A flux calibration device for the SuperNova Integral Field Spectrograph (SNIFS)
Simona Lombardo, Greg Aldering, Akos Hoffmann, Marek Kowalski, Daniel Kuesters, Klaus Reif, Mickael Rigault
AA flux calibration device for the SuperNova Integral FieldSpectrograph (SNIFS)
Simona Lombardo a , Greg Aldering b , Akos Hoffmann a ,Marek Kowalski a , Daniel K¨usters a , Klaus Reif c , Mickael Rigault aa Physikalisches Institut, Nussallee 12, Universit¨at Bonn, Bonn, Germany b Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, USA c Argelander Institute f¨ur Astronomie, Auf dem H¨ugel 71, Universit¨at Bonn, Bonn, Germany
ABSTRACT
Observational cosmology employing optical surveys often require precise flux calibration. In this context wepresent SNIFS Calibration Apparatus (SCALA), a flux calibration system developed for the SuperNova Inte-gral Field Spectrograph (SNIFS), operating at the University of Hawaii 2.2 m telescope. SCALA consists ofa hexagonal array of 18 small parabolic mirrors distributed over the face of, and feeding parallel light to, thetelescope entrance pupil. The mirrors are illuminated by integrating spheres and a wavelength-tunable (from UVto IR) light source, generating light beams with opening angles of 1 ◦ . These nearly parallel beams are flat andflux-calibrated at a subpercent level, enabling us to calibrate our “telescope + SNIFS system” at the requiredprecision. Keywords:
Cosmology, calibration, flat field, standard star network
1. INTRODUCTION
A major goal of modern cosmological surveys is to better constrain the properties of dark energy. After havingproven their importance in detecting the accelerated expansion of the universe,
1, 2 type Ia Supernovae (SNe Ia)remain the strongest demonstrated technique for measuring the dark energy equation of state parameter, w .However, current studies are limited by systematic uncertainties, among which the flux calibration is the dominantsystematic uncertainty.
3, 4
To reduce this uncertainty, two issues must be addressed: reaching the 1% precisionphotometry required by modern imaging surveys, and refining the primary standard star network which currentlyrelies heavily on models of white dwarf stars. Accordingly, there is an ongoing effort to develop new techniquesand instruments for calibration.
7, 8
Here we present a new approach to the problem. [email protected] a r X i v : . [ a s t r o - ph . I M ] N ov he SNIFS Calibration Apparatus (SCALA) is a flux calibration device developed for the SuperNova IntegralField Spectrograph (SNIFS), built by the Nearby Supernova Factory, and mounted on the 2.2 m telescope ofthe University of Hawaii. SNIFS has a fully filled 6” ×
6” spectroscopic field-of-view subdivided by a microlensarray into a grid of 15 ×
15 contiguous square spatial elements (spaxels) forming a 2D grid of spectra. Thedual-channel spectrograph simultaneously covers 3200–5200 ˚A and 5100–10 000 ˚A with 2.8 and 3.2 ˚A resolution,respectively. The purpose of SCALA is to provide an accurate measurement of the instrumental response of the“telescope + SNIFS system” for each of SNIFS spaxels. This project is part of the Nearby Supernova Factory(SNfactory) which is producing a large set of flux-calibrated spectrophotometric timeseries of SNe Ia. To achieve the 1% precision, one needs to control two important factors in the measurement of the brightnessof SNe. The first is the atmospheric extinction, which has already been studied in detail by the SNfactoryusing a large set of nightly spectroscopic observations of standard stars. The second is the instrumentalresponse function of SNIFS and the UH 2.2 m telescope, which is currently solved for using the above mentionedobservations of standard stars, and solving for the instrument response function. SCALA is an attempt tomeasure the instrument response function in-situ.In the next sections we discuss the motivations behind the SCALA concept (Sec. 2) and give a detaileddescription of the device including first test results (Sec. 3). We conclude in Sec. 4.
2. MOTIVATIONS AND MAIN CONCEPT
The use of SNe Ia as distance indicators in cosmology is based on the ability to standardize the flux with anuncertainty of 10-15 % for individual objects. The flux ratio between nearby and high redshift SNe enablescosmological parameters to be constrained. Photons from distant SNe are redshifted, hence, while the absoluteflux calibration cancels in the ratio of brightnesses, one has to establish a reliable (relative) flux calibration asa function of wavelength. SCALA’s main purpose is to calibrate the instrumental response of the “telescope +SNIFS” system to 1% precision. Such a device will provide an independent verification of the flux calibrationby refining the standard star network. Driven by the requirement for accurate SNe Ia distance indicators, ourcurrent efforts are focused on determining the relative wavelength dependent flux calibration; the absolute fluxcalibration is left for future work.We built a device that produces a uniform and homogeneous illumination of the focal plane, or in other wordsa flat-field, which needs to be flat at a sub-percent level in order to be a subdominant effect. Ideally, a flat-fieldwould be a parallel light beam with the size of the entrance pupil of the telescope (2.2 m). Given the difficultiesrelated with the production of such a light beam we opted for many smaller parallel light beams. SCALA consistsigure 1: Hexagonal arrangment of the six submodules of SCALA. This structure has been mounted in front ofthe entrance pupil of the telescope.of 18 f/4 parabolic mirrors with diameters of 20 cm, distributed over the entrance pupil in a nearly hexagonalarrangement (see Fig. 1). Small integrating spheres, fed by a wavelength-tunable (from UV to IR) light source,illuminate the mirrors, producing 18 parallel and collimated beams with opening angles of 1 ◦ . The combinationof these beams allows us to achieve an illumination of the 2.2 m telescope focal plane that is flat to within 1%.A large fraction of the entrance pupil (about 20%) is sampled uniformly with this configuration. Hence, largescale gradients in the reflectivity of the primary mirror – as well as small scale variations – will average out toa large extent. Uniform illumination of the entrance pupil (averaged over roughly 1 m patches) is particularlyimportant for the purpose of SNIFS’ calibration. The optics of this spectrograph are such that the PSF atwhich the individual spectra are imaged on the CCD corresponds to an image of the entrance pupil. Usingsimulations, we have verified that the chosen sampling reproduces the PSF obtained in the case of entirelyuniform illumination of the telescope pupil to a few percent. . DESCRIPTION AND FIRST RESULTS In this section, the light source, integrating spheres, projector modules and photodiode systems, and the resultsof the first tests are discussed.
The requirement for the light source used in our device is to produce mono-chromatic light of sufficient intensityto allow a calibration measurement with better than 1% statistical precision over the SNIFS wavelengths rangefrom 320 nm to 1000 nm using a calibrated photodiode (see Sec. 3.4). We have chosen to use a monochromator(Cornerstone 260) for its ease of use, and a 150 W Xenon arc lamp together with a custom-made tungstenhalogen lamp to illuminate the monochromator. The former lamp is used from 320 nm to 700 nm and the latterfor redder wavelengths. Both lamps are used in order to avoid bright and narrow emission lines produced bythe Xenon lamp above 700 nm, obtaining a continuum spectrum from the near UV to the near IR region. Abandpass of 3 nm was chosen in order to achieve a good balance between resolution and sufficient light level.The light source is entirely computer controlled.
The beam produced by SCALA and imaged in the focal plane of the telescope is an image of the light emitted bythe integrating spheres (IS). SCALA’s ISs have been designed and produced to achieve a flat field with variations < (cid:29) τ = I e I i = ρf e − ρ (1 − f i ) (1)where I e is the emitted flux, I i is the entering flux, ρ is the reflectivity, f e is the ratio between the area of the exitport and the area of the sphere, f i is the ratio between the area of all ports and the area of the sphere. In thecase of SCALA, we have f i = 0 .
02: the diameter of the spheres is 8 cm and the exit ports are 1.4 cm each. Thisvalue of f i is better than the required 0.05 (which is an upper limit) necessary to have homogeneous illuminationof the exit ports. Fig. 3 shows the efficiency of SCALA’s ISs as a function of wavelength, where we see that τ is on average 12%. Thus, according to Eq. 1 ρ = 98% . SCALA is composed of six projector modules (see Fig. 4) arranged in a hexagonal pattern shown in Fig. 1. Asingle fiber bundle, manufactured to fit the slit width and height of the monochromator splits into six bundlesthat feed the IS. Each of them illuminates three parabolic mirrors, which are mounted on a triangular projectormodule back structure. Every projector module is therefore composed of an IS and three parabolic mirrors,tilted to produce parallel beams of light with opening angles of 1 ◦ . The narrowness of the beams reduces theamount of stray light compared to conventional flat fields.Most components of SCALA are constructed of aluminium (for a total weight of 150 kg), and the hexagonalaluminium profiles that hold the projector modules are attached to an iron ladder in the dome of the telescope.Because of this robust construction, SCALA can preserve its position in an environment exposed to wind loadsand earthquakes.igure 3: Transmissivity of a SCALA prototype integrating sphere. The decrease in the IR is expected becausethe PTFE becomes partially transparent for large wavelengths.Figure 4: One of six projector modules for calibration of SNIFS. Left: A projector module with some of thefeatures of the device: the three mirrors, the integrating sphere (the cylindrical object), and the projector holder.Right: A projector module in the laboratory, the small object mounted in front and towards the upper edge ofone of the mirrors is the calibrated photodiode.igure 5: Simulation of the illumination of the focal plane of the UH 2.2 m telescope produced by the superpositionof the 18 parallel beams. The variations are smaller than 1% over its full width.The off-axis illumination of the 18 mirrors produces a gradient in the single reflected light beam of about5%. This gradient is cancelled out because of the symmetrical location of the three mirrors of the projectormodules. The overall beam, composed of the overlap of the 18 beams in the focal plane of the telescope, willbe even flatter due to the symmetrical illumination of the entrance pupil. The flatness of this design has beenverified through simulation (using the photon engineering software FRED ). The simulation result of the finalconfiguration of SCALA is shown in Fig. 5. The beam produced is flat with variations smaller than 1% over itsfull width. Considering that a 1 ◦ beam has a diameter of about 400 mm in the UH 2.2 m telescope focal plane,we can assume that the variations are much smaller over the area of SNIFS ( < × In order to monitor the amount of light entering the telescope we use two Cooled Large Area Photodiodes(CLAPs ) placed in the beams produced by the mirrors. The CLAPs have been developed for (Sn)DICE (Direct Illumination Calibration Experiment) and calibrated against a NIST-calibrated photodiode. They havebeen optimised for our low light levels. The various combinations of fiber bundles, mirrors and integratingpheres result in small spectral variations in the different beams and hence we have to control each of the beamsseparately. The use of 18 CLAPs to calibrate every beam individually was beyond the scope of this project,so we use two CLAPs: the first is used in a stationary configuration to continuously monitor the light level ofone of the beams (the reference beam) and a second CLAP is used to calibrate every other beam against thereference beam. We remind the reader that the goal of SCALA is the relative spectral calibration and our focusis on controlling chromatic effects. Therefore, we do not try to extrapolate the photon flux observed by CLAPto the total photon flux in the beam, which would be required for any attempt of an absolute flux calibration. It is crucial to demonstrate that the overall beam produced in the focal plane of the telescope is flat to within1%. Since the beam seen by SNIFS will be an image of the IS exit port, the flatness requirement translates intoverifying the uniformity of the light emitted by our integrating sphere. To ensure this uniformity, we replacedthe fiber bundle in the input of the IS with a LED and reimaged the light from the exit port onto a Atik 383L+(KAF8300 monochrome) CCD. We aligned an exit port with the CCD, and, after processing the image, we foundthat the variations are 0 .
4% over the full width of the beam (see Fig. 6). Considering that SNIFS will see onlya very small fraction of this beam, this result shows that the beam fulfils the flatness requirement.
4. CONCLUSION
We have described SCALA, a device for the in-situ calibration of the SuperNova Integral Field Spectrograph(SNIFS) mounted on the University Hawaii 2.2 m telescope. The purpose of SCALA is to calibrate the spectraof SNIFS by providing monochromatic flat fields of known chromatic intensity. SCALA consists of 18 mirrorsof 20 cm diameter, illuminated by six integrating spheres that are fed by a tunable monochromatic light source.Located at the telescope pupil, it provides narrow beams of 1 degree width that illuminate the focal planehomogeneously at the sub-percent level with a minimum amount of stray light. The photon flux entering thetelescope is monitored using calibrated photodiodes.We have shown through simulation and tests that the beam produced in the focal plane of the UH 2.2 mtelescope will be flat at the sub-percent level and have verified experimentally that the light emitted by ourintegrating spheres fulfils the same requirement. All components (lamp, monochromator, mirrors, integratingspheres and fiber bundle) have been characterised in the laboratory. SCALA commissioning is currently ongoingat the UH 2.2 m telescope, where all the properties of the device will again be verified.igure 6: Top: the exit port of the IS is aligned with a Atik 383L+ (KAF8300 monochrome) CCD and imaged(the axis are in number of pixels). Bottom: zoom on the averaged intensity of the beam that shows variationsof about 0 .
4% over its full width
CKNOWLEDGMENTS
We are grateful to Laurent Le Guillou and Nicolas Regnault from LPNHE for providing the CLAP modules andsupport. Furthermore we thank Manuel Danner from the Hamburger Sternwarte for realuminising the mirrors.This work was supported by the German Science Foundation through TRR33 “The Dark Universe” as well asthrough a grant of the BMBF-Verbundforschung (“Erasmus-F”) and by the Director, Office of Science, Office ofHigh Energy Physics, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
REFERENCES [1] S. Perlmutter, et al., “ Measurements of Ω and Λ from 42 High-Redshift Supernovae,”
ApJ , pp. 5–65,1999.[2] A.G. Riess, et al., “Observational Evidence from Supernovae for an Accelerating Universe and a CosmologicalConstant,” AJ , pp. 1009–1038, 1998.[3] A. Conley, et al., “ Light-curve parameters from the SNLS (Conley+, 2011),” ApJS , pp. 1, 2011.[4] M. Betoule, et al., “Improved cosmological constraints from a joint analysis of the SDSS-II and SNLSsupernova samples,” arXiv:1401.4064 , accepted, 2014.[5] C. W. Stubbs and J. L. Tonry, “Toward 1% Photometry: End-to-End Calibration of Astronomical Telescopesand Detectors,” AJ , pp. 1436–1444, 2006.[6] R. C. Bohlin, “Spectrophotometric Standards From the Far-UV to the Near-IR on the White Dwarf FluxScale,” AJ , pp. 1743, 1996.[7] C. Juramy, et al., “SNDICE: a direct illumination calibration experiment at CFHT,” in Society of Photo-Optical Instrumentation Engineers , Proc. SPIE , 2008.[8] C. Regnault, et al., “(Sn)DICE: A Calibration System Designed for Wide Field Imagers,” arXiv:1208.6301 ,2012.[9] B. Lantz, et al., “SNIFS: a wideband integral field spectrograph with microlens arrays,” in
Optical Designand Engineering , Proc. SPIE , pp. 146–155, 2004.[10] G. Aldering, et al., “Overview of the Nearby Supernova Factory,” in
Survey and Other Telescope Technologiesand Discoveries , Proc. SPIE , pp. 61–72, 2002.[11] C. Buton, et al., “Atmospheric extinction properties above Mauna Kea from the Nearby SuperNova Factoryspectro-photometric data set,”
A& A , pp. 8, 2013.[12] A. Ducharme, “Design of an Integrating Sphere as a Uniform Illumination Source,”
IEEE Transactions oneducation40