HRC and WFC Internal Tungsten and Deuterium Lamp Count Rates

AUTHORS : A.R. Martel and G. Hartig

GOAL :

The main goal is to measure and characterize the count rates of the internal tungsten and deuterium lamps through all the filters of wheels 1 and 2 for the flight build detectors HRC#1 and WFC#4. The default lamps for internal calibration are tungsten 4 for HRC and tungsten 2 for WFC and the deuterium lamp for the HRC F220W and F250W filters. All the measurements were made on ground data in ambient or vacuum environments.

DATA :

1. Tungsten lamps

HRC and WFC frames illuminated with the tungsten lamps were compiled from the ACS preflight database for the period Jan-Aug 2001, when the two flight build detectors were in their final configuration in ACS. Only the frames with the default parameters were considered (HRC : gain=2, offset=3, amp C read-out, WFC : gain=1, offset=3, amps ABCD read-out). For the small filters detectable on WFC (F892N, POLVIS, POLUV), the frames were read out as quadrants with amplifier C. No significant differences are expected in the count rates on the amp B quadrant. The ramp filters are analyzed at their zero-offset positions only i.e. central wavelength. Obviously, the count rates will vary along each segment.

Bias frames from the dark environment of the TB/TV 3 campaign (HRC-C : ID 32200-32201, WFC-ABCD : ID 32208-32209, WFC-C : ID 32460) were subtracted from each flat field and the bias level adjusted with the virtual overscan for the HRC frames and the leading physical overscan for the full-frame WFC images and the amp C subarrays. The choice of these overscan regions is justified in WFC#4 Overscan Analysis and Bias Subtraction and HRC#1 Bias Subtraction and Overscan Analysis (Jun 2000). For the WFC, a small residual offset between quadrants may remain visible at low electron levels because the bias frames were taken with the lamp off and possibly with a non-matching shutter blade. Ideally, a bias frame acquired with the tungsten lamp turned on through the same filter and same shutter blade would be subtracted for a consistent illumination and absolute levels. Such data are not available.

All the data were acquired while ACS was in SSDIF i.e. the few TB/TV 3 data are not considered. We make no distinction between MEB sides 1 and 2, given that the CCD performance is controlled by the CEB (comparison of the levels and noise of bias frames acquired on MEB sides 1 and 2 show no differences, so there is no reason to believe that any differences will be observed in the count rates). Note also that in TB/TV 3, the CEB temperature varied between plateaux (CS#2, HS#2, etc...), thus introducing measurable differences between TB/TV 3 and SSDIF count rates and making any comparison between the two environments very difficult.

2. Deuterium lamp

Similarly, HRC#1 F220W and F250W frames illuminated with the deuterium (D2) lamp were compiled from the ACS preflight database with no preference for the gain and read-out amplifier. Only gain=1 data with offset=0 and amp B or C read-out were found. The total integration time of the Amp C, F220W data is 2000 sec and for F250W, 1000 sec. On the other hand, the Amp B integrations were only 100 sec and so were not considered. Bias frames acquired at the same epoch and with the same gain and amplifier were subtracted.

RESULTS :

1. Count Rates

The results are tabulated in Tables 1, 2, and 3. Only the HRC is sensitive to the blue wavelengths of the F220W, F250W, F330W, F344N, and PR200L filters. In columns 2 and 7 of Tables 1 and 2 and column 2 of Table 3, the ID number of the frames are listed and displayed. The images are shown in DNs, hence the offset in counts between the WFC quadrants, which would be removed if normalized to e- with the proper gain values. In columns 3-4 and 8-9 of Tables 1 and 2 and column 3-4 of Table 3, the maximum and median count rates are tabulated in e-/sec/pixel and in DN/sec/pixel, for the standard configurations of gain=2, Amp C read-out for the HRC and gain=1, Amps ABCD read-out for the WFC. Digital saturation is reached at 65545 DN/pixel for both detectors so the maximum count rate will determine the longest integration time for a particular lamp and filter combination.

The statistics were performed on the full data regions of the HRC and WFC frames, including the HRC Fastie finger and the WFC central gap. If more than one frame is available for one filter, the average is listed. The maximum counts are evaluated by fitting a 2nd order polynomial to the upper tail of the image histogram and determining the counts where the histogram has a value of 1. When these limits could not be located automatically, they were evaluated by eye. Note that the WFC maximum (in DN) is located in the upper-left corner on the Amp B quadrant and for the HRC, the minimum is under the Fastie finger. The counts are then divided by the integration times and converted to electron rates with the appropriate gains tabulated in HRC#1 : Gain, Linearity, Saturation, Noise (Amp C : gain=1.18 e-/DN and 2.22 e-/DN) and WFC#4 : Gain, Linearity, Saturation, Noise (Amps ABCD : gain=1.00 e-/DN).

The CEI specification for the internal lamp calibration (see CEI Specifications below) states that a S/N ratio of 100 must be reached in less than 10 hours (36000 sec) at a 2x2 pixel resolution over both detectors. To verify this, we calculated the time required to reach this S/N ratio with a 2x2 resolution element using the median count rates following the recipe outlined on pp. 103-104 of the ACS Instrument Handbook (Version 2.1, July 2001), using a read noise of 4.85 e- for the WFC (gain=1) and 4.62 e- for the HRC (gain=2) and dark currents of 5.8 and 8 e-/pix/hour for the WFC and HRC, respectively. The resulting integration times are tabulated in columns 5 and 10 of Tables 1 and 2 and column 5 of Table 3. Note that the minimum integration time for the HRC is 0.1 sec and for the WFC, 0.5 sec (and 0.6 sec is not permitted).

Table 1 : HRC Tungsten 3 and 4 Count Rates

Note : The count rates in DN/sec are listed for the standard Amp C, gain=2 read-out. A gain of 2.22 e-/DN is used for the conversion.

Table 2 : WFC Tungsten 1 and 2 Count Rates

Note : The count rates in DN/sec are listed for the standard Amp ABCD, gain=1 read-out. A gain of 1.0 e-/DN is used for the conversion.

Table 3 : HRC Deuterium Count Rates

Note : The count rates in DN/sec are listed for the standard Amp C, gain=2 read-out (gain=2.22 e-/DN).

2. Comparison of the tungsten lamps

The ratio of the full-field median count rates for the HRC tungsten 3 and 4 lamps and the WFC tungsten 1 and 2 lamps are plotted as a function of wavelength in Figs 1 and 2 for the broad, medium and narrow-band filters. For HRC tungsten 3 and 4, the mean ratio for these filters is 1.328 and for the WFC , 1.199. There is a strong wavelength dependence : the TUNG1/TUNG2 and TUNG3/TUNG4 ratios increase to the blue (excluding F220W and F250W for the HRC), except perhaps for the HRC narrow-band fluxes. For the HRC, the increase is ~12% and for the WFC, ~18% from the blue to the red ends. We conclude that the tungsten 1 and 3 lamps are "bluer" than their counterparts, perhaps due to different power settings.

• Figure 1 : Ratio of the HRC tungsten 3 and 4 count rates as a function of the central wavelength of the filters. The dashed line is the mean of all the medium-, broad-, and narrow-band ratios.
• Figure 2 : Ratio of the WFC tungsten 1 and 2 count rates as a function of the central wavelength of the filters. The dashed line is the mean of all the medium-, broad-, and narrow-band ratios.

3. Comparison of the D2 and tungsten 3/4 lamps for HRC F220W and F250W

Flat fields through the F220W and F250W filters on HRC can be obtained with both the deuterium or tungsten 3 or 4 lamps. We recommend using the D2 lamp for three reasons : (1) the count rates are higher for the D2 lamp for both filters (factor of ~2.5 for F250W), hence the integration times are shorter, (2) the illumination of the D2 lamp is more uniform than the tungsten lamps, in particular at the field center (see Figs 3 and 4), and (3) the tungsten lamp flux in the F220W and F250W wavelength range is probably negligible - the observed tungsten illumination on the HRC is likely dominated by scattered red light around the shutter blade.

• Figure 3 : F220W : Ratio of D2 over tungsten 3 flat fields. Large-scale variations across the frame are typically less than 20%.
• Figure 4 : F250W : Ratio of D2 over tungsten 3 flat fields.

CEI SPECIFICATION :

As mentioned above, the CEI specification for the internal lamp calibration states that a S/N ratio of 100 must be reached in less than 10 hours (36000 sec) with 2x2 pixel resolution elements over the entire detector areas (see paragraph 4.6 of CEI Specifiation STE-50, Nov. 1995). The longest integration time for the HRC is through F250W (tungsten 4) for ~1.1 hours and for the WFC, through FR388N for ~55 sec. The CEI specification is easily met for both detectors and all filters.

CONCLUSION :

The internal tungsten lamp count rates were measured for all filters for the HRC and WFC. The tungsten 1 and 3 lamps are bluer than the tungsten 3 and 4 lamps by ~15% from 4300 Ang to 8900 Ang. The CEI specification of S/N=100 in <10 hours in 2x2 resolution elements is easily met for all filters. For internal flat fields through F220W and F250W on the HRC, we recommend using the deuterium lamp instead of the tungsten lamps.

MISCELLANEOUS :

• the HRC#1 : Gain, Linearity, Saturation, Noise results
• the WFC#4 : Gain, Linearity, Saturation, Noise results
• the ACS Instrument Handbook, Version 2.1, Pavlovsky, C., et al. 2001 (Baltimore: STScI).