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Instrument Science Report WFC3 2008-13

WFC3 TV3 Testing: UVIS-1' Gain Results
S. Baggett June 20, 2008

ABSTRACT Thermal-vacuum (TV3) level tests using the integrated WFC3 instrument, with the UVIS1' detector in place, were performed at Goddard Space Flight Center (GSFC) during the spring of 2008. Flatfield data were acquired to allow an absolute gain measurement using the mean-variance method at the nominal gain 1.5 setting, in both unbinned and binned modes. The CCDs were at the nominal operating temperature of -83C. The gain in unbinned mode was measured at 1.56, 1.56, 1.58, and 1.57 e-/DN for quadrants A,B,C, and D, respectively, with errors ~0.01 e-/DN; these values are within 1-2% of those measured for UVIS-1 during TV1. The TV3 gain values for the binned modes were within 1% of the unbinned mode gain, specifically, 1.56, 1.55, 1.58, and 1.57 e-/DN for 2x2 mode and 1.55, 1.55, 1.56, and 1.56 e-/DN for 3x3 mode, for A,B,C, and D, respectively, with errors ~0.01 e-/DN.

Introduction
The WFC3 instrument, with the UVIS-1' flight detector in place, recently underwent thermal-vacuum (TV) testing in the GSFC Space Environment Simulator chamber. Flatfield illumination was provided by the WFC3 optical stimulus (CASTLE), an apparatus that simulates the HST OTA, including the major aberrations. Flatfield data for the gain determinations were taken at the nominal setting (1.5 e-/DN) in unbinned and binned modes (1x1, 2x2 and 3x3). The gain 1.5 setting is the default setting as it provides the best compromise between sampling the entire full-well of the detector, given the 16-bit A to D

Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration


Instrument Science Report WFC3 2008-13 converter, and providing a reasonable sampling of the low read noise. The three off-nominal gain settings (1.0, 2.0, and 4.0 e-/DN) were measured during a previous TV test with UVIS-1 (Baggett, 2005); they were not remeasured at this time as these modes will be unavailable to observers. The binned modes have been measured, however, as they will be supported and available for use. While binning, the co-addition of either 2x2 or 3x3 pixels before reading out the effective pixel value, reduces the spatial resolution, it improves the signal-to-noise ratio by minimizing the readout noise, which can be useful with faint targets for example. This report presents the results of the TV3 gain analysis.

Method
The gains were measured via the standard mean-variance technique: the inverse slope of the mean signal level plotted versus the variance yields the gain. As presented previously (Baggett, 2005), the derivation is based upon the assumption that there are only two sources of noise: photon shot noise and detector read noise. In that case, the total noise can be written as N = p + R , with all parameters in units of e-. Converting to units of DN
2 2 2 2 2

via the gain (e-/DN), the equation can be rewritten as ( N / g ) = ( p / g ) + ( R / g ) , where the noise squared term is the variance. When the mean signal, µ, is measured in DN, the photon noise, p, is simply g µ which when substituted into the noise equation yields 2 = (1/g) *µ + (R/g)2. The inverse slope is the gain. The images used were taken March 12, 2008 and April 10/11, 2008 (listed in Table 2, Appendix A). All images were processed through calwf3 version 0.8 (21-Dec-2007 release); the CCDTAB and OSCNTAB were those in place in the pipeline in early 2008 (r6p1618ci_ccd.fits and q911321oi_osc.fits, respectively). Only the overscan correction was applied. Average and difference images were formed for each pair of flatfields at a given exposure level and the mean-variance plots were constructed from the means of the average images and the variances of the difference images (standard deviation squared and divided by two). Statistics in the unbinned data were measured in twenty-six 400x400 pixel regions per image pair per detector quadrant, for a total of >300 regions per quad. No individual pixels were masked although a small number of regions (< 5 out of the more than 300) were discarded due to excessively high variance, traced to cosmic ray events. Statistics for the binned data were computed in twenty-six 200x200 pixel regions (2x2 binned images) and twenty-six 133x133 pixel regions (3x3 binned images). In the initial mean-variance plots for the binned data, there were obvious outliers found to be due to the influence of e.g. bad columns or cosmic ray events on the now-smaller image sections being used. For this rea-

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Instrument Science Report WFC3 2008-13 son, 3 iterations of 3 sigma clipping were done to mask pixels (for the binned images only).

Results
Figure 1 shows the quad C results for the unbinned mode; the final fit has been restricted to ~30K DN to avoid any potential non-linearity effects at the next highest exposure level, 39K DN (expanding the fits to 39K DN raised the resulting gains by ~1%). The final TV3 gains for UVIS-1' were 1-2% higher than those measured during TV1 testing with UVIS1 (Baggett, 2005). One of the binned-mode mean-variance plots is shown in Figure 1 (quad C, 3x3 binning). In the binned modes, the quadrants showed good mean-variance behavior out to ~60-62K DN, quadrant C out to ~55 KDN. To stay well within the linear regime and treat all quads equally, the gain was measured on data out to ~50K DN; values are tabulated in Table 1. The binned mode gains are within 1% of the unbinned mode gains, within the noise of the analysis. Table 1. UVIS-1' gain results for the commanded gain 1.5 setting.
unbinned quadrant A B C D gain 1.56 1.56 1.58 1.57 fit error <0.01 <0.01 <0.01 <0.01 2x2 binning gain 1.56 1.55 1.58 1.57 fit error <0.01 <0.01 <0.01 <0.01 3x3 binning gain 1.55 1.55 1.56 1.56 fiterror <0.01 <0.01 <0.01 <0.01

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Instrument Science Report WFC3 2008-13

Figure 1: Mean-variance plots for quad C, for unbinned mode (left) and 3x3 binned mode (right).
quadC quadC

20000

30000

15000

variance

10000

variance

20000

10000 5000

0 0 10000 mean (DN) 20000 30000

0 0 10000 20000 30000 mean (DN) 40000 50000

Acknowledgements
Thanks are due to all the WFC3 team members, at GSFC and STScI, who supported the thermal vacuum testing.

References
Baggett, S., "WFC3 Thermal Vac Testing: UVIS Gain Results," ISR 2005-08, Feb 2005.

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Instrument Science Report WFC3 2008-13

Appendix A.
Table 2. Files used for absolute gain determinations. Images were all full-frame, fouramp readouts, with offset=3, CCD temperature of -83o C, gain setting of 1.5. Unbinned data were done with WFC3 electronics side 1 (MEB1) while binned data were done with side 2 (MEB2). Flats were taken with F606W, using the CASTLE tungsten lamp with OSFILT0 and OSFILT2 set to OPEN; the OSFILT1 setting is recorded in the osfilter column. Tabulated are the imagenames, tv number, observation date and time, exposure time, CASTLE OSFILT1, binning mode, and median level (in DN).

imagename iu051b02r_08072025818 iu051b04r_08072031956 iu051b05r_08072031956 iu051b07r_08072034134 iu051b08r_08072034134 iu051b0ar_08072040312 iu051b0br_08072040312 iu051b0dr_08072042450 iu051b0er_08072042450 iu051b0gr_08072044126 iu051b0jr_08072051343 iu051b0lr_08072053459 iu051b0mr_08072053459 iu051b0or_08072055615 iu051b0pr_08072055615 iu051b0rr_08072061240 iu051b0ur_08072063305 iu051b0wr_08072065348 iu051b0xr_08072065348 iu051b0zr_08072071710 iu051b14r_08072074105 iu051b29r_08072120132 iu051b16r_08072075347 iu051b17r_08072075347

tvnum 50946 50947 50948 50949 50950 50951 50952 50953 50954 50955 50957 50958 50959 50960 50961 50962 50964 50965 50966 50967 50970 50997 50971 50972

observation date 2008-03-12 2:53:25.06 2008-03-12 3:09:51.05 2008-03-12 3:15:03.05 2008-03-12 3:31:29.06 2008-03-12 3:36:41.06 2008-03-12 3:53:07.05 2008-03-12 3:58:19.05 2008-03-12 4:14:45.06 2008-03-12 4:19:57.06 2008-03-12 4:36:23.05 2008-03-12 5:08:30.06 2008-03-12 5:24:44.04 2008-03-12 5:29:45.06 2008-03-12 5:46:00.04 2008-03-12 5:51:01.06 2008-03-12 6:07:16.04 2008-03-12 6:29:23.05 2008-03-12 6:44:06.06 2008-03-12 6:47:34.06 2008-03-12 7:04:47.05 2008-03-12 7:37:52.04 2008-03-12 11:57:13.06 2008-03-12 7:46:58.06 2008-03-12 7:50:16.04

exptime 146.1 146.1 146.1 146.1 146.1 146.1 146.1 146.1 146.1 146.1 167.2 167.2 167.2 167.2 167.2 167.2 76.4 76.4 229.3 229.3 48.4 48.4 64.4 64.4

osfilter ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND2 ND2 ND2 ND2 ND2 ND2 ND1 ND1 ND1 ND1 OPEN1 OPEN1 OPEN1 OPEN1

binning 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

mean (DN) 75.92 75.74 75.785 75.82 75.87 75.73 75.605 75.52 75.775 75.575 784. 783.7 783.85 783.2 783.1 783.55 3753. 3750.5 11276.5 11275. 22231. 22293.5 29609. 29597.

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Instrument Science Report WFC3 2008-13

imagename iu051b1ar_08072081153 iu051b1cr_08072083015 iu051b1fr_08072084853 iu051b1gr_08072084853 iu050202r_08101205248 iu050204r_08101205858 iu050206r_08101210351 iu050208r_08101210842 iu05020ar_08101211335 iu05020cr_08101211828 iu05020er_08101212321 iu05020gr_08101212814 iu05020ir_08101213307 iu05020kr_08101213800 iu05020nr_08101215240 iu05020pr_08101215823 iu05020rr_08101220249 iu05020tr_08101220715 iu05020vr_08101221141 iu05020xr_08101221607 iu05020zr_08101222413 iu050210r_08101223226 iu050214r_08101224357 iu050215r_08101225009 iu050217r_08101225009 iu050224r_08102002805 iu05021br_08101231022 iu05021cr_08101231022 iu05021er_08101231022 iu05021fr_08101231022 iu05021hr_08101231022 iu05021ir_08101231022

tvnum 50974 50975 50977 50978 57311 57312 57313 57314 57315 57316 57317 57318 57319 57320 57322 57323 57324 57325 57326 57327 57328 57329 57331 57332 57334 57362 57336 57337 57339 57340 57342 57343

observation date 2008-03-12 8:08:06.06 2008-03-12 8:22:54.06 2008-03-12 8:41:16.04 2008-03-12 8:44:58.06 2008-04-10 20:50:28.04 2008-04-10 20:56:38.06 2008-04-10 21:01:31.05 2008-04-10 21:06:22.04 2008-04-10 21:11:15.06 2008-04-10 21:16:08.06 2008-04-10 21:21:01.05 2008-04-10 21:25:54.04 2008-04-10 21:30:47.06 2008-04-10 21:35:40.06 2008-04-10 21:50:15.06 2008-04-10 21:55:58.04 2008-04-10 22:00:24.06 2008-04-10 22:04:50.04 2008-04-10 22:09:16.06 2008-04-10 22:13:42.04 2008-04-10 22:18:06.04 2008-04-10 22:23:48.06 2008-04-10 22:35:44.06 2008-04-10 22:38:20.06 2008-04-10 22:43:32.06 2008-04-11 0:20:55.05 2008-04-10 22:56:02.04 2008-04-10 22:57:44.06 2008-04-10 23:01:08.06 2008-04-10 23:02:55.06 2008-04-10 23:06:29.05 2008-04-10 23:08:19.06

exptime 80.6 80.6 88.7 88.7 48.1 48.1 48.1 48.1 48.1 48.1 48.1 48.1 48.1 48.1 53.1 53.1 53.1 53.1 53.1 53.1 265.5 265.5 76. 76. 151.8 151.8 21.6 21.6 27. 27. 29.7 29.7

osfilter OPEN1 OPEN1 OPEN1 OPEN1 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND2 ND2 ND2 ND2 ND2 ND2 ND2 ND2 ND1 ND1 ND1 ND1 OPEN1 OPEN1 OPEN1 OPEN1 OPEN1 OPEN1

binning 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

mean (DN) 37058.5 37028. 40825. 40794.5 95.93 96.095 95.915 95.995 96.07 95.855 95.6 95.995 95.875 95.92 955.5 955.3 955.55 954.55 954.9 954.75 4780. 4781.5 14400.5 14390.5 28832. 28796.5 38274.5 38216.5 47845.5 47885. 52644. 52564.5

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Instrument Science Report WFC3 2008-13

imagename iu050227r_08102002805 iu050229r_08102003428 iu05022br_08102003742 iu05022dr_08102004056 iu05022fr_08102004410 iu05022hr_08102004724 iu05022jr_08102005038 iu05022lr_08102005352 iu05022nr_08102005706 iu05022pr_08102010020 iu05022sr_08102010822 iu05022ur_08102011140 iu05022wr_08102011424 iu05022yr_08102011708 iu050230r_08102011952 iu050232r_08102012236 iu050234r_08102012708 iu050235r_08102012957 iu05023br_08102014135 iu05023dr_08102014503 iu05023fr_08102015044 iu05023gr_08102015305 iu05023hr_08102015526 iu05023jr_08102015747 iu05023nr_08102020315 iu05023pr_08102020559 iu05023tr_08102020854 iu05023vr_08102021149

tvnum 57364 57365 57366 57367 57368 57369 57370 57371 57372 57373 57375 57376 57377 57378 57379 57380 57381 57382 57385 57386 57387 57388 57389 57390 57392 57393 57395 57396

observation date 2008-04-11 0:26:41.06 2008-04-11 0:33:04.04 2008-04-11 0:36:18.06 2008-04-11 0:39:32.04 2008-04-11 0:42:46.06 2008-04-11 0:46:00.04 2008-04-11 0:49:14.06 2008-04-11 0:52:28.04 2008-04-11 0:55:42.06 2008-04-11 0:58:56.04 2008-04-11 1:06:56.04 2008-04-11 1:10:14.06 2008-04-11 1:12:58.06 2008-04-11 1:15:42.06 2008-04-11 1:18:26.06 2008-04-11 1:21:10.06 2008-04-11 1:23:54.06 2008-04-11 1:26:43.05 2008-04-11 1:39:59.05 2008-04-11 1:43:27.05 2008-04-11 1:46:21.06 2008-04-11 1:48:20.04 2008-04-11 1:50:19.05 2008-04-11 1:52:40.04 2008-04-11 1:57:22.06 2008-04-11 2:00:06.06 2008-04-11 02:05:34.06 2008-04-11 02:08:29.06

exptime 21.4 21.4 21.4 21.4 21.4 21.4 21.4 21.4 21.4 21.4 23.6 23.6 23.6 23.6 23.6 23.6 118. 118. 33.7 33.7 67.4 67.4 89.9 89.9 112.4 112.4 123.6 123.6

osfilter ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND3 ND2 ND2 ND2 ND2 ND2 ND2 ND2 ND2 ND1 ND1 ND1 ND1 ND1 ND1 ND1 ND1 ND1 ND1

binning 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

mean (DN) 96.435 96.465 96.525 96.58 94.745 96.19 96.755 96.36 96.225 96.49 955.75 955.45 955.6 955.85 955.8 956.45 4781.5 4783.5 14388.5 14399.5 28880.5 28880.5 38569.5 38559.5 48234. 48209.5 53058.0 53035.5

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