In-Flight Performance and Calibration of the LOng Range Reconnaissance Imager (LORRI) for the New Horizons Mission
H. A. Weaver, A. F. Cheng, F. Morgan, H. W. Taylor, S. J. Conard, J. I. Nunez, D. J. Rodgers, T. R. Lauer, W. M. Owen, J. R. Spencer, O. Barnouin, A. S. Rivkin, C. B. Olkin, S. A. Stern, L. A. Young, M. B. Tapley, M. Vincent
DDraft version January 13, 2020
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In-Flight Performance and Calibration of the LOng Range Reconnaissance Imager (LORRI) for the
New Horizons
Mission
H. A. Weaver, A. F. Cheng, F. Morgan, H. W. Taylor, S. J. Conard, J. I. Nunez, D. J. Rodgers, T. R. Lauer, W. M. Owen, J. R. Spencer, O. Barnouin, A. S. Rivkin, C. B. Olkin, S. A. Stern, L. A. Young, M. B. Tapley, and M. Vincent Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA NSF’s National Optical-Infrared Astronomy Research Laboratory, P. O. Box 26732, Tucson, AZ 85726, USA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA (Accepted PASP, January 2020)
ABSTRACTThe LOng Range Reconnaissance Imager (LORRI) is a panchromatic (360–910 nm for the wave-lengths where the responsivity falls to 10% of the peak value), narrow-angle (field of view = 0 . ◦ . (cid:48)(cid:48)
02) visible light imager used on NASA’s
New Horizons (NH)mission for both science observations and optical navigation. Calibration observations began severalmonths after the
New Horizons launch on 2006 January 19 and have been repeated approximatelyannually throughout the course of the mission, which is ongoing. This paper describes the in-flightLORRI calibration measurements, and the results derived from our analysis of the calibration data.LORRI has been remarkably stable over time with no detectable changes (at the ∼
1% level) in sen-sitivity or optical performance since launch. The point spread function (PSF) varies over the FOVbut is well-characterized and stable, enabling accurate deconvolution to recover the highest possiblespatial resolution during observations of resolved targets, especially when multiple, overlapping imagesare obtained. By employing 4 × ∼
30 s, and co-addition of ∼
100 images, LORRI can detect unresolvedtargets down to V ≈
22 with a signal-to-noise ratio (SNR) of ∼
5. LORRI images have an instanta-neous dynamic range of ∼ ∼
2% (1 σ ) for targets with solar-type spectral energydistributions (SEDs). The accuracy of the absolute calibration for targets with other SEDs shouldbe comparably good when employing synthetic photometry techniques, which we do when derivingLORRI’s photometry keywords. We also describe various instrumental artifacts that could affect theinterpretation of LORRI images under some observing circumstances. Keywords:
Astronomical Instrumentation — telescopes — instrumentation: detectors — methods:data analysis — space vehicles: instruments — techniques: photometric — INTRODUCTION TO LORRI
Corresponding author: H. A. [email protected] a r X i v : . [ a s t r o - ph . I M ] J a n Weaver et al.
The LOng Range Reconnaissance Imager (LORRI) is a narrow angle (FOV=0.291 ◦ ), high spatial resolution(IFOV=1 . (cid:48)(cid:48) ◦ C to − ◦ Cwithout the use of a focus mechanism when mounted within the
New Horizons spacecraft. LORRI’s only movingpart is a once-open aperture cover mounted to the
New Horizons spacecraft, which was opened on 2006 August 29,after LORRI was allowed to outgas for decontamination purposes at a temperature of approximately +50 ◦ C forapproximately 7 months. After the door opened, the OTA mirrors have generally remained within several degrees of − ◦ C, except when a heater is turned on to promote additional decontamination, when the OTA mirror temperaturesrise to approximately − ◦ C. Table 1 details how many times the 10 W decontamination heater has been activatedduring the course of the mission.Figure 2 shows the LORRI OTA during laboratory testing, LORRI’s location within the
New Horizons spacecraft,and the locations of all the instruments on the
New Horizons spacecraft. The boresights of the three remote sensinginstruments (LORRI, Ralph, and Alice) are approximately co-aligned.A 1024 × ∼ ∼
90% at wavelengths near the peak efficiency (see further discussionlater). For the highest resolution observations, all optically active pixels are read out from the CCD (“1 ×
1” format).But the pixels can be also be re-binned by a factor of 4 in each direction (i.e., column and row directions) during
Figure 1.
Computer Aided Design (CAD) views of LORRI. “M1” refers to the primary mirror, “M2” refers to the secondarymirror, “FPA” refers to the focal plane assembly, “Be” refers to beryllium components, and “SAS” refers to the system aperturestop. “L1”, “L2”, and “L3” refer to the 3 separate lenses of the field-flattening lens assembly.
Table 1.
LORRI decontamination activitiesYear Number of decontamination activities2006 2 ∗ ∗ After the LORRI front aperture door wasopened on 2006 August 29, there were two ad-ditional decontamination activities that year.From launch on 2006 January 19 until the doorwas opened, LORRI was allowed to outgas fordecontamination purposes at a temperature ofapproximately +50 ◦ C. Note —A “decontamination activity” involvesturning on the 10 W decontamination heaterfor at least 24 hr, and sometimes for as long as ∼ Figure 2.
The image on the left shows a completely assembled LORRI on a lab bench during ground testing. The middleimage shows where LORRI is mounted within the
New Horizons spacecraft. The image on the right is a drawing showing thelocations of all the instruments on the
New Horizons spacecraft. The diameter of the high gain antenna, which feeds the REXinstrument, is 2.1 m. Short descriptions of all the
New Horizons instruments can be found in Weaver et al. (2008).
CCD readout (“4 ×
4” format), which reduces the data volume by a factor of 16 and results in an effective pixel size of
Weaver et al.
IFOV=4 . (cid:48)(cid:48)
08. During readout in either format, the analog signals are processed using correlated double-sampling andconverted to data numbers (DNs) using a 12-bit analog-to-digital converter, yielding a valid DN integer range from 0to 4095.In addition to the optically active pixels, four columns (one column in 4 × ∼
540 DN but varies linearly with the temperature of the focal plane electronics board. The readout of the extra CCDcolumns enables accurate tracking of the bias level for each image. Further discussion of how the calibration pipelinedetermines the bias level is provided below.The electronics noise, which includes the CCD read noise, is ∼ ×
25 pixelsubarrays (7 × × ∼ × × × × × × ∼ ◦ , the ratiosof the median signal rates was ∼ . × × − ) appear to be ∼
8% larger in 4 × × . − and 19 . − for 1 × × New Horizons mission, the CCD temperature was warmed to ∼ ◦ C. Periodically throughout the mission (seeTable 1), a 10 W decontamination heater is used to warm the CCD to approximately − ◦ C to prevent accumulationof any outgassed contaminants. Otherwise, the LORRI CCD temperature has been stable at approximately − ◦ Cthroughout the mission, which is low enough that dark current is negligible for all 1 × × <
1% of the total number of pixels.As is the case for all imagers using CCDs, the charge transfer efficiency (CTE) is less than 100%, which can result inlower signal rates for targets located at high row numbers (i.e., farthest from the serial transfer output) and potentialdegradation of the PSF. We did not independently measure the CTE, but the manufacturer’s specification sheet givesCTE values of 0 . . ∼ Readnoise Histogram in Jul 2009: 1x1 format
Mean RN = 1.22 ± N ( ) Readnoise Histogram in Jul 2009: 4x4 format
Mean RN = 1.13 ± N ( ) Readnoise Histogram in Jul 2019: 1x1 format
Mean RN = 1.11 ± N ( ) Readnoise Histogram in Jul 2019: 4x4 format
Mean RN = 1.30 ± N ( ) Figure 3.
These figures show in-flight electronics noise histograms for LORRI at two different epochs separated by 10 years:July 2009 data are shown along the top row and July 2019 data are shown along the bottom row. The figures on the left are for1 × × time, followed by a “frame transfer” in which the CCD rows are sequentially transferred from the optically active areato the image storage region, followed by a readout of the image storage region to the downstream electronics, whichconvert the detected electrons to DNs. The digitized image is then transferred to the spacecraft’s solid state recorder(SSR). The spacecraft’s command and data handling (CDH) computer can either losslessly compress the full image, Weaver et al.
Gain Histogram: 1x1 format
Mean = 21.11 ±
16 18 20 22 24 26 28Gain (e/DN)0500100015002000 N Gain Histogram: 4x4 format
Mean = 20.54 ± N Figure 4.
These figures show gain histograms obtained during ground testing for each of the two CCD formats. For both 1 × × . − for 1 × . − for 4 × lossy compress the full image, or window and losslessly compress arbitrarily selected portions of the image. Whencommanded to do so, the CDH computer can send selected images from the SSR to the spacecraft’s telecommunicationshardware for downlink to the antennas of NASA’s deep space network (DSN). For observations conducted in theKuiper belt (i.e., when the heliocentric distance is ≥
30 AU), data downlink rates are typically ∼ ∼ We recently (July 2019) discoveredan error in the exposure times reported by the LORRI flight software. As a consequence, the actual exposure timeis ∼ ≤
10 ms. Wenote also that using the actual exposure time is important for correcting the smear produced during the frame scruband frame transfer process, as described later in more detail. The LORRI data will be re-processed to account for thiserror in the exposure times for a future delivery to the Planetary Data System archive.A flexible autoexposure mode is available whenever the scene being imaged has unknown intensities, but autoexposurecan only be used for exposure times ≤
967 ms. Although we exercise the autoexposure capability at every annualcheckout, we have generally preferred to use two different manual exposure times that span the dynamic range ofinterest, rather than relying on autoexposure mode for in-flight observations of targets with unknown, or poorlycharacterized, brightnesses.A “trigger” mode is also available that enables LORRI to determine autonomously when a target has entered theFOV, via analysis of a 32-bin histogram of the image, and then send images to the spacecraft for a specified durationafter the trigger condition has been satisfied. Trigger mode was developed in 2011 and required uplinking to thespacecraft new LORRI flight software. The primary potential use of trigger mode was to enable taking many (tens to New LORRI flight software was uploaded in July 2019 that enabled exposure times up to 64,967 ms. Prior to that, the longest availableexposure time was 29,967 ms. Observations taken in September 2019 have verified nominal performance with exposure times of 64,967 ms. hundreds) 1 × extended target could be identified that mimicked the conditions of the Pluto flyby. For that reason, the New Horizons
Projectdecided not to use trigger mode during the Pluto flyby. Trigger mode was also not used during the
New Horizons spacecraft flyby of the Kuiper belt object 2014 MU (hereafter “MU69”) because that target was so faint that thenecessary trigger condition could not be satisfied with the coarse (32 bins) image histogram available.The New Horizons spacecraft has two main guidance and control (GNC) states: either spinning at ∼ New Horizons spacecraft fires hydrazine thrusters to maintain the pointing within a specified attitude “deadband”.(Owing to power constraints,
New Horizons doesn’t have reaction wheels to stabilize the pointing.) The deadbandsused during LORRI 1 × ± . (cid:48)(cid:48) ± (cid:48)(cid:48) , with thruster firings approximately every 3 s andevery 6 s, respectively. Typical pointing drift rates during 3-axis observations are ∼ (cid:48)(cid:48) s − ; typical 1 × − ≤ × ∼
80 times per minute to keep the LORRIboresight at a fixed inertial pointing location to an accuracy of ± (cid:48)(cid:48) (1 σ ) for exposure times up to ∼
65 s. RCM hasenabled LORRI deep imaging observations that can reach V ≈ . × V ≈ New Horizons can be found in Cheng et al. (2008). Engineeringdetails on the construction of LORRI can be found in Conard et al. (2005). The ground-based calibration of LORRI isdescribed in Morgan et al. (2005). LORRI’s performance through the Jupiter flyby is described in Noble et al. (2009),and LORRI’s performance during the Pluto flyby is described in Conard et al. (2017). LORRI’s in-flight straylightperformance is described in Cheng et al. (2010), and a detailed comparison of LORRI’s optical design and its actualperformance is described in McMichael & Bentley (2012). Zemcov et al. (2017) describes attempts to measure theextragalactic background optical light with LORRI, and Zemcov et al. (2018) describe LORRI’s potential future usesfor various astrophysical objectives. Below we focus on the results from the in-flight LORRI calibration observations,their trending with time over the entire
New Horizons mission to date, and how the calibration results are used toderive the various photometry keywords needed to transform LORRI images from engineering units to absolutelycalibrated scientific units. CALIBRATION STEPS
Weaver et al.
Figure 5.
This deep LORRI image of the MU69 background field was produced by combining 96 individual 30 s imagestaken on 2017 September 21. Celestial north is up, and east points to the left. The pixels are subsampled by a factor of two(using sinc function interpolation) to remove the pixelated appearance of the raw 4 × − V ≈
22 inthis composite. This composite image was used as a template that could be subtracted from LORRI images of MU69 takenin late-2018, thereby removing nearby stars and enabling the detection of MU69 for optical navigation and deep searches forsatellites and dust in the vicinity of MU69.
Before discussing the in-flight LORRI calibration program and its results, we first describe the steps in the LORRIcalibration pipeline, which is the software that removes instrumental artifacts in the raw images and creates the
Figure 6.
This is a deconvolved LORRI image of the al-Adrisi mountains on Pluto taken on 2015 July 14 when the spacecraftwas 78,600 km from the surface (0.390 km pixel − ). The exposure time was 0.15 s, which is in the typical range used for scienceobservations when LORRI is the prime instrument, with its boresight fixed relative to the target. The region to the right of theframe is the northwest corner of Sputnik Planitia (SP), a giant ice sheet composed primarily of N that displays a geometricalpattern attributed to rising (near the center of a pattern) and falling (at the boundaries of the patterns) icy material heatedfrom below. The mountains are the blocky structures at the edge of SP and are thought to be giant water icebergs floating inthe nitrogen ice. Many other interesting geological features are evident in this image, as discussed in Moore et al. (2016). calibrated images that can be used for scientific analysis. The calibration pipeline makes use of reference files thatare derived from either ground or flight calibration measurements, as described further below. Critically, the pipeline0 Weaver et al.
Figure 7.
This is a deconvolved LORRI image showing even finer detail on Pluto’s surface, in this case the border between theSputnik Planitia ice sheet (the smooth geometrical patterns below and to the right) and the chaotic terrain called the al-Adrisimountain range. This image was taken on 2015 July 14 when the spacecraft was 17,000 km from the surface (0.084 km pixel − ).The LORRI images were taken while scanning the MVIC imager across Pluto at 1000 µ rad s − , which required using LORRIexposure times of only 0.010 s to minimize pointing smear during the scan. Although LORRI uses a standard frame transferCCD, while MVIC uses a CCD in time delay integration (TDI) mode (“push broom mode”), LORRI’s high sensitivity enabledits use during scanning observations. populates various photometry FITS header keywords based on the results from LORRI observations of absolutecalibration standard stars, as discussed in Section 3.2.1. Bias and Dark Subtraction − ◦ C. At this temperature, the CCD dark current is usually negligible for most LORRI observations.The process used to subtract the CCD electronic offset signal (which is called the “bias” level and is used to preventsending negative analog signals to the analog-to-digital converter) automatically includes any dark current that maybe present. From a series of long exposure time images (up to 64.967 s) taken during a test in July 2019 when the CCDtemperature was − ◦ C, we determined an upper limit for the dark current in 1 × − pixel − ,corresponding to 0.040 e s − pixel − at the nominal gain of 21 e pixel − , which is approximately 10 times larger thanthe dark current calculated using data from the CCD manufacturer’s specification sheet. During long exposure ( ≥
10 s)4 × × .
968 ( ± . .
239 ( ± . ∗ T board (1)where “bias” is the bias level in DN and T board is the FPE board temperature. For 4 × ∼ ◦ C to 29 ◦ C.We have recently (in 2019) noticed that the bias level may also display a “start-up” feature, whereby the bias levelstarts at a slightly higher value and gradually plateaus to its expected value over the course of several minutes. Thisbehavior is still being characterized, but the magnitude of the effect is at the sub-DN level, which is inconsequentialfor most
New Horizons science applications. We do, however, want to understand this effect better because it willlikely affect LORRI’s ability to accurately measure the extragalactic background light (Zemcov et al. 2017).As implemented in the LORRI pipeline, the bias+dark subtraction is a two-step process. First, the overall bias+darklevel for an image is subtracted from the raw (“Level 1”) image. Specifically, for 1 × × − − × ×
256 pixels), the median intensity of pixelsin column 257 (which is the re-binned version of CCD columns 1032 − × ≤ ∼ × × × × ∼ × × × × New Horizons mission. No systematic change in the bias images has ever been seen. LORRI images (both bias images and regularimages) sometimes exhibit a small even-odd column signal offsets (“jail bars”), but the effect is generally ≤ Weaver et al.
Figure 8.
This “delta-bias” image, which shows the pixel-to-pixel variations in the bias level, was created by combining 1001 × − in magnitude, which is smaller than the typical electronics noise ( ∼ ≤ ∼ Desmear
As already mentioned above, LORRI does not have a shutter. Thus, the target being observed illuminates the activeregion of the CCD whenever LORRI is pointed at the scene. In particular, the CCD continues to record the sceneas the “scrub” is performed (before the nominal start of the exposure) and as charge is transferred from the activeportion to the storage area (after the nominal end of the exposure). Both of these processes result in a smearing ofthe observed scene along a CCD column.The smear process can be visualized as pulling a sheet of light sensitive film, whose size is exactly the size of theCCD’s active area, uniformly across a scene whose size is also exactly the size of the CCD’s sensitive area. The flushbegins as the film is pulled across the scene starting from the top of the CCD (i.e., starting at the highest CCD row)until the film exactly covers the full scene. The total flush time is the duration of that process, and the rate of motion,which is also the row transfer time, determines how much signal from pixels at higher rows is transferred to anyparticular pixel in the same column. After the flush process is completed, the film stays in place for the commandedexposure time, and each pixel accumulates signal only from the scene imaged at that pixel. After the exposure iscompleted, the film is pulled in the same direction as before, but this time each pixel will accumulate signal from thescene in rows below that pixel as the film is moved to the image storage region of the CCD, which is also completelyoutside the illuminated scene. The total transfer time is the duration of that process, and the rate of rate of motionduring the transfer determines how much signal from pixels at lower rows is transferred to any particular pixel in thesame column. All of the pixels in the CCD storage region are then transferred row-by-row to the CCD serial readoutregister, where the analog signals are amplified and then digitized.For images with no over-exposed pixels and that have fixed pointing during the exposure, the smear can be essentiallycompletely removed using the algorithm described in Cheng et al. (2008). Once a pixel is over-exposed, however,information on its true intrinsic level is lost and the smear correction will be incomplete for that column of the image.As the de-smear algorithm works on a column by column basis, any significant motion of the camera during theexposure in the row direction (i.e., perpendicular to the columns), as can occur when LORRI is used in ride-alongmode with the
New Horizons
MVIC or LEISA instruments, may also limit the accuracy of the correction. Of course,the de-smear correction cannot remove the extra shot-noise associated with the smeared light. For long exposures this isonly a modest effect, but for images of extended targets with short exposure times (i.e., similar to, or smaller than, thetotal frame transfer time of ∼
12 ms), the signals generated during the scrub and transfer processes are comparable to,or even larger than, the signal levels accumulated during the nominal exposure time. In this case, the shot noise fromthe smear signal produces significant degradation of the SNR. For example, when the signal accumulated during theexposure time is comparable to that accumulated during the scrub and transfer, the SNR is reduced by approximately √ ∼
10 compared to performing the matrix multiplications given in Cheng et al. (2008). This faster technique,which produces results essentially identical to those using the full matrix multiplication, has been used in the LORRIdata pipeline since 2014.We have recently investigated two alternative formulations for the desmear step, which are simpler and easier toimplement than the algorithm given in Cheng et al. (2008). One of the algorithms (Owen et al. 2019, in press) adoptsapproximations to accelerate the computation by avoiding matrix operations. Another algorithm, discussed furtherbelow, is exact but involves a matrix inversion for each image. The new algorithms are still being evaluated and mustbe tested extensively before either can be used in the LORRI calibration pipeline.2.2.1.
An Exact Simple De-smear Algorithm Weaver et al.
We are presently investigating an alternative algorithm for the de-smear step, which is simpler and easier to imple-ment than the algorithm given in Cheng et al. (2008). As with the Cheng et al. (2008) approach, the new algorithmis applied on a per column basis, since the smeared charge associated with any pixel stays within its column. Thesolution is derived with a single matrix multiplication that transforms the column of data values as generated byLORRI to a column of pixel values corrected for charge-transfer smear.For any column of pixels in the LORRI CCD, neglecting dark current, the rate at which signal is accumulated in aLORRI CCD pixel within that column can be written as:S k t exp = n (cid:88) i , i > k R i I i t scrub t exp + n (cid:88) i , i < k R i I i t transfer t exp + R k I k (2)where:S k is the measured signal in a pixel in row k (electrons, or “e”)R x is the intrinsic responsivity of pixel x ([e s − pixel − ] / [photons s − pixel − ])I x is the input photon flux at pixel x (photons s − pixel − )t exp is the total exposure time (s), which is the commanded time plus 0.6 mst scrub is the CCD row transfer time during the frame scrub (s)t transfer is the CCD row transfer time during the frame transfer (s)n is the number of rows in the image (1024 for 1 × × larger row numbers compared to the pixel of interest, and is accumulated during theframe scrub process. The second term on the right-hand side of the equation represents the smear contribution to theobserved signal from pixels in the same column, but at smaller row numbers compared to the pixel of interest, and isaccumulated during the frame transfer process. The third term is the actual desired quantity, the signal accumulatedat the pixel of interest during the commanded exposure time.Equation 2 can be recast as a matrix equation: D i = n (cid:88) j=1 g ij F j (3) → D = G F (4)where in any CCD column:D i is the detected signal (including smear) at row i (DN)F j is the true signal (not including smear) at row j (DN)g ij are coefficients determined as described belowThe coefficients of G are analogous to, but not identical to, the (cid:15) coefficients in the smear formulation of Chenget al. (2008). For LORRI, the frame scrub takes a total of 12.15 ms, which means each row transfer (t scrub above)takes 0.0119 ms for 1 × × transfer above) takes 0.0109 ms for 1 × × ij = 1 whenj = i, g ij = t scrub / t exp when j > i, and g ij = t transfer / t exp when j < i.In summary, the measured count rates for the pixels in a column (represented by the column matrix D ) can berepresented by the matrix multiplication of the smear matrix and a column matrix of the actual count rates (i.e., F ,the count rates after the smear contributions have been removed). Thus, the desmear problem reduces to finding theinverse of the smear matrix: F = G − D (5)Note that the matrix inversion need only be performed once for each image because the smear matrix is identicalfor each column in the image. In fact, the smear matrix depends only on the ratios t scrub / t exp and t transfer / t exp . In5principle, the matrix inversion could be performed in advance for each exposure time employed, in which case thematrix inversion within the calibration pipeline could be replaced by a simple lookup of the appropriate invertedmatrix, which could be stored in a reference file directory (e.g., similar to what is done for the delta-bias and flat field).2.3. Flat-Field
Flat-fielding refers to the process of removing the pixel-to-pixel sensitivity variations in the image. An exposureobtained by illuminating the LORRI aperture uniformly with light is called a flat-field image. During ground calibrationtesting, flat-fields were obtained by using an integrating sphere to provide uniform illumination (Morgan et al. 2005).The light source was a xenon arc lamp with a spectrum similar to that of the Sun. The absolute intensity of theinput illumination was measured using a calibrated photodiode. For the panchromatic case, which is the one mostrelevant for flat-fielding LORRI images, the light from the xenon lamp was unfiltered. Flat-field images were alsoobtained by passing the light through bandpass filters centered at five different wavelengths spanning the range overwhich LORRI is sensitive, prior to injection into the reference sphere, to estimate the sensitivity of the flat-fieldsto the spectral distribution of the source. The spatial patterns in the flat-field images change significantly withwavelength. However, the variation in panchromatic flat-fields caused by differences in the spectral distribution of theillumination source are much less significant. Indeed, panchromatic flat-field images produced using a tungsten lampwere virtually indistinguishable from those produced by the xenon lamp. Flat-fields were obtained at four differentsets of thermal environments (at standard laboratory room temperature, and at the lowest, nominal, and highesttemperatures predicted for in-flight conditions), but no significant variations in the flat-field images were detected.No suitable flat field astronomical target was available after launch. The flat-field reference file used in the LORRIcalibration pipeline was produced by averaging 100 flat-field images taken at room temperature during ground testingusing the xenon arc lamp as the light source, debiasing and desmearing the average image as described earlier, andnormalizing the intensities in the active region to a median value of 1 (Figure 9). If S (units are DN) is an image of atarget that has already been desmeared and debiased, and if FF is the reference flat-field image, then the flat-fielded(i.e., photometrically-corrected) target image (C; units are DN) is given by C = S/FF.The LORRI flat-field has been monitored throughout the
New Horizons mission by taking exposures with onboard“cal lamps”, which refer to two small tungsten filament lamps mounted on opposite sides of the CCD, in close proximityto the CCD. The primary function of these onboard lamps is to provide illumination of the CCD during “functional”tests, which verify basic operation of the CCD and its electronics, but not the OTA. The illumination provided bythe lamps is highly non-uniform spatially (see the top row of Figure 10) but is highly stable over time. The temporalstability of the lamps has been monitored annually throughout the mission. Ratios of lamp images taken at differenttimes provide a sensitive measure of the stability of the lamps (see the bottom row of Figure 10). The histogram forthe lamp
Conversion to Scientific Units
The software pipeline that performs the calibration steps defined above does not perform the conversion from DNto physical units because that conversion requires knowledge of the spectral energy distribution (SED) of the target.(The SED is related to the “color” of the target in standard astronomical usage.) Instead, various LORRI FITS headerkeywords (“photometry” keywords) are provided that allow users to convert from DN to physical units depending onthe spectral type and spatial distribution (diffuse vs point source) of the target. Photometry keywords are provided fortargets having spectral distributions similar to Pluto, Charon, Pholus, Jupiter, MU69, and the Sun. The units adoptedfor the radiance (also called “intensity”) of diffuse targets are ergs cm − s − ˚A − sr − . The units adopted for theirradiance (also called “flux”) of point (i.e., unresolved) targets are ergs cm − s − ˚A − . The latest (i.e., current) valuesof the photometry keywords are provided in the header of the calibrated (called “Level 2”) FITS file for the imagebeing analyzed. The photometry keywords derived from the in-flight calibration campaign conducted in July 2016,using the star HD 37962 (with a solar-type SED) as the absolute calibration standard, is provided in Table 2. Allof these keywords enable conversion of raw signals in engineering units to absolute signals at the so-called “pivot”wavelength ( λ pivot ), which is one way of characterizing the “effective” wavelength for a broadband optical instrument.6 Weaver et al.
Figure 9.
The LORRI 1 × NewHorizons mission (as documented from images of the internal calibration lamp). The dark streaks in the lower right quadrantwere apparently produced when a camel hair brush was moved across the CCD (in an attempt to remove the flakes). There aretwo diagonal-shaped artifacts, one in the upper left (the larger one) and one in the upper right, which are the residuals fromour attempt to remove the stray light seen in those regions. Figure 10.
Two “cal lamps” provide illumination of the LORRI CCD during in-flight functional tests. The upper left frameshows a 1 × × ± Weaver et al.
Table 2.
LORRI photometry keywordsKeyword Value (1 × × . × . × RPLUTO 2 . × . × RCHARON 2 . × . × RJUPITER 2 . × . × RMU69 2 . × . × RPHOLUS 2 . × . × PSOLAR 9 . × . × PPLUTO 9 . × . × PCHARON 9 . × . × PJUPITER 8 . × . × PMU69 1 . × . × PPHOLUS 1 . × . × Note —The keywords starting with“R” are diffuse target sensitivity key-words and their values have units of(DN s − pixel − ) / (ergs cm − s − ˚A − sr − ).The keywords starting with “P”are point target sensitivity key-words and their values have units of(DN s − ) / (ergs cm − s − ˚A − ). For pointtargets the signal refers to values integratedover the entire instrumental PSF. The pivot wavelength is defined as: λ pivot = (cid:115) (cid:82) QE ∗ λ dλ (cid:82) QE /λ dλ (6)where “QE” is the total system quantum efficiency (see the next section). The pivot wavelength for LORRI is calculatedto be 6076 ˚A.For convenience to users, we provide a prescription for converting LORRI signal rates to standard V magnitudes inthe Johnson photometric system: V = − . / t exp ) + ZPT + CC − AC (7)where V is the magnitude in the standard Johnson V band (i.e., specifies the target’s flux at 5500 ˚A), S is the measuredsignal in the selected photometric aperture (DN), t exp is the exposure time (s), ZPT is the photometric zero point(18.78 for 1 × × New Horizons team, which are basedon published spectra. For typical LORRI observations of point sources, the SNR is optimized by integrating over acircular aperture with a radius of 5 pixels (1 × × Table 3.
LORRI colorcorrectionsSpectral Type CCO, B, A stars − . . . . − . − . − . . . Note —For targets of thespecified spectral type,CC provides the colorcorrection term in the for-mula used for convert-ing LORRI count rates(DN s − ) to Johnson V magnitude. calibrated) file should be used to convert from the observed count rate in a pixel to a radiance value at LORRI’s pivotwavelength: I = S / t exp / RPLUTO (diffuse target) (8)where:I is the diffuse target radiance (ergs cm − s − ˚A − sr − ) at λ pivot S is the measured signal in a pixel (DN)t exp is the exposure time (s)RPLUTO is the LORRI diffuse photometry keyword for targets with Pluto-like SEDsSince the solar flux (F (cid:12) ) at a heliocentric distance of 1 AU at the LORRI pivot wavelength is 176 ergs cm − s − ˚A − ,the value for the radiance can be converted to I/F (where π F = F (cid:12) ), which is a standard photometric quantity usedin planetary science, using: I / F = π Ir / F (cid:12) (9) → I / F = (S / t exp / RPLUTO) ∗ π r / F (cid:12) (10)where “r” is the target’s heliocentric distance in AU.For unresolved targets (e.g., planetary targets observed at large ranges), the absolutely calibrated flux (also calledthe irradiance) at the LORRI pivot wavelength can be determined using the point source photometry keywords. For atarget with an SED similar to that of MU69, the observed count rate integrated over the LORRI PSF can be relatedto the flux (not to be confused with “F” in “I/F”) at the LORRI pivot wavelength by:F = S total / t exp / PMU69 (point target) (11)where:F is the point target flux, or irradiance (ergs cm − s − ˚A − )S total is the total signal from the target integrated over the PSF (DN)0 Weaver et al. t exp is the exposure time (s)PMU69 is the LORRI point source photometry keyword for targets with MU69-like SEDsWhen observing stars, it is more common to convert the absolute flux to a magnitude in a standard photometricsystem. For an A-type star observed by LORRI, the V magnitude is given by:V star = − . total / t exp ) + ZPT − .
060 (12)where V star is the star’s magnitude in the standard Johnson V band, S total is the total signal integrated over theLORRI PSF (DN), t exp is the exposure time (s), ZPT is the photometric zero point (18.78 for 1 × × − . IN-FLIGHT CALIBRATION MEASUREMENTS AND RESULTSIn-flight LORRI calibration measurements started shortly after the launch of the
New Horizons spacecraft on2006 January 19 and have continued at regular intervals throughout the mission. The relevant calibration measure-ments conducted during the mission are listed in Table 4. The results from these observations are summarized in thefollowing sub-sections. 3.1.
Optical Performance
LORRI’s optical performance has been monitored throughout the mission by observing star clusters. Measurementsof the two-dimensional spatial distributions of individual stars enable characterization of the LORRI PSF across theentire FOV. Photometry of the individual stars is performed to monitor LORRI’s sensitivity, both across the FOVand as a function of time. By using clusters whose stars are catalogued in one or more astrometric surveys, LORRI’sgeometrical distortion can also be measured and monitored as a function of time.The galactic open cluster Messier 7 (M7; alternate names are NGC 6475 and the Ptolemy cluster) was used forLORRI’s optical performance calibrations from 2006 through 2013. In 2013, we switched to the galactic open clusterNGC 3532 (alternate names are the Wishing Well cluster, the Pincushion cluster, and the Football cluster), primarilybecause NGC 3532 has a higher density of stars (Figure 11), which allows better areal coverage over the full CCDand improved mapping of LORRI’s geometrical distortion. We observed both clusters during ACO-7 in 2013 so thatlater observations of NGC 3532 (i.e., those taken after 2013) could be compared to earlier observations of M7 (i.e.,those taken between 2006 and 2013), thereby enabling systematic monitoring of LORRI’s performance over the entiremission.The optical design of LORRI has a small amount of pin-cushion distortion, which is clearly detected in measurementsof the star clusters (Figure 12). However, LORRI achieved its design goal of keeping the geometrical distortion ≤ σ level). The LORRI geometrical distortion is described in detail in the SPICE instrumentkernel for LORRI, which is archived at the Small Bodies Node (SBN) of the Planetary Data System (PDS). Thegeometrical distortion coefficients described within the Simple Image Polynomial (SIP) framework, which is commonlyused in astronomy, are captured in the LORRI FITS header keywords. We note also that the archived LORRI FITSfiles employ World Coordinate System (WCS) keywords to enable accurate transformations between native LORRI[x,y] pixel locations and standard astronomical coordinates (e.g., [RA,DEC]).High SNR PSFs for both 1 × × × Table 4.
LORRI in-flight calibration observationsCal ID Date SAP ID Target Objective(1) (2) (3) (4) (5)ACO-0 2006 Apr 23,24 025 N/A Bias images, CR monitoring (4 × × × ≤
10 ms)2006 Sep 29 013 N/A Solar scattered light (3-axis)2006 Sep 24 020 M7 Mosaic and geometric distortion2007 Jan 10 023 Callirrhoe First test of RCM (t exp = 5 ,
10 s)ACO-1 2007 Sep 29 013 N/A Solar scattered light (3-axis)2007 Oct 29 050 N/A Functional testACO-2 2008 Oct 13 047 N/A Solar scattered light (spinning)2008 Oct 15 043 N/A Functional testACO-3 2009 Jul 21 050 N/A Functional testACO-4 2010 Jun 24 047 N/A Solar scattered light (spinning)2010 Jun 25 043 M7 Optical performance, FunctionalACO-5 2011 May 23 050 N/A Functional testACO-6 2012 May 23 050 N/A Functional test2012 Jun 01 055 M7 Optical performanceACO-7 2013 Jun 22 050 N/A Functional test2013 Jul 02 080 M7 Optical performance2013 Jul 03 081 NGC 3532 Optical performanceACO-8 2014 Jul 05 050 N/A Functional test2014 Jul 22 082 NGC 3532 Optical performanceCal Campaign 2016 Jul 03 050 N/A Functional test2016 Jul 11 081 NGC 3532 Optical performance2016 Jul 14 102 HD 37962 Absolute calibration2016 Jul 16 103 HD 205905 Absolute calibration2017 Sep 18 081a NGC 3532 Verify performance after 100 days without decontamination2017 Dec 05 081c NGC 3532 Verify performance after 180 days without decontamination
Note —“ACO” stands for “Annual Check Out”. “SAP” stands for “Science Activity Plan”. “CR” stands for “CosmicRay”. “RCM” stands for “Relative Control Mode”. “Functional” tests include bias images (0 ms exposure times), lampexposures, and autoexposure images. All listed dates are in UTC in the spacecraft frame. of Pluto’s satellite Kerberos, when four separate images could be combined to create a composite with significantlyhigher resolution than the individual images (Figure 14). Deconvolution can also be used to remove motion smear forLORRI images taken during scanning observations of the Ralph instrument (e.g., Figure 7).Both the shapes and intensities of the stars in the calibration fields are used to determine whether there has beenany degradation in the optical performance over the course of the mission. For example, contamination (e.g., iceaccumulation) anywhere along LORRI’s optical path could manifest as a reduction in photometric sensitivity and/ora broadening of the PSF. We monitor the PSF shape by fitting 2-dimensional Gaussians to the stellar images eachtime the calibration fields are observed. We monitor LORRI’s sensitivity by comparing the signals for the same starsmeasured at multiple epochs.2
Weaver et al.
Figure 11.
LORRI 1 × − Figure 15 shows the results of the Gaussian fits to the stars measured during the observations of NGC 3532 duringthe calibration campaign in July 2016. By comparing the PSFs from stars falling in five different regions of the CCD,we show how the PSF varies across the LORRI FOV. The PSF behavior exhibited in this figure is typical of what hasbeen observed throughout the mission. Figure 16 explicitly compares the LORRI PSFs measured on M7 stars over a5-year period (2008-2013); there is no significant variation in the shape of the PSF over this period.The Gaussian fits applied to the star cluster images employed a ±
10 pixel region centered on each star’s peak pixel,which includes both the core and a significant fraction of the wings of the spatial brightness distribution. Restrictingthe fit to the core only, which is a better measure of LORRI’s spatial resolution, results in a narrower full width at halfmaximum (FWHM). For example, using the same Gaussian fitting routine on the 1 × ±
10 pixel region is fit, and (XFWHM,YFWHM)=(1.87,2.47) pixelswhen a ± ∼ Absolute Calibration
Since the SEDs of virtually all solar system targets are produced by scattered sunlight, we searched for absolutestandard stars that are solar analogs to calibrate LORRI. Fortunately, two solar-type standard stars with absolutefluxes measured to an accuracy of ∼
1% (1 σ ) by the Hubble Space Telescope ( HST ) have V magnitudes that are suitablefor high SNR LORRI measurements, and they are also visible from the New Horizons spacecraft at reasonably largesolar elongation angles for the entire mission.HD 37962 has V = 7 .
85 and B − V = 0 .
65, which is identical to the solar color. This star was observed by LORRIon 2016 July 14, as part of the special post-Pluto calibration campaign. The solar elongation angle was 56 ◦ , and thesolar scattered light level was negligible in all images. We obtained 5 different 100 ms exposures in 1 × Figure 12.
LORRI 1 × × Weaver et al.
Figure 13.
LORRI composite PSFs, valid for locations near the center of the CCD, are displayed for both 1 × × ×
32 pixel regions are displayed using a hyperbolic sine (sinh) intensitystretch to show more clearly the full dynamic range of the image. Diffraction spikes from the three legs of the OTA spider areclearly evident. Encircled energy (EE) plots are displayed below each image; each data point is labeled with the fraction of lightwithin the plotted radius. In 1 × ∼
14% of the total intensity. In 4 × ∼
32% of the total intensity. Figure 14.
Demonstration of the image processing steps performed to produce a deconvolved LORRI image of Kerberos, one ofPluto’s four small satellites. (A) A single calibrated LORRI image, one of four taken. (B) The interlaced and Nyquist-sampled“superimage” generated by combining the four calibrated images – the pixel scale is twice as fine as the native LORRI scale.(C) The superimage after applying Lucy-Richardson deconvolution. (D) The deconvolved image up-sampled by an additionalfactor of 4 for a final scale 8 times finer than the native LORRI scale. This latter step removes the pixelated appearance of theprevious image for improved clarity, and is mathematically justified since the superimage is Nyquist-sampled. Adapted fromWeaver et al. (2016). × ≥
140 in all 10 images. We usedaperture photometry and the gain values discussed previously to calculate the total fluxes in electrons s − . As shownin Figure 19, the measured fluxes for the 1 × × V = 6 .
74 and B − V = 0 .
62, which is slightly bluer than solar color. This star was observed byLORRI on 2016 July 16, also as part of the special post-Pluto calibration campaign. We obtained 5 different 100 msexposures in 1 × × ≥ ◦ , and the solar scattered light level was negligible in all images.The photometry results for HD 205905 are fully consistent with the results obtained from HD 37962.Figure 20 shows a comparison of the stellar and solar spectra. Given the remarkably solar-like SED for HD 37962,we decided to use that star as the primary LORRI absolute calibration standard. Assuming that LORRI’s relativeresponsivity (see the next section) as a function of wavelength was accurately measured during the ground calibration(Morgan et al. 2005), we adjusted the absolute scale of the responsivity curve using a single constant multiplicativefactor to force the calculated signal from HD 37962 to match the observed signal. The calculated signal is the integralover all wavelengths of the product of the star’s SED in absolute units and LORRI’s responsivity curve. The newLORRI responsivity curve produced in this way is discussed in the next section. For targets having solar-type SEDs,the absolute accuracy of the values derived from LORRI data should be comparable to the accuracy of the HST measurements (i.e., ∼ σ ) because the LORRI calibration is tied to the absolute flux from HD 37962. However,the accuracy of the LORRI calibration also depends on the accuracy with which the total measured LORRI signal fromHD 37962 is determined. The peak pixel during these measurements has SNR ≥ × × relative photometry of different solar-type stars is set by the SNR of the individual measurements, which can be considerablymore accurate than the absolute flux measurement.For targets with SEDs significantly different than solar-type, the LORRI calibration is dependent on the accuracyof the shape of LORRI’s responsivity curve, as determined from the ground calibration (Morgan et al. 2005). Asdiscussed there, we assumed that the wavelength dependence of the responsivity followed the prescriptions providedby the various vendors contributing to the LORRI hardware. While these prescriptions usually involved measurementsmade on actual LORRI hardware (e.g., transmittance measurements for the optical components), we assumed a QEcurve for the CCD that followed the “typical” response specified by the manufacturer for the anti-reflection layerordered for the LORRI CCD. The shape of the CCD QE curve is thought to be accurate to a few percent in theregions of high QE (i.e., for most of the wavelength range covered by LORRI), but larger variations in QE are possiblein the steeply sloped portions of the QE curve. Given the wide bandpass of LORRI, these latter variations will likelynot significantly affect the measured signals from LORRI’s targets. For all these reasons, we suggest that the absolute6 Weaver et al.
Figure 15.
Two dimensional Gaussian fits to the spatial brightness distributions of stars in NGC 3532 observed during thecalibration campaign in July 2016 are plotted. The LORRI boresight was moved around in a 3 × ×
256 pixel region on the CCD, as indicated in the figure. The CCDhas an optically active region of 1024 × Figure 16.
Two dimensional Gaussian fits to the spatial brightness distributions of multiple stars in the open galactic clusterM7 are plotted for four different epochs. The +x-dimension corresponds to the direction of increasing CCD columns, and the+y-dimension corresponds to the direction of increasing CCD rows. The average full width half maximum (FWHM) in eachdimension, and the standard deviation of the FWHM, are listed and plotted for each epoch. ACO is an acronym for “AnnualCheck Out”. See the text for further discussion. accuracy for LORRI observations of non-solar-type targets is probably ∼ Responsivity Curves
For a photon counting optical system like LORRI, the signal detected (S e in electrons) in an image pixel withexposure time t exp (in seconds) can be expressed as:S e = t exp ∗ A ∗ Ω ∗ (cid:90) λ I ∗ QE d λ (diffuse target) (13)and: S e = t exp ∗ A ∗ EE ∗ (cid:90) λ F ∗ QE d λ (point target) (14)8 Weaver et al.
Figure 17.
LORRI aperture photometry of stars in M7 at four different epochs is compared to that measured on 2006-August-31 (ACO-0), immediately after the telescope door was opened. For each epoch, a histogram is plotted showing the ratio of theobserved stellar fluxes to the fluxes measured for those same stars in 2006. The number of matched stars is displayed on eachplot. The histogram bin widths are 0.02, and the bin location of the peak in the distribution is listed on each plot. ACO is anacronym for “Annual Check Out”. See the text for further discussion. Figure 18.
LORRI aperture photometry of stars in NGC 3532 at four different epochs is compared to that measured on2013-July-03 (ACO-7). For each epoch, a histogram is plotted showing the ratio of the observed stellar fluxes to the fluxesmeasured for those same stars in 2013. The number of matched stars is displayed on each plot. The histogram bin widths are0.02, and the bin location of the peak in the distribution is listed on each plot. ACO is an acronym for “Annual Check Out”.See the text for further discussion. where:A is the unobscured aperture area of the OTA (339.8 cm )Ω is the IFOV (2 . × − sr for 1 × . × − sr for 4 × − s − ˚A − sr − )F is the point target flux (photons cm − s − ˚A − )EE is the fraction of the point source flux captured in the peak pixelQE is the total system quantum efficiencyThe noise (N in electrons) in a LORRI image pixel can be expressed as (for both diffuse and point targets):N = (cid:113) S e + SL + FT + (I d ∗ t exp ) + RN (15)where:0 Weaver et al.
HD 37962 : LORRI Fluxes
Mean 1x1 value is 506.41 Mean 4x4 value is 506.31 C o un t R a t e ( e l e c t r o n s s - ) Figure 19.
The total fluxes from the absolute calibration standard star HD 37962 are plotted for five different 1 × × × × × × (cid:46) ∼
30 s, which is the spacing between consecutive images for these observations.
SL is the signal produced by solar scattered light (e)FT is the signal produced by the CCD frame scrub and transfer process (i.e., smear) (e)I d is the CCD dark current (e s − pixel − )RN is the electronics noise (e, including the CCD read noise)The LORRI solar scattered light level (SL) has been measured multiple times during the mission (Table 4). Forsolar elongation angles (SEAs) smaller than ∼ ◦ , the scattered light level varies as a function of the spacecraft rollangle. Figure 21 shows a model for the I/F of the solar scattered light level as a function of SEA. The model falls1 Figure 20.
The spectral energy distribution (SED) of the two absolute calibration standard stars used by LORRI (HD 37962and HD 205905) are compared to the solar spectrum. All spectra are normalized to their peak values. The SED of HD 37962is particularly close to that of the Sun, so this star was selected as the primary standard star for LORRI absolute sensitivitycalibrations. roughly halfway between the smallest and largest observed scattered light values. At small SEAs, the actual I/F of thesolar scattered light could be up to a factor of two times smaller or larger than the model value. The model I/F valuesare independent of the spacecraft’s heliocentric distance ( r ), but the CCD signal rate (DN/s) has an r − dependence.The SEA at which the solar scattered light produces a signal rate of 10 DN s − pixel − (i.e., ∼ × larger than theelectronics noise) is shown in the figure for the heliocentric distances of the New Horizons flybys (i.e., at Jupiter,Pluto, and MU69).The LORRI team created an exposure time calculator (ETC) that uses the above formalism to estimate SNRs forplanned observations. The ETC can also be used estimate target signals in absolute units by comparing observed SNRsto SNRs reported by the ETC. For point sources with SNR ≥ ∼ obscure ∗ T optics ∗ QE CCD (16)where:L obscure is the loss factor associated with obscuration by the secondary mirror and OTA spiderT optics is the total transmittance of all optical elementsQE
CCD is the quantum efficiency of the CCDAlthough not explicitly stated, all quantities in the equation above are a function of wavelength. Prior to any systemcalibration measurements, LORRI’s QE was estimated from equation 16 using component level measurements frommanufacturers. The original LORRI ETC also relied on these component level measurements. The system level QEwas determined from ground calibration measurements (Morgan et al. 2005), and those measurements were used to2
Weaver et al.
Figure 21.
A model for the LORRI scattered light level is plotted as a function of solar elongation angle (SEA). The SEAat which the solar scattered light produces a signal rate of 10 DN s − pixel − (i.e., ∼ × larger than the electronics noise) isshown in the figure for the heliocentric distances of the New Horizons flybys (i.e., at Jupiter, Pluto, and MU69). update the ETC. Subsequently, in-flight measurements of absolute calibration standard reference stars (see the previoussection) were used to further refine LORRI’s absolute responsivity. The final LORRI absolute calibration is essentiallya hybrid product, with the wavelength dependence determined from the ground calibration (when filters could be usedto restrict the wavelengths sampled) and with the absolute sensitivity determined by scaling the wavelength-dependentcurve by whatever factor is needed to force a match between the predicted and observed LORRI signals for observationsof the absolute calibration standard stars.Figure 22 shows the LORRI system QE as a function of wavelength, as determined from the absolute calibrationmeasurements of the solar-type reference star HD 37962. The LORRI system QE is approximately 50% over much ofthe visible wavelength range (e.g., 480–700 nm). LORRI is a panchromatic instrument, which means that its outputsignal is proportional to the integral over all wavelengths of the product of the QE and the target’s SED.Note that the commonly used quantity “effective area” (A eff ) of the optical system is just the area of LORRI’sinput aperture (A = π ∗ . / . ) multiplied by the system QE. Given the estimated LORRI obscurationof ∼
11% (i.e., L obscure = 0 . ∼
300 cm . Figure 23 shows the LORRIeffective area as a function of wavelength.3 Figure 22.
LORRI’s system quantum efficiency (QE) is plotted as a function of wavelength. The mean QE over the wavelengthrange 400 −
850 nm is ∼ V and R band wavelengths. The LORRI absolute responsivity curve, which is used to calculate the photometry keywords, can be derived fromthe QE curve using the following equation: R λ = A eff ∗ Ω ∗ λ/ gain / hc (17)where:R λ is the responsivity ([DN s − pixel − ] / [ergs cm − s − sr − ])A eff is the effective area as defined above (cm )Ω is the solid angle of a single pixel (sr) λ is the wavelength of interest (nm)gain is the CCD gain (21 e DN − for 1 × . × − J-nm)Figure 24 shows the LORRI responsivity curve as a function of wavelength for 1 × × × Weaver et al.
Figure 23.
LORRI’s effective area is plotted as a function of wavelength. Given the estimated LORRI obscuration of ∼ Figure 24.
LORRI’s absolute responsivity is plotted as a function of wavelength. The locations of the LORRI pivot wavelengthand the standard visible photometric bands are also shown. Weaver et al.
Table 5.
Summary of LORRI key parametersItem DescriptionOptical Telescope Assembly (OTA) L3H-SSG Ritchey-Chr´etien optical design with 3-element field flattener lens assemblySilicon Carbide (SiC) structure, SiC mirrors coated with high reflectance dielectric20.8 cm primary mirror diameter, ∼
11% central obscurationFocal length = 261.908 cmPlate scale = 0.381813 µ radians µ m − = 787 . (cid:48)(cid:48)
546 mm − NH OTA in-flight operating temperature is approximately − ◦ CNo moving parts, except for once-open telescope cover mounted to spacecraftFocal Plane Characteristics Teledyne-e2v 47-20 frame transfer CCD detectorCCD frame transfer time ≈
12 ms1024 × µ m square pixelsAR-coated, backside-thinned, backside-illuminated CCD1 × × × µ radians = 1 . (cid:48)(cid:48) . . ◦ ≈ ≈
24 eDynamic range ≈ − (1 × − (4 × ≤ − pixel − (1 × − ◦ C)Available exposure times: 0 ms to 64,967 ms at 1 ms spacings1 Hz maximum frame rate (minimum time between consecutive images is 1 s)Wavelength Range Panchromatic (no filters) with ∼
50% peak QE435–870 nm at 50% of peak QE360–910 nm at 10% of peak QEPhotometric Accuracy ∼
2% (1 σ ) absolute for solar-type SED ≤
10% (1 σ ) absolute for non-solar-type SEDs ≤
1% (1 σ ) relative for SNR ≥ ∼
1% level for ≥
13 years
Note —“NH” stands for “New Horizons”, “CCD” stands for “Charge Coupled Device”, “IFOV” stands for “Individual pixelField of View”, “FOV” stands for “Field of View”, and “AR” stands for “Anti-Reflection”. “CDS” refers to “CorrelatedDouble Sampling”, “ADC” refers to analog-to-digital converter, “QE” stands for “Quantum Efficiency”,“SED” stands for“Spectral Energy Distribution”, and “SNR” stands for “Signal-to-Noise Ratio”.4.
SUMMARYLORRI’s key design parameters are summarized in Table 5. LORRI has played a major role in the success ofthe
New Horizons mission by serving as the primary optical navigation camera, by providing the highest resolutionmeasurements of the mission’s flyby targets, and by performing high sensitivity observations of remote targets atunique geometries.Assuming that the wavelength variation of LORRI’s sensitivity is accurately described by the ground-based calibra-tion, LORRI’s absolute sensitivity should be accurate to ∼
2% (1 σ ) for targets with solar-type SEDs. The accuracy ofthe absolute calibration for targets with other SEDs should be comparably good when employing synthetic photometrytechniques, which we do when deriving LORRI’s photometry keywords.7LORRI’s sensitivity and optical performance are essentially unchanged since the launch of the New Horizons missionin January 2006, more than 13 years ago. Although LORRI is a “single string” instrument, susceptible to a singlepoint failure to one of its critical components, its longevity is testimony to its simple, yet powerful, design. Indeed,the next generation of LORRI is currently being built to serve similar functions on NASA’s
Lucy mission, which isscheduled to launch in October 2021 when it will begin a program to perform the first flyby measurements of six JovianTrojans (Levison et al. 2017). ACKNOWLEDGMENTSWe thank the scientists, engineers, and managers at APL who participated in the design, construction, and testing ofLORRI prior to launch: J. Boldt, K. Cooper, H. Darlington, M. Grey, J. Hayes, P. Hogue, T. Magee, E. Rossano, and C.Schlemm. We thank G. Rogers for his outstanding and innovative support of the spacecraft pointing operations, whichenabled the LORRI observing program. We thank the
New Horizons system engineering team (D. Kusnierkiewicz,C. Hersman, V. Mallder, G. Rogers) for its excellent work in maintaining the health and safety of the spacecraft.We thank S. Williams and A. Mick for their outstanding support of the
New Horizons command and data handlingsystem, including their management of complex solid state recorder operations. We thank the
New Horizons
MissionOperations team (especially A. Bowman, K. Whittenburg, and H. Hart) for its expert implementation of the LORRIobservational program. We thank the
New Horizons
Science Operations team (E. Birath, A. Harch, D. Rose, andN. Martin) for its expert scheduling of the LORRI observations and data downlink. We thank M. Holdridge for hisleadership during the Pluto and MU69 flyby campaigns. We thank the personnel at NASA’s Deep Space Networkfor their support of communications with the
New Horizons spacecraft, including the downlinking of the missionengineering and science data. We thank the
New Horizons
Project Managers, G. Fountain and H. Winters, for theirsteadfast support of LORRI throughout the mission. We thank B. Carcich for developing a computational shortcut forthe original desmear calculations. We thank R. Bohlin for discussions regarding absolute calibration standards. Wethank the personnel at SSG Precision Optronics (now L3-Harris SSG) who played major roles in building the LORRIOTA: F. Azad, K. E. Kosakowski, and D. Sampath.
Software:
Interactive Data Language (IDL), licensed by the Harris Corporation8
Weaver et al.