calibration plans rashoms feb2001 flats |
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Last updated
20 December 2001 10:44:56
Maintained by
martel
PROCEDURE : RAS/HOMS : 34w, 34h
Internal : 36w, 36h
TITLE : External and Internal Continuum ("White Light") Flat Fields : HRC, WFC
CATEGORY : Calibration
PURPOSE :
The primary goal is to acquire the highest priority broad-band (or "white light") flat fields with RAS/HOMS coupled to a 100 Watt lamp. RAS/HOMS provides external, OTA-like illumination above its refractive cutoff wavelength of ~3500 Ang. The flat fields will serve as the primary ground-based flat-field dataset for the flight-build detectors HRC#1 (gain=2) and WFC#4 (gain=1).
Additionally, "quarter-million-electron" (QME) flats will be acquired through blue and red filters with the internal Tungsten lamps. These flat fields will serve as references for future ground- and on-orbit monitoring of the internal lamp fluxes and for verification of flat field stability and repeatability.
FREQUENCY :
Execute until all the flats are acquired.
DETECTOR :
Flight build detectors HRC#1 (amp C) and WFC#4 (amps ABCD) at gains of 2 and 1 e-/DN, respectively, and bias offsets of 3.
PREREQUISITES/BACKGROUND :
The illumination and exposure times must be adjusted before the execution of each SMS so that the average counts in each HRC and WFC frame are ~30000 DN/pix.
HARDWARE REQUIREMENTS :
RAS/HOMS is equipped with a Mylar flat field diffuser at the pupil mask location so that the "white light" illumination from the 100 Watt lamp correctly simulates the OTA beam angles over the entire field.
SOFTWARE REQUIREMENTS :
IDL and/or IRAF routines to perform basic statistics on the images (counts, median, mean, sigma).
COMMAND MODE : SMS/RT
PROGRAM/EXPOSURES :
The entire "white light" flat-field program consists of two separate campaigns : (1) the bulk of the flat fields will be obtained with external, OTA-like illumination of RAS/HOMS and (2) the QME flat fields will be acquired with the internal Tungsten lamps. The program is integrated in a suite of SMS spreadsheets. Since the illumination levels, and hence the correct exposure times, will not be exactly known until the beginning of the campaign, particularly for the RAS/HOMS flats, the spreadsheets will need to be updated on-site and SMS outputs generated with a rapid turnover. If this proves impossible, the flat fields will be acquired in real time with CCL.
The observations are a refinement of the general "wish list" compiled by Sparks et al. in TIR 2000-0004 : ACS Flat-Field Ground Calibration Planning. For completeness, the "blue" HRC and SBC flat fields which can't be acquired with RAS/HOMS because of its refractive cutoff below ~3500 Ang are listed in separate SMSs. These apply to filters F220W, F250W, F330W, F344N for HRC and all of the SBC filters. The blue HRC flats will need to be acquired in a separate, non-RAS/HOMS campaign while the SBC flats have essentially been obtained in the STUFF campaign (Feb-Mar 1999) (with slightly non-OTA illumination). We note that the internal D2 lamp illumination does not follow an OTA-like path.
In general, three consecutive WFC exposures, each at a level of ~30000 DN/pix, are taken through each filter to reach the full well (80000 e-/pix) at a gain of 1, while still remaining below the A/D digitization limit of 65536 DN/pix per exposure. Similarly, for the HRC, two exposures at levels of ~30000 DN/pix are sufficient to reach the full well (120000 e-/pix) at a gain of 2. Single bias frames are acquired at the beginning and end of each flat-field sequence.
Although the absolute exposure times of the external, RAS/HOMS flats can not be determined before the campaign, the relative exposure times through different filters can be estimated with the ACS Imaging Exposure Time Calculator (ETC) available on the STScI Web site. For these calculations, we use the known spectrum of the 250 Watt QTH lamp which is approximated by a blackbody at a temperature of 3200 K (lambda_peak=9055 Ang). In Tables 1 and 2, the integration times required to reach a S/N ratio of 10 are compiled and normalized to an arbitrary exposure of 100 sec in F555W. For the HRC coronagraphic mode, the integration times were simply doubled from their direct mode counterparts to achieve the same S/N ratio. In the individual spreadsheets, the shortest integration time is set at 1 sec and the other exposures are scaled relative to it. The ETC does not support the G800L grism, the ramp filters, and the HRC crossed acquisition filters in "white light" so we use a dummy exposure time of 1 sec in the spreadsheets JRHW34E, JRHH34I, JRHH34J, and GRISM.
At their nominal positions i.e. with no specified offsets, the small filters fall at the centre of the WFC and HRC chips. But to avoid the "buttcrack" between the two WFC chips, offsets are specified to move the filter on one of the two chips (WFC1 : Amps A+B, WFC2 : Amps C+D). For a small filter on wheel 1, an offset of -61 steps places it in the amp C quadrant (WFC2), and an offset of +57 moves it in the amp B quadrant (WFC1). Since wheel 2 moves in the opposite direction, a +61 steps offset moves the small filter in the amp C quadrant and -57 steps in the amp B quadrant. For the ramp filters, all in filter wheel 2, offsets must be applied for full coverage of the central segment (edge-to-edge) and for partial coverage of the inner and outer segments when the WFC chips are fully read-out. The smaller HRC full field is located on the central segment only and three wheel positions are required to span the entire segment. More details for the ramp filters are given below in Priority II.
The flat-field spreadsheets are listed in the following tables and the full image sequences are listed in the Appendix below.
1. Basic Flats
| WFC | HRC | SBC |
|---|---|---|
| JRHW34A Broad-Band Filters |
JRHH34A Broad-Band Filters |
JGCS34A SBC Filters |
| JRHW34B Medium+Narrow-Band Filters |
JRHH34B Medium+Narrow-Band Filters |
|
| JGCH34A Blue Filters |
2. Polarizers with Crossed Filters
| WFC | HRC |
|---|---|
| JRHW34C Visual Polarizers + Broad/Narrow-Band Filters |
JRHH34C Visual Polarizers + Broad/Narrow-Band Filters |
| JRHW34D UV Polarizers + Broad-Band Filters |
JRHH34D UV Polarizers + Broad-Band Filters |
| JGCH34B UV Polarizers + Blue Filters |
3. HRC Coronagraphic Flats
| HRC |
|---|
| JRHH34E Broad-Band Filters |
| JRHH34F Medium+Narrow-Band Filters |
| JGCH34C Blue Filters |
4. HRC Polarizers + Coronagraphic Flats
| HRC |
|---|
| JRHH34G Visual Polarizers + Broad/Narrow-Band Filters |
| JRHH34H UV Polarizers + Broad-Band Filters |
| JGCH34D UV Polarizers + Blue Filters |
5. HRC Crossed Filters for Acquisitions
| HRC |
|---|
| JRHH34I Crossed Filters |
WFC and HRC Ramps
WFC full field ramp flat fields in white light are acquired at three positions, nominal and offsets of +/- 62 steps, resulting in full coverage of the middle segment, from edge to edge, and partial coverage of the inner and outer segments. These flats will serve to verify the effectiveness of flat fielding with a full field flat of similar wavelength through a broad-band filter (see Item 4 : External Monochromatic Flat Fields) or of using an interpolation tool and will also provide information on QE changes along the segments. At the nominal position, the central wavelength of the middle ramp segment is centered on the WFC FOV i.e. on the "buttcrack". An offset of -62 steps brings it to the WFC1/Amp B quadrant and an offset of +62 steps moves it to the WFC2/Amp C quadrant. The central wavelength of the outer segment is located on WFC2 (Amp D quadrant) and that of the inner segment on WFC1 (Amp A quadrant). A full WFC chip read-out is performed at all positions. HRC full field ramps are also taken at three positions : nominal and offsets of -84 steps (WFC1 Amp B direction) and +88 steps (WFC2 Amp C direction). These only cover the central segment.
For the WFC ramp data, great care must be exercised in selecting the exposure time since the flux through each segment of a given ramp filter can vary considerably. For example, a reasonable level of ~30000 DN/pix in one segment could produce saturation in the neighboring segment. None of the segments should saturate. Ideally, three separate sets, one for each segment, would be acquired, with each set consisting of three exposures of equal exposure times to reach the full well. This would then result in nine images per wheel position or a total of 27 images for one ramp filter. This is specified in the spreadsheets.
| WFC | HRC |
|---|---|
| JRHW34E WFC full field ramps at 3 positions |
JRHH34J HRC full field ramps at 3 positions |
WFC, HRC (and SBC) Prisms and Grisms
With RAS/HOMS equipped with the 100 Watt lamp, acquire a set of flat fields with the G800L grism for WFC and HRC in real time; no SMS needs to be generated. The HRC and WFC observations are formally listed in the standard SMS spreadsheet GRISM. For completeness, flats with the PR200L prism for HRC and with the PR110L and PR130L prisms for SBC are listed.
| HRC, WFC, SBC |
|---|
| GRISM G800L (HRC, WFC) |
Polarizer Flat Fields in Non-Supported Modes
Flat fields are acquired with the visual and UV polarizers as in Priority I, Section 2, but with filter combinations that are not officially supported by STScI. Flat fields in coronagraphic mode are also included for the HRC.
| WFC | HRC |
|---|---|
| JRHW34F UV + Visual Polarizers on WFC1 and WFC2 |
JRHH34K UV + Visual Polarizers |
| JRHH34L UV + Visual Polarizers with Coronagraph |
High S/N-ratio flat fields will be acquired with the internal WFC/T2 lamp and the HRC/T4 lamp through blue and red filters. The choice of filters is based on the GTO filter usage compiled by G. DeMarchi (STScI). For the WFC, the Sloan filters are the most popular but for the HRC, there is no clear preference between the filter sets. The red F814W and F850LP filters for the HRC and WFC, respectively, and the bluer F475W filter for both detectors offers a good compromise.
The count rates through these filters can be determined from data taken in past calibration campaigns. In the June 2000 BATC campaign (HRC#1,WFC#3), the search for dust features on the filters produced internal T2 flat fields on the WFC and their count rates were measured (see WFC and HRC Internal Lamp Count Rates). At a gain of 1 e-/DN, a median count rate of 5930 DN/sec is found for WFC/F850LP and 4885 DN/sec for WFC/F475W. Therefore, exposure times of 5 and 6 sec will yield counts of ~30000 DN/pix per exposure for F850LP and F475W, respectively, and 9 consecutive exposures in each filter will result in a total of ~250000 e-/pix. For the HRC, images 14117 and 14118 in F814W yield median count rates of 34260 DN/sec after bias subtraction (ID 14113) at a gain of 1 or 17130 DN/sec at a gain of 2. Similarly, for F475W, image 5595 gives a median count rate of 1716 DN/sec at a gain of 1 after bias subtraction (ID 5593), which corresponds to 858 DN/sec at a gain of 2 e-/DN. The exposure times to reach ~30000 DN/pix are therefore 1.8 and 35 sec for F814W and F475W, respectively, at a gain of 2, and 9 consecutive exposures in each filter will result in a total of ~250000 e-/pix.
| WFC | HRC |
|---|---|
| JRHW36A Internal WFC/T2 Flat Fields |
JRHH36A Internal HRC/T4 Flat Fields |
TOTAL TIME :
If run as formal SMSs, the number of images generated by each procedure and their total execution time are listed below. We assume 8 min for a read-out and dump of a WFC full frame, 2 min for one quadrant, and 2 min for an HRC full frame. The entire continuum flat field program requires a grand total of ~1.8 days. This is a minimum time estimate; overheads for test exposures to determine the correct integration times and contingencies, for example, will likely contribute significantly.
The grism flats yield these images :
ANALYSIS :
The flat fields will be examined and manipulated with IDL and IRAF (bias subtraction, cosmic-ray rejection, normalization, statistics, etc...). They will be archived appropriately for future reference and transmitted to the STScI pipeline team.
REFERENCES :
APPENDIX :
Complete listings of the spreadsheets are available in the Appendix.
