Photometry using the Infrared Array Camera on the Spitzer Space Telescope
Joseph L. Hora, Sean Carey, Jason Surace, Massimo Marengo, Patrick Lowrance, William J. Glaccum, Mark Lacy, William T. Reach, William F. Hoffmann, Pauline Barmby, S. P. Willner, Giovanni G. Fazio, S. Thomas Megeath, Lori E. Allen, Bidushi Bhattacharya, Manuel Quijada
aa r X i v : . [ a s t r o - ph ] S e p To appear in the Publications of the Astronomical Society of the Pacific
Preprint typeset using L A TEX style emulateapj v. 08/22/09
PHOTOMETRY USING THE INFRARED ARRAY CAMERA ON THE SPITZER SPACE TELESCOPE
Joseph L. Hora , Sean Carey , Jason Surace , Massimo Marengo , Patrick Lowrance , William J. Glaccum ,Mark Lacy , William T. Reach , William F. Hoffmann , Pauline Barmby , S. P. Willner , Giovanni G. Fazio ,S. Thomas Megeath , Lori E. Allen , Bidushi Bhattacharya , Manuel Quijada To appear in the Publications of the Astronomical Society of the Pacific
ABSTRACTWe present several corrections for point source photometry to be applied to data from the InfraredArray Camera (IRAC) on the Spitzer Space Telescope. These corrections are necessary because ofcharacteristics of the IRAC arrays and optics and the way the instrument is calibrated in-flight. Whenthese corrections are applied, it is possible to achieve a ∼
2% relative photometric accuracy for sourcesof adequate signal to noise in an IRAC image.
Subject headings: infrared, instrumentation, calibration, IRAC, Spitzer Space Telescope INTRODUCTION
The Infrared Array Camera (IRAC) was built at theNASA Goddard Space Flight Center under the direc-tion of a team led by Giovanni Fazio at the SmithsonianAstrophysical Observatory (Fazio et al. 2004). IRAC isthe mid-infrared camera on the Spitzer Space Telescope(Werner et al. 2004), with four arrays, or “channels”, si-multaneously taking data in two separate fields of view.The four channels are referred to in this paper with theirstandard labels of 3.6, 4.5, 5.8, and 8.0 µ m for channels 1,2, 3, 4, respectively, although as described by the IRACdocumentation and by Fazio et al. (2004); Reach et al.(2005) the nominal wavelengths differ from these labels.The absolute calibration of the camera was performedin flight by comparison to a set of stars that had beenselected and characterized before launch (Megeath et al.2003; Cohen et al. 2003). Reach et al. (2005) presentedthe IRAC in-flight calibration results, including the ob-serving strategy, the predictions and measurements, andan assessment of the calibration accuracy and stability ofthe instrument and the pipeline-processed data providedby the Spitzer Science Center (SSC) to observers.There are several characteristics of the IRAC instru-ment that affect the accuracy of the photometry obtainedfrom the images. The effects considered in this paper are:1. The IRAC science pipeline generates images inunits of surface brightness, but because of distor-tion, the pixels do not subtend constant solid angle.This causes errors in point source photometry thatvary over the field of view (FOV) in each channel. Electronic address: [email protected] Harvard-Smithsonian Center for Astrophysics, 60 GardenStreet, MS-65, Cambridge, MA 02138 Spitzer
Science Center, MS 220-6, California Institute of Tech-nology, Pasadena, CA 91125 Steward Observatory, Univ. of Arizona, Tucson, AZ 85721 Department of Physics & Astronomy, University of WesternOntario, 1151 Richmond St, London, ON N6A 3K7, CANADA Ritter Observatory, mail drop 113, The University of Toledo,2801 West Bancroft Street, Toledo, Ohio 43606 NASA/Herschel Science Center, California Institute of Tech-nology, Pasadena, CA 91125 Goddard Space Flight Center, Optics Branch, Code 551,Greenbelt, MD 20771 http://ssc.spitzer.caltech.edu/irac/
2. The IRAC spectral response varies over the field ofview, and therefore the color corrections are fielddependent.3. The electron rates in the 3.6 µ m and 4.5 µ m chan-nels depend slightly on pixel phase (the positionof the star relative to the nearest pixel center).The pipeline calibration factors are correct on av-erage, as is appropriate for sources observed multi-ple times at multiple dither positions. For the mostprecise photometry, however, pixel phase should betaken into account.4. A correction must be applied for the size of theaperture and background region used in aperturephotometry, if different from that used by Reach etal. (2005) to derive the calibration from standardstars.The corrections described in this paper should be ap-plied to the photometry in a manner consistent withthose applied by Reach et al. (2005) (which includes us-ing the centroiding technique and the aperture and annu-lus background sizes described by Reach et al., applyingthe point source gain correction described in Section 2.2below, and the pixel phase correction described in Sec-tion 3 below), in order for the absolute calibration toremain valid and to achieve <
2% photometric accuracyreported. FIELD-OF-VIEW (FOV) DEPENDENT EFFECTS
A portion of the IRAC optical layout is shown in Fig-ure 1, and the details are described by Fazio et al. (2004).This figure shows the relationship between the channels– the two separate fields of view are shared by pairs ofchannels (channels 1 & 3 and channels 2 & 4). Each pairshares the same doublet lens and is divided at the beam-splitter where the short wavelength light is reflected andlong wavelength transmitted. In both channels, the lightthen passes through filters before entering the detectorarrays at an angle that depends on the position in thefield of view. The tilted elements and the variable angleof incidence on the filters and array causes some of theeffects on the IRAC photometry described below. Hora et al.
Distortion and Pixel Area
The IRAC images have distortion in each channel dueto its optical design, resulting in a pixel displacementof ∼ Point Source Gain Correction
The pipeline gain or “flat” correction is determinedfrom observations of the zodiacal background (zodi)emission. Regions of high and low zodi background (nearthe ecliptic plane and poles, respectively) are observedduring an IRAC campaign. The flat image in each bandis obtained by rejecting point sources in the ditheredframes and differencing the high and low zodi images.The illumination by the zodi is assumed to be uniformover the FOV, so dividing each science image by a nor-malized version of the flat corrects most of the pixel-to-pixel gain variations that exist in the arrays. The flatcorrection has been found not to vary over time duringthe mission within the measurement noise, so data fromthe whole Spitzer mission have been combined into a “su-perflat” which is used to correct the data for the entiremission. The SSC will soon reprocess all of the IRACdata with the S18 pipeline, which will use the superflatconstructed from the first four years of Spitzer operation.The flat correction works very well for extended sourceswith colors similar to the zodi. The changes in pixel areaover the FOV due to the geometric distortion are part ofthe flat, as are the effective wavelength variations, andthe BCD are calibrated to units of MJy/sr. In addi-tion, the spectrum of the zodi is quite different than thatof a typical star (Figure 3), so essentially the oppositecorrection for effective wavelength is applied by the flatto data from normal stars. Furthermore, scattered lightfrom extended emission outside of the FOV could affectthe flat measurement near the array edges, and light orcharge spreading within the array could cause differencesbetween extended and point source photometry. All ofthese effects and possibly others lead to a variation inthe photometry of a point source at different locationson the array. We can use the photometry of a star atmany points on the array to derive a correction to applyto remove the variation.
Data and Analysis
We derived the point source gain correction fromcharacterization data taken during IOC. The obser-vations were taken after telescope cooldown and fi-nal focus adjustment. A standard star (BD+671044 = SAO 17718) was observed on a 5x5 grid(a square grid with equal spacings of roughly 50 pixels) across the arrays using Astronomical Observ-ing Requests (AORs) ADS/Sa.Spitzer phot command in IRAF . A radius of 10 IRAC pix-els was used. In each channel, the photometry variessystematically across the FOV. The variations can be fitby a quadratic surface across the arrays, a different onefor each IRAC channel. The point source photometrycorrection factor F psp was found by fitting the function F psp ( x, y ) = A + B ( x − C ( y − D ( x − y − E ( x − + F ( y − (1)where x and y are the pixel coordinates in the BCDframe, and the centers of the pixels run from 1 to 256 inboth axes. The convention used by Reach et al. (2005) forthis correction in the absolute calibration was to define itrelative to pixel (128,128). The polynomial coefficientsof the fit are given in Table 1. The fit was performedrelative to pixel (128,128), and then the array of correc-tion values was normalized so that the median value ofthe correction factor over the array is 1. In the case ofAORs designed to obtain photometry for a source usinga small dither pattern near the center of the array, thecorrection is near 1.0 in all channels. A cubic fit did notimprove the quality of the correction significantly. Thefitted surfaces are shown in Figure 4, and the coefficientsare listed in Table 1. Correction images that can be ap-plied to IRAC data are supplied on the SSC website .To correct the data, F corr ( x, y ) = F psp ( x, y ) × F measured .In channels 1 and 2, the pattern is “bowl”-shaped, withthe uncorrected photometry having the smallest valuenear the center of the field. For channels 3 and 4, the pat-tern is dominated by a gradient mostly left-right acrossthe images with opposite signs. The maximum ranges(maximum-minimum correction values) are 4.7%, 5.9%,13%, and 9% for channels 1-4, respectively. Since theextreme values are at the edges or corners of the arrays,the errors for uncorrected stars closer to the centers ofthe arrays are much smaller. For example, for objects inthe central 128 ×
128 pixel area, the range of correctionsare 1.6%, 1.9%, 5.6%, and 4.0%.Because of the way this correction was derived, it cor-rects for several of the point source gain errors at once.These include the pixel area difference over the FOV, thechanging effective wavelength over the FOV (see section IRAF is distributed by the National Optical Astronomy Ob-servatory, which is operated by the Association of Universities forResearch in Astronomy (AURA) under cooperative agreement withthe National Science Foundation. http://ssc.spitzer.caltech.edu/irac/locationcolor/ hotometry with IRAC 3below), and any extended/point source illumination dif-ferences. Therefore, the correction is only strictly truefor stars of the same spectral type as the standard used inthese observations. However, in practice the color termof the correction is relatively smaller than the other ef-fects, so the photometry is in general improved when thecorrection is applied to all point sources. Effective Wavelength Variations over the Field ofView
As a consequence of the wide-field and compact opti-cal design of the cameras, the light at each point of theFOVs passes through the filters at a different averageangle, as shown in Figure 1. In addition, the filters aretilted with respect to the optical axis in order to minimizeabberations introduced by other optical elements and theoff-axis design. The range in angles as a function of po-sition results in a change in effective wavelength over theFOV. The filters were designed to have the desired nom-inal wavelength and bandpass for the center of the field,given the average filter tilt for the position as specified inthe optical design. In addition to the filters, the trans-mission of the beamsplitters produces angle-dependentreflection variations in channels 1 and 2, and transmis-sion variations in channels 3 and 4. The primary effectis a change in the total transmission or reflection overthe band as a function of angle at the beamsplitter. Thedesign of the filters and beamsplitters and the measure-ments of their transmission and reflectance is detailed byQuijada et al. (2004).Based on the Quijada et al. (2004) results, we have con-structed models of the instrument transmission for eachpixel of the four channels. The angles of transmissionthrough the filter and transmission or reflection of thebeamsplitter were determined for each pixel, and the to-tal relative system response (RSR) was calculated. Thisalso includes the assumed telescope transmission and thedetector quantum efficiency. Then, for a source with aspectrum significantly different than that of the standardstars, the color correction K i,j can be calculated for anobject at a particular location in an IRAC frame as de-scribed by Reach et al. (2005), where the color correctionis defined as: K i,j ≡ R ( F ν /F ν ) ( ν/ν ) − R i,j dν R ( ν/ν ) − R i,j dν . (2)where F ν is the source spectrum, F ν is the referencespectrum (assumed to be νF ν = constant ), R i,j is theinstrumental response as a function of frequency at arraylocation ( i,j ), and ν is the nominal frequency.To illustrate the changes in transmission across theFOV, we have calculated the nominal wavelength at eachpixel for each channel, as described by Fazio et al. (2004)for the original instrument response curves. The nominalwavelength was calculated using the following expressionat each pixel, integrated over the bandpass: λ = R Rdλ R λ − Rdλ . (3)Figure 5 shows the variation of the nominal wavelengthacross the FOV for each of the channels. The nominalwavelength varies from 3.5406 to 3.5512 µ m for channel 1, 4.4680 to 4.4949 µ m for channel 2, 5.6718 to 5.7458 µ m for channel 3, and 7.6212 to 7.8929 µ m for chan-nel 4. The dominant change over the fields is a shiftof the entire transmission pattern, although there aresome small changes in the details of the relative responsecurves as one moves around the FOV, and in Channels3 and 4 there is a significant difference in the averagetransmission over the field, mainly due to variations inthe beamsplitter transmission as noted by Quijada et al.(2004). To first order this transmission change shouldbe compensated for by the flat correction in the BCDpipeline.By applying the photometry correction described insection 2.2 above, one is also implicitly applying a cor-rection for the wavelength variation effect because thecorrection was derived based on standard stars. There-fore, if a source of interest has a spectrum similar tothe standard star, no further wavelength correction isnecessary. If the source spectrum is different from thestandard, the correction for the standard star must bebacked out before the correction is applied to the datafor the specific source of interest.Based on the instrument response curves for the indi-vidual pixels, we also calculated average response curvesfor the entire array and for the subarray. These areshown in Figures 6 – 9. Each figure shows the trans-mission curves averaged over the entire array, the sub-array region, and also plots of the pixels with extremesin nominal wavelength for that channel. These data areavailable on the SSC website , including the full 3-d dat-acube with the instrument response for each pixel for allchannels. INTRA-PIXEL GAIN EFFECTS
The optical point spread function (PSF) is slightly un-dersampled by the IRAC pixel scale. The optical modelpredicted image FWHM sizes of 1.6, 1.6, 1.8, and 1.9 arc-sec for channels 1-4 respectively, with a pixel size of 1.2arcsec in all channels. The small size of the PSF causesthe IRAC photometry to be sensitive to intra-pixel gainvariations, due to variations of the quantum efficiencyacross a pixel area or gaps between pixels.The intra-pixel gain effects were investigated by exam-ining photometry of stars at many different positions onthe array. The photometry was extracted and the pointsource gain correction, as described in Section 2.2 above,was applied. Then for each measurement, the photome-try relative to the median value for that star for all po-sitions was plotted against the distance from the sourcecentroid to the center of the nearest pixel. The resultsare shown in Figure 10. For channels 1 and 2, there isa correlation between the source location relative to thepixel center (or “pixel phase”) and the extracted pho-tometry. As expected, the magnitude of this correlationis dependent on the wavelength (or size of the PSF) –the effect is greatest in channel 1, less in channel 2, andnot detected in channels 3 and 4.If the correction is defined to be unity for the me-dian location of a source in a pixel (for randomly placedsources this location is 1 / √ π pixels from the center) thecorrection is given by http://ssc.spitzer.caltech.edu/irac Hora et al. f IP G = 1 + A (1 / √ π − p ) (4)where p is the distance (in pixels) from the source cen-troid to the nearest center (0 ≤ p ≤ √ / A = 0 . A = 0 . h f phase i = 1 . µ m observations of the transiting planet which they wereable to reduce the residual RMS of their time series to0.27%. APPLYING THE PHOTOMETRY CORRECTIONS
Correcting photometry extracted from the BCD
In the case where the point source is sampled at suf-ficient signal to noise in a single exposure, the moststraightforward approach is to apply corrections to theBCD since the corrections are based on position in theFOV of each frame. For a point source in the frame,after it is extracted using photometry software, the PSFflat field correction is applied based on the centroid ofthe object. For channels 1 and 2, the intra-pixel gaincorrection can also be applied, based on the pixel phase.If a color correction is necessary, then the correction iscalculated based on the source spectrum and the PSFposition and applied to the photometry.Figure 11 shows an example of a test case where thepixel phase and point source gain corrections were ap-plied. The top plot shows the relative Channel 1 photom-etry of stars as a function of distance from the center ofthe photometric correction pattern in Figure 4. A linearfit to the data illustrates the trend of lower photometrynear the center of the pattern. The second plot shows thesame data after the gain corrections have been applied.The linear fit to the relative photometry now shows notrend with position on the array, and the scatter for manyof the positions is noticeably lower. Figure 12 shows theeffect of the phase correction on the photometry. Forstars of several fluences, the standard deviation of the photometry at a particular point on the array was calcu-lated from the available measurements. Also shown arethe results from the same data after the intrapixel gaincorrection was applied. The reduction in the scatter ofthe photometry was about 0.5% for the stars examined.
Correcting photometry extracted from mosaics
Correcting photometry extracted from mosaics is morecomplicated because the image at any point source lo-cation is the combination of images at several differentFOV positions in the individual frames. The pixels con-tain some mixture of extended and point source emission.Therefore if one applies the correction before mosaicing,the correction is wrongly applied to the extended emis-sion, and that will create artifacts in the images.One possible way to apply the point source gain cor-rection to mosaics is to extract the photometry from theuncorrected mosaic and also to calculate a separate cor-rection map. The correction map is calculated by succes-sively offsetting the BCD correction image to the samelocations as the science images and calculating the meancorrections at each location in the final mosaic. The BCDcorrection image, however, includes the correction for thechange in pixel area over the array, and the mosaics havebeen reprojected to a constant pixel area. Therefore wehave to divide out the pixel area normalization from theBCD correction image before we use it to make the mo-saic correction map.An example of such a correction map for a mappingAOR with dithers is shown in Figure 13. For each source,one would need to determine its pixel coordinates and usethe value at the same location in the correction mosaicto correct the photometry. This approach will work incases where either the target is of high surface bright-ness relative to the local extended emission, includingthe zodi, or the extended emission has a spectrum simi-lar to the zodi emission. The wavelength-dependent partof the correction depends on the spectral slope of thesum of the extended and pointlike emission (i.e. whatactually arrived at the detector), not the spectral slopeof the point source alone. If the local extended-sourcespectrum differs from the zodi, the extended emission isalso in need of correction, and so the method will haveproblems in proportion to the strength of the extendedemission.The correction mosaics themselves will have seams allthrough them as seen in Figure 13, and it isn’t obvious foran object whose photometry is derived from many pixelsjust how to derive a single correction factor applicableto the catalog entry except by duplicating the extractionprocedure. The correction would be even more difficultto derive for PSF fitting. A PSF-weighted average ofthe pixels in the correction mosaic might closely approx-imate the correction value for a particular position in themosaic.For a data set such as an extragalactic survey likeSWIRE that has a relatively uniform background andfew bright, extended emission sources, the data can besplit into a nearly constant “background” and residual“object” components, and the corrections in image spaceapplied to the “object image”. Then the zodi-flattened(zodi-colored) background can be put back in. The re-sulting mosaics would be seamless and fully calibratedfor either point sources or extended emission. However,hotometry with IRAC 5this technique is possible only where the background isuniform and has a color similar to the zodi emission.A fully generic implementation, which is under develop-ment at the SSC, will involve pixel-wise corrections thattake into account the pixel colors and thus be correct forboth point sources and extended emission.
Aperture corrections
The IRAC calibration described by Reach et al. (2005)used a 10 pixel aperture and an aperture with an innerradius of 12 pixels and outer radius of 20 pixels. This isappropriate for measuring standard stars since they werechosen to be extremely bright relative to background ob-jects within this distance on the sky. The aperture con-tains a large fraction of the total flux from the object,but is small enough so that it does not extend off the ar-ray when extracting the photometry from BCDs wherethe star is offset at different positions on the array. How-ever, for many other applications where one is extractingsources in crowded fields or where there is significant ex-tended emission near the source, a smaller aperture islikely to produce more accurate photometry. In order tocalibrate the photometry using aperture sizes differentfrom that of the standard star observations, one mustperform an extraction of the same star(s) using the dif-ferent parameters, and a correction factor can then bedetermined. This analysis was performed based on obser-vations of a standard star at many different positions onthe array. The procedure aper in IDLPHOT was used toextract the photometry from the BCD, and the averagecorrections were determined. The results are reported inthe IRAC Data Handbook (IDH; see Table 5.7 of theHandbook). Note that for extracting photometry fromthe BCD, there might be a position dependence due tothe distortion, especially for small apertures. This hasnot been taken into account in the IDH where the aver-age value for all positions with the same pixel aperturedimensions are reported.The aperture correction values in the IDH were deter-mined from the BCD of a star with high S/N observationsand many positions on the array. Many observers insteadextract photometry from mosaics for which the numberof dithers at each position and the reprojected pixel sizeis different from the instrumental pixels. The data setand the choices one makes in the reduction parameterscan make a difference in the aperture photometry correc-tions. In order to illustrate this, we measured the aper-ture corrections for two different datasets using a few dif-ferent mosaic parameters. The datasets that were usedwere an AOR from the Extragalactic First Look Survey(FLS; Lacy et al. 2005, - ADS/Sa.Spitzer ◦ apart. The location of the SAGEmosaic was chosen to be in a region of good coverage in http://ssc.spitzer.caltech.edu/irac/dh/ a corner of the field, far from the dense LMC stellar dis-tribution and any extended emission.The data for each survey field were mosaicked to a pixelscale of 0 . ′′ . ′′ daofind and phot tasks in IRAF to find and extract pho-tometry for the sources in the field. We used only thosesources with a S/N > CONCLUSIONS
The IRAC camera has FOV-dependent transmissioncharacteristics that affect the measurement of astronom-ical sources. After correction of these effects for standardstars, Reach et al. (2005) found that the calibration hasa relative accuracy of 1.8%, 1.9%, 2.0%, and 2.1% inchannels 1 (3.6 µ m), 2 (4.5 µ m), 3 (5.8 µ m), and 4 (8 µ m), respectively. To measure fluxes at this level of ac-curacy requires several photometric corrections: arrayposition dependence (due to changing spectral responseand pixel solid angle over the camera of view), pixel phasedependence (due to nonuniform quantum efficiency overa pixel), color correction (due to the different system re-sponse integrated over the passband for sources of differ-ent color), and aperture correction (due to the fractionsof light included within the measurement aperture andlost in the background aperture). The same accuraciesare possible for sources with spectra similar to the A-type standards used in the absolute calibration with thearray position dependence, pixel phase, and aperture cor-rections. For sources with spectra different from A stars,a knowledge of the source spectrum is necessary to makethe necessary color corrections to achieve the same pho-tometric accuracy.This work is based on observations made with theSpitzer Space Telescope, which is operated by the Jet Hora et al.Propulsion Laboratory, California Institute of Technol-ogy under NASA contract 1407. Support for this workwas provided by NASA through an award issued by JPL/Caltech. Facilities:
Spitzer (IRAC)
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Astronomical Data Analysis Software and Systems XIV hotometry with IRAC 7 (cid:1)(cid:0)(cid:2) (cid:3)(cid:4)(cid:5)(cid:6)(cid:9)(cid:7)(cid:8)(cid:10) (cid:11)(cid:12)(cid:13)(cid:14)
Fig. 1.—
The IRAC optical design layout (interior to the camera body), showing the side view and top view. There are two fieldsviewed simultaneously, with channels 1 and 3 viewing one field and channels 2 and 4 the other. In each pair, the light is reflected from thesurface of the beamsplitter and passes through a filter to the InSb detector (channels 1 and 2). The longer wavelength light passes throughthe beamsplitter and filters to the Si:As detectors (channels 3 and 4). The range of angles of incidence on the filters and beamsplittersdepending on position in the field of view is apparent from the rays traced through the system.
Hora et al.
Fig. 2.—
Distortion correction images for each of the IRAC channels. Darker regions represent regions where response to a point sourcehas a lower value than in the lighter regions. The full range of variations are 2.5%, 3.2%, 2.4%, and 3.8% for channels 1-4, respectively. Thearrays are shown in “BCD orientation”, with the first pixel in the FITS file shown in the lower left of each image, with the most rapidlyvarying index from left to right.
Fig. 3.—
The relative band transmissions of the four IRAC channels are shown compared to the spectra of an A0V standard star, anda model of the zodi emission in the ecliptic pole region (Kelsall et al. 1998). The vertical axis is the logarithm of the total instrumentaltransmission (Fazio et al. 2004), or for the star and zodi it is the logarithm of the flux density (W cm − µ m − ) scaled to fit on the plot. hotometry with IRAC 9 Fig. 4.—
The point source photometry correction images for each IRAC band. Darker regions represent regions where the photometry ofa point source has a lower value than in the lighter regions. The photometry therefore should be divided by the value given in the imagesto correct the measurement relative to the center of the array.
Fig. 5.—
Changes to the nominal wavelength over the FOV for each of the IRAC bands. The images are shown in the BCD orientation.Darker areas represent regions of shorter (lower) wavelength. The nominal wavelength varies from 3.5406 to 3.5512 µ m for channel 1,4.4680 to 4.4949 µ m for channel 2, 5.6718 to 5.7458 µ m for channel 3, and 7.6212 to 7.8929 µ m for channel 4. Channel 1 Transmission T r a n s m i ss i on New Full ArrayMin. wvlthSubarrayPeak wvlth
Fig. 6.—
The instrument response function for various locations on the array for channel 1 (“3.6 µ m”). The full array average is shownin black. The 32 ×
32 subarray response curve is shown in green. The pixel with the lowest nominal wavelength is shown in blue, and thelocation with the highest wavelength in red. hotometry with IRAC 11
Channel 2 Transmission0.00.10.20.30.40.50.63.9 4.1 4.3 4.5 4.7 4.9 5.1Wavelength (microns) T r a n s m i ss i on New Full ArraySubarrayMin. wvlthPeak wvlth
Fig. 7.—
The instrument response function for various locations on the array for channel 2 (“4.5 µ m”). The full array average is shownin black. The 32 ×
32 subarray response curve is shown in green. The pixel with the lowest nominal wavelength is shown in blue, and thelocation with the highest wavelength in red.
Channel 3 Transmission T r a n s m i ss i on New Full ArraySubarrayPeak wvlthMin. wvlth
Fig. 8.—
The instrument response function for various locations on the array for channel 3 (“5.8 µ m”). The full array average is shownin black. The 32 ×
32 subarray response curve is shown in green. The pixel with the lowest nominal wavelength is shown in blue, and thelocation with the highest wavelength in red.
Channel 4 Transmission0.00.10.20.36.0 7.0 8.0 9.0Wavelength (microns) T r a n s m i ss i on New Full ArraySubarrayPeak wvlthMin. wvlth
Fig. 9.—
The instrument response function for various locations on the array for channel 4 (“8.0 µ m”). The full array average is shownin black. The 32 ×
32 subarray response curve is shown in green. The pixel with the lowest nominal wavelength is shown in blue, and thelocation with the highest wavelength in red. hotometry with IRAC 13
Ch1 Photometry
Distance from center of pixel (pixels) R a t i o t o m e d i a n Ch2 Photometry
Distance from center of pixel (pixels) R a t i o t o m e d i a n Ch3 Photometry
Distance from center of pixel (pixels) R a t i o t o m e d i a n Ch4 Photometry
Distance from center of pixel (pixels) R a t i o t o m e d i a n Fig. 10.—
Intra-pixel gain effects on the photometry in each channel. The extracted photometry of a source is plotted against thepixel phase. The strongest effect is in the shortest wavelength channel, there is no significant effect detected in channels 3 and 4 for thephotometric standard star data examined.
BCD Photometry
Distance from Center of Pattern (pixels) R a t i o t o m e d i a n Corrected BCD Photometry
Distance from Center of Pattern (pixels) R a t i o t o m e d i a n Fig. 11.—
The top plot shows the relative photometry of stars in a test field in Channel 1 (the ratio of the measurement to the medianvalue at that array position) as a function of the distance from the center of the photometric correction pattern in Figure 4. Individualstars have unique colors and symbols, several stars have multiple measurements at different array positions. A linear fit to the medianvalues shows the trend of lower values towards the center. The lower plot shows the results after the gain corrections have been applied.The overall trend with array position is removed, and the scatter of individual positions is reduced. hotometry with IRAC 15
Photometric Repeatability - Pixel Phase Correction
Star fluence (ADU) S t a nd a r d D ev i a t i on ( % ) CorrectedUncorrected
Fig. 12.—
The standard deviation of photometry of several stars observed in Channel 1, plotted against the fluence in ADUs. The darkblue points are uncorrected, the magenta points are after the intrapixel gain correction has been applied. The standard deviations arereduced by about 0.5% for most stars.
Fig. 13.—
Point source photometry correction image for a channel 1 mosaic made from a mapping AOR that used three dithers per mapposition. The range of correction values is -1.2% (the dark regions) to +3.4% (the ligher areas).
TABLE 1IRAC Photometry correction fit coefficients.
IRAC Coefficients a Channel
A B C D E F a The coefficient labels are defined in equation 2.2.1
TABLE 2IRAC Channel Transmission Summary
IRAC Nominal Central a Average c Channel Wavelength Wavelength Bandpass Bandpass Transmission( µ m) ( µ m) ( µ m) (percent)1 3.544 3.543 0.747 21.1 0.4302 4.479 4.501 1.018 22.7 0.4693 5.710 5.711 1.412 24.8 0.1254 7.844 7.905 2.910 37.0 0.280Subarray:1 3.534 3.538 0.740 20.9 0.4262 4.489 4.506 1.009 22.5 0.4573 5.679 5.687 1.383 24.3 0.1224 7.884 7.912 2.861 36.4 0.289 a The central wavelength is defined as the midpoint between the half-power points of the bandpass. The half-power points are defined asbeing the wavelengths where the transmission is 50transmission. b The bandpass is the distance in wavelength between the half-power points of the transmission. c The average transmission is determined by averaging the transmission between the half-power points of the bandpass.
TABLE 3Channel 1 Aperture Correction Factors aperture annulusradius a range IDH b FLS 0.6 c FLS PBCD d LMC 0.6 e a aperture and annulus ranges are in native pixel units (1 . ′′ b IRAC Data Handbook values c First Look Survey data, 0.6 arcsec/pixel mosaic d First Look Survey data, 1.2 arcsec/pixel post-BCD mosaic e SAGE LMC data, 0.6 arcsec/pixel mosaic, both epochs
TABLE 4Channel 2 Aperture Correction Factors aperture annulusradius a range IDH b FLS 0.6 c FLS PBCD d LMC 0.6 e a aperture and annulus ranges are in native pixel units (1 . ′′ b IRAC Data Handbook values c First Look Survey data, 0.6 arcsec/pixel mosaic d First Look Survey data, 1.2 arcsec/pixel post-BCD mosaic e SAGE LMC data, 0.6 arcsec/pixel mosaic, both epochs hotometry with IRAC 17
TABLE 5Channel 3 Aperture Correction Factors aperture annulusradius a range IDH b FLS 0.6 c FLS PBCD d LMC 0.6 e a aperture and annulus ranges are in native pixel units (1 . ′′ b IRAC Data Handbook values c First Look Survey data, 0.6 arcsec/pixel mosaic d First Look Survey data, 1.2 arcsec/pixel post-BCD mosaic e SAGE LMC data, 0.6 arcsec/pixel mosaic, both epochs
TABLE 6Channel 4 Aperture Correction Factors aperture annulusradius a range IDH b FLS 0.6 c FLS PBCD d LMC 0.6 e a aperture and annulus ranges are in native pixel units (1 . ′′ b IRAC Data Handbook values c First Look Survey data, 0.6 arcsec/pixel mosaic d First Look Survey data, 1.2 arcsec/pixel post-BCD mosaic ee