UV-optimized delta-doped EMCCD is the firstflight test of two truly mission-enabling technologies. These detectors will play key roles in future missions. Additionaltechnology, such as the anamorphic grating, guidance system, and use of a multi-object spectrograph on a balloonpayload have also been significant improvements over the existing state of the art.Finally, FIREBall has always been a pathfinder for a larger program to observe and map the faint circumgalactic andintergalactic media. While the observations in the visible bandpass have vindicated the concept of directly observingthe diffuse gas from the CGM, we have not yet realized the potential of doing these observations in the UV. The 2021flight of FIREBall-2 will detect this emission at a redshift of 0.7, but a future mission concept to explore the CGM inthe UV from redshifts of z < Hamden et al.
For excellent advice and assistance on detector testing and optimization, we sincerely thank N¨uv¨u especially OlivierDaigle and Yoann Gosselin. We also thank Teledyne-e2v, for their continued support in detector development andtesting.
Facilities:Software:
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