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WFC4

Bias, Overscan
Gain, Noise

HRC1

Bias, Overscan
Gain, Noise
CTE

SBC

Darks
Flats
Throughput

Shutter

Shading, accuracy

PSF

Encircled energy

Internal Count Rates

HRC1, WFC4
SBC

FLASH

HRC1, WFC4

Dispersers

Grism
Prism

Stray Light

Light leak
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Last updated
18 December 2001 10:25:45

Maintained by
martel


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WFC#4 : Gain, Noise, Linearity, Saturation

AUTHORS : A.R. Martel, G. Hartig, and M. Sirianni

GOAL :

The main goal is to measure the primary system gains of 1, 2, 4 and 8 e-/DN with the photon-transfer method as well as the read noise for each amplifier of the WFC flight build 4 detector. The full well of the detector can also be estimated.

LOCATION AND DATE :

A thorough set of flat fields at gain=1 was obtained on 4 Apr 2001 with SMS JGCW32A and the gain=2,4 data were acquired on 9 Apr 2001 at GSFC/SSDIF with SMS JGCW32B. During the Thermal Balance/Vacuum Campaign 3, additional internal flat fields were acquired at gain=1 with the CCD basic monitor SMS JTVW01A in Cold Soak#1 (Jul 13) and in the subsequent calibration phase, JTVW01A, JTVW01B, JTVW01C, and JTVW01D were executed (Jul 18). All the thermal vac flat fields were acquired with amps ABCD and a bias offset of 3.

Pairs of bias frames were acquired as part of SMSs JGCW32A and JGCHW2B as well as in the calibration phase of TB/TV 3 campaign on Jul 20 (gain=1, 2) and Jul 21 (gain=4, 8) for ABCD, AD, and BC read-outs.

INSTRUMENT CONFIGURATION :

For the gain=1, 2, and 4 measurements, the instrument is configured as in the post-acoustics RAMP+RAS/Cal session (see Pre- and Post-Acoustics Alignment with RAMP+RAS/Cal) with the flight build detectors WFC#4 and HRC#1. ACS is on the optics table in SSDIF (GSFC) and is covered with lumalloy blankets to minimize straylight from the overhead room lights. RAS/Cal is mounted on RAMP in front of the instrument but is not used for the gain measurements. The instrument configuration for the TB/TV 3 calibration session is described in Thermal Balance/Vacuum Campaign 3.

METHOD :

The SSDIF images were acquired with the SMSs JGCW32A (gain=1) and JGCW32B (gain=2,4). The illumination is provided by the internal tungsten 2 lamp through the F625W filter. A brief description of the photon-transfer method is given in the appendix. Essentially, a sequence of flat field pairs is acquired at each gain setting, over as wide a range in count levels as possible, from the read noise at low levels (usually at the shortest integration time of 0.5 sec), through the linear part of the curve, and up to non-linearities and saturation at the highest levels. Full ABCD frames are read out, so information on a particular amplifier can be derived by analyzing its quadrant. The bias level of each image was subtracted from the leading physical overscan only. We find that nearly identical results are obtained when a full median bias frame is subtracted from each flat field and the residuals removed with the leading physical overscan. The statistics were performed on twenty 20x20 boxes distributed over each quadrant. The gain is measured from a linear fit to the photon-transfer curve. Since the on-orbit gain will be set to 1 e-/DN, SMS JGCW32A has the most complete coverage of the photon-transfer curve while SMS JGCW32B only samples a few points at gains of 2 and 4 for comparison.

The gains were also measured with the flat fields acquired in TB/TV 3 with the basic CCD monitor SMSs. These SMSs are not as thorough as the formal gain SMSs but still serve as a verification of gains=1,2,4 and in particular, allow a measurement of gain=8. The read-out noise is derived from pairs of bias frames acquired either in SSDIF or in the dark environment of TB/TV 3. For these, the WFC temperature was -79 C in SSDIF and -76 C in TB/TV 3. The ACS enclosure is completely light-tight (see Enclosure Light Leak Characterization) so the bias frames acquired in SSDIF are unlikely to suffer from straylight contamination from the ceiling lights during read-out.

RESULTS :

The system gains and read noise of each amplifier for gain=1, 2, 4, and 8 e-/DN are listed in Tables 1 to 4 below. For comparison, the measurements for both the SSDIF and TB/TV 3 environments are tabulated. Plots of the photon-transfer curves are shown in Figs 1-3 (SSDIF) and 4-7 (TB/TV 3). Some points of interest :

Table 1 : Gain in e-/DN (SSDIF)

Amplifier Gain="1" Gain="2" Gain="4"
A 1.001 +/- 0.006 2.02 +/- 0.02 4.00 +/- 0.03
B 0.966 +/- 0.005 1.99 +/- 0.02 3.95 +/- 0.03
C 1.021 +/- 0.006 2.02 +/- 0.01 3.98 +/- 0.03
D 1.013 +/- 0.006 2.00 +/- 0.02 4.03 +/- 0.03
Average 0.997 +/- 0.003 2.008 +/- 0.007 3.99 +/- 0.01

Table 2 : Gain in e-/DN (TB/TV 3)

Amplifier Gain="1" Gain="2" Gain="4" Gain="8"
Jul 13 Jul 18
A 1.00 +/- 0.01 1.01 +/- 0.01 2.01 +/- 0.03 4.01 +/- 0.04 8.09 +/- 0.09
B 0.95 +/- 0.01 0.97 +/- 0.01 1.93 +/- 0.02 3.92 +/- 0.05 7.72 +/- 0.09
C 1.01 +/- 0.01 1.02 +/- 0.01 2.03 +/- 0.02 4.01 +/- 0.05 7.99 +/- 0.08
D 1.01 +/- 0.01 1.01 +/- 0.01 2.02 +/- 0.02 4.03 +/- 0.04 8.05 +/- 0.07
Average 0.986 +/- 0.006 1.004 +/- 0.005 2.00 +/- 0.01 3.99 +/- 0.02 7.98 +/- 0.04


Table 3 : Read-Out Noise in e- (SSDIF)

Amplifier Gain="1" Gain="2" Gain="4"
A 4.86 +/- 0.20 5.24 +/- 0.22 6.20 +/- 0.25
B 4.60 +/- 0.17 5.00 +/- 0.23 5.98 +/- 0.20
C 5.14 +/- 0.18 5.42 +/- 0.24 6.36 +/- 0.28
D 4.81 +/- 0.14 5.13 +/- 0.22 6.08 +/- 0.22
Average 4.85 +/- 0.08 5.19 +/- 0.11 6.12 +/- 0.12

Note : The read noise is measured in DN and is converted to e- using the gains measured in SSDIF.

Table 4 : Read-Out Noise in e- (TB/TV 3)

Amplifier Gain="1" Gain="2" Gain="4" Gain="8"
A 4.81 +/- 0.21 5.28 +/- 0.17 6.06 +/- 0.26 8.82 +/- 0.34
B 4.72 +/- 0.21 5.12 +/- 0.20 6.04 +/- 0.23 8.83 +/- 0.43
C 5.20 +/- 0.23 5.42 +/- 0.26 6.18 +/- 0.25 9.17 +/- 0.29
D 4.73 +/- 0.21 5.15 +/- 0.26 6.17 +/- 0.24 9.29 +/- 0.40
Average 4.85 +/- 0.11 5.23 +/- 0.10 6.11 +/- 0.12 9.05 +/- 0.18

Note : The read noise is measured in DN and is converted to e- using the gains measured in SSDIF and in TB/TV 3 (for gain=8 only).

CEI SPECIFICATIONS :

In STE-50, "Information End Item Specification (Part II)", Table 4-5 states that the requirement for the RMS noise per pixel, which includes both the read noise and noise from the dark current, is <5.8 e- and the goal is <4.6 e- for a reference integration time of 21 min (1260 sec). The read noise was measured from the imaging area of the bias frames and is listed in Tables 3 and 4 above for different amplifier and gain combinations. The dark count rate measured in TB/TV 3 is approximately 5 e-/pix/hour, which amounts to ~1.8 e-/pix for a 1260 sec exposure. The noise on the dark rate is taken as shot noise i.e. the square root of the dark current (as in the ACS Exposure Time Calculator, see p. 103 of the ACS user manual), so the total noise is the square root of the sum of the read noise squared and the dark rate (1.8 e-/pix). From Table 4, we find that all the amplifiers at gains=1 and 2 meet the CEI requirement (<5.8 e-/pix) but not for gains=4 and 8. None of the amplifier/gain combinations meet the goal of <4.6 e-/pix. In the same CEI table, the full well is specifed as >50000 e-. Our measured full well of ~77000 e- meets this requirement.

CONCLUSION :

The system gain (e-/DN) and total noise (e-) of the primary gain settings of 1, 2, 4, and 8 for the WFC build 4 detector were measured. The full well is estimated at ~77000 e-. The gains from SSDIF (Table 1) and the noise measurements from TB/TV 3 (Table 4) are the primary reference.

APPENDIX :

Here, we give a basic description of the photon-transfer method which we implemented in IRAF scripts.