WFC and HRC Internal Lamp Count Rates

AUTHORS : A. Martel, G. Hartig, J. McCann.

PURPOSE :

The main goal is to measure the count rates in flat fields acquired with the internal T2 (WFC), T4 (HRC), and D₂ (HRC) lamps through all the filters with no cross filters. These will serve as references for future calibration campaigns and for CEI verification.

DATA :

The bias frames and flat fields were acquired as part of the dust filter search with the internal tungsten and deuterieum lamps (T2 for WFC and T4, D₂ for HRC) and WFC Build#3 (amplifiers B and C, gain=1) and HRC Build#1 (amplifier C, gain=1). All the images were acquired with SMS procedures (JGCW27A-E for WFC and JGCH27F for HRC) and include all the filters, thus providing a wide range in wavelengths and exposure levels. The F892N filter and the polarizing filters were read out as 1/4-field subarrays and all others as full arrays. The F220W and F250W filters were imaged with a D₂ lamp and HRC. Only the count rates of the flats obtained at the nominal filter positions (FWoffset1=0, FWoffset2=0) are considered.

METHOD :

The reduction is fully explained in the HRC and WFC Bias Subtraction and Overscan Analysis and we summarize it here. We first subtracted the bias frame from the individual flat fields and then removed any residual in the leading physical overscan (WFC) or trailing physical overscan (HRC). The minimum (MIN) and maximum (MAX) pixel values in the data regions were evaluated from the histogram of each image while the median (MED) was calculated with the 'msstat' task in IRAF after constructing pixel masks for each chip over the bad pixels/columns. The count rates are simply obtained by dividing the counts by the exposure time.

RESULTS :

In Tables 1 and 2, the count rates are tabulated for each filter imaged with the WFC and HRC, respectively, after subtraction of the bias frames. For the WFC narrow-, medium-, and broad-band filters, we take the average values for Chips 1 and 2. The ramp filters were imaged at three consecutive wheel positions on the WFC to cover most of the surface of the three segments. Since each image shows great variations in intensity between each segment, average values between the two chips would be misleading, hence we list the MIN, MAX, and MED count rates of the full mosaiced WFC field at each wheel position.

The full well of the WFC Build#3 CCDs is >78000 e- (see WFC Builds) and its gain of 1 corresponds to 1.7 e-/DN, so saturation sets in around ~46000 DN. The polarization filters are very near this limit and in fact become non-linear in their central parts around 38000 DN. This is apparent in the ratioed images shown in the dust filter search, which are identically equal to one in the middle. The POL flat fields will have to be taken again with shorter exposure times for the same WFC detector.

The exposure time required to reach a median count of 20000 DN/pixel is indicated in the last column of each table. This value is a useful guide since no HRC or WFC detectors saturate at this level at gains of 1 or 2 and sufficient S/N ratios are reached.

Table 1 : WFC Build#3

FILTER	MIN	MAX	MED	T_20K

F435W	1320	1570	1465	13.6
F475W	4260	5285	4885	4.1
F555W	5780	7280	6650	3.0
F606W	12230	15430	14135	1.4
F625W	7560	9790	8940	2.2
F775W	5300	6760	6145	3.2
F814W	8500	10450	9575	2.1
F850LP	5310	6635	5930	3.4

F550M	3355	4240	3850	5.2

F502N	185	250	220	90.3
F658N	390	485	445	44.8
F660N	150	190	170	117
F892N	480	595	540	37.0

FR459M 1	45	3025	720	27.8
FR459M 2	10	2985	380	52.2
FR459M 3	15	2850	1005	19.9
FR914M 1	195	2810	1125	17.8
FR914M 2	20	2765	885	22.6
FR914M 3	15	2700	535	37.2

FR388N 1	8	255	30	700
FR388N 2	2	195	20	1060
FR388N 3	6	335	45	445
FR505N 1	155	830	495	40.4
FR505N 2	35	830	465	43.1
FR505N 3	9	835	515	38.8
FR656N 1	160	785	600	33.4
FR656N 2	20	855	590	34.0
FR656N 3	10	730	530	37.8

G800L	16000	21800	19635	1.0

POL0UV	12000	14000	13325	1.5
POL60UV	12840	14260	13700	1.5
POL120UV	12320	13840	13205	1.5

POL0V	11860	13720	12785	1.6
POL60V	11785	13605	12725	1.6
POL120V	11785	13750	12810	1.6

Note :

All the WFC flats were illuminated with the T2 lamp.
MIN (DN/sec) : Minimum count rate in the data region after bias-subtraction.
MAX (DN/sec) : Maximum count rate in the data region after bias-subtraction.
MED (DN/sec) : Median count rate in the data region after bias-subtraction.
T_20K (sec) : Exposure time to reach a median count of 20000 DN.
The small filters were read out as subarrays : F892N, POL0UV, POL60UV, POL120UV, POL0V, POL60V, and POL120V. The polarization filters are slightly saturated in their centers.

Table 2 : HRC Build#1

FILTER	MIN	MAX	MED	T_20K

F220W	0.64	0.96	0.82	24400
F250W	1.44	2.14	1.83	10930
F330W	7.50	12.5	10.4	1915

F344N	4.21	8.42	6.31	3165

PR200L	-	-	-	-

Note :

All the HRC flats were illuminated with the T4 lamp except for filters F220W and F250W which were imaged with the internal D₂ lamp.
The PR200L images were saturated with 2.6 sec exposures.
MIN (DN/sec) : Minimum count rate in the data region after bias-subtraction.
MAX (DN/sec) : Maximum count rate in the data region after bias-subtraction.
MED (DN/sec) : Median count rate in the data region after bias-subtraction.
T_20K (sec) : Exposure time to reach a median count of 20000 DN.

CEI SPECIFICATIONS :

Update.

CONCLUSION :

The count rates were determined for the different filters and internal lamps. Measurements for the polarization filters need to be repeated with non-saturated images.

REFERENCES :

the ACS filters