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Last updated
20 December 2001 09:14:04
Maintained by
martel
PROCEDURE : -
TITLE : Monochromatic Flat Fields : HRC, WFC
CATEGORY : Calibration
PURPOSE :
The primary goal is to secure the highest priority monochromatic flat fields for the flight-build detectors HRC#1 (gain=2) and WFC#4 (gain=1) with RAS/HOMS equipped with a QTH lamp coupled to a tunable monochromator. RAS/HOMS provides external, OTA-like illumination above its refractive cutoff wavelength of ~3500 Ang. The monochromatic flats will be used to verify the accuracy of the ramp flat-fielding procedure and to correct for fringing longward of 7000 Ang.
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 monochromatic illumination from the QTH lamp correctly simulates the OTA beam angles over the entire field. The QTH lamp is coupled to the monochromator and the slit is adjusted to provide a 20 Ang bandpass at each tuned wavelength (a ~1.3 mm slit corresponds to a ~40 Ang bandpass).
SOFTWARE REQUIREMENTS :
IDL and/or IRAF routines to perform basic statistics on the images (counts, median, mean, sigma).
COMMAND MODE : Real time.
PROGRAM/EXPOSURES :
1. Monochromatic Flats through Broad-Band Filters : No Fringing
In ISR ACS 00-10 : Flats : Preliminary WFC Data and Plans for Flight Flats, Bohlin, Hartig, & Tsvetanov (2000) show that below 7000 Ang, where fringing is not important, flat fields acquired with continuum light (or "white light") through the broad band filters nearest in wavelength (F330W, F435W, F475W, F555W, F606W, and F625W) can be used to flat field the ramp filters. Such "white light" flats will be acquired in Item 3 : External "White Light" Flat Fields of this campaign. Below, we list all the ramp segments (even at >7000 Ang) and possible broad-band filters that best match their wavelength range.
| SEGMENT | RANGE | WAVELENGTH | BROAD-BAND FILTER |
|---|---|---|---|
| FR462N | 4420-4820 | 4620 | F475W |
| FR388N | 3710-4050 | 3880 | F435W |
| FR423N | 4050-4420 | 4235 | F435W |
| FR782N | 7470-8160 | 7815 | F775W |
| FR656N | 6270-6850 | 6560 | F625W |
| FR716N | 6850-7470 | 7160 | F775W |
| FR931N | 8910-9720 | 9315 | F850LP |
| FR914M | 7570-10710 | 9140 | F850LP |
| FR853N | 8160-8910 | 8535 | F814W |
| FR1016N | 9720-10610 | 10165 | F850LP |
| FR459M | 3810-5370 | 4590 | F475W |
| FR647M | 5370-7570 | 6470 | F606W |
| FR601N | 5750-6270 | 6010 | F625W |
| FR505N | 4820-5270 | 5045 | F475W |
| FR551N | 5270-5750 | 5510 | F555W |
To evaluate the accuracy of this flat-fielding method, high S/N-ratio monochromatic flat fields at test wavelengths through the broad-band filters are necessary. Bohlin, Hartig, & Tsvetanov (2000) suggest flat fields at the central wavelength of the F330W, F435W, F475W, F555W, F606W, and F625W filters, and at intermediate wavelengths in case interpolation is required to remove wavelength dependent effects. A F625W white light flat field can serve as a G800L flat field but a monochromatic flat field at the central wavelength of F625W (~6320 Ang for WFC and 6301 Ang for HRC) should be acquired for G800L as well. For fringing in the G800L grism at long wavelengths (>7000 Ang), monochromatic fringe flats (see section (2) below) can be used.
The list of monochromatic flat fields through the broad-band filters below 7000 Ang are listed below for the WFC and HRC. All illumination bandpasses are 20 Ang wide. The blue F330W (3346 Ang) flat fields can not be observed with RAS/HOMS. The flat fields at intermediate wavelengths are acquired through one of the two bracketing filters with the highest transmission (the monochromatic flats are essentially independent of the filter), although preference is given to the Sloan filters because of their extensive use in the GTO program. All the central (and the derived intermediate) wavelengths are extracted from the task 'bandpar' in IRAF/SYNPHOT. Monochromatic flats through G800L are also obtained, as well as at 3880 Ang in F435W, the central wavelength of FR388N, which departs the most from its neighboring filters (F330W and F435W).
For consistency with the continuum flat-field program, the monochromatic flat fields are listed in SMS spreadsheets, even though full-blown SMSs will not be generated. These are called : MONO_HRC and MONO_WFC. As for the continuum flat fields (Item 3), three consecutive WFC flats and two HRC flats are acquired, respectively, bracketed by bias frames. Typically, counts of ~30000 DN/pix should be reached in each frame for their sum to reach the full well. The integration times will need to be determined on-site.
|
HRC : MONO_HRC
|
WFC : MONO_WFC
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2. Monochromatic Flats through Broad-Band Filters : Fringing
At wavelengths greater than 7000 Ang, numerical modeling of the fringe pattern is necessary and will be performed by the ECF group. W. Freudling has provided a "wish list" of wavelengths for monochromatic flat fields to be obtained with 20 Ang bandpasses through the appropriate broad-band filters for HRC and WFC. The affected filters are :
| Wheel | Filter | Range (Ang) |
|---|---|---|
| Wheel 1 | F606W | 4600-7200 |
| F625W | 5400-7100 | |
| F775W | 6800-8700 | |
| F850LP | 8000-10500 | |
| F892N | 8750-9050 | |
| G800L | 5500-10000 | |
| Wheel 2 | F814W | 6900-9700 |
The fringe pattern is primarily dictated by the CCD and the bandwidth of the illumination and is independent of the filter. For example, a 7000 Ang monochromatic flat through F775W is also applicable to the F625W filter. Therefore, it is necessary to use only the broad-band filters with the highest transmission at the wavelength of interest. The images are obtained in the same manner as those of section (1) above, with 20 Ang bandpasses, and are implemented in the SMS spreadsheets FRINGES_HRC and FRINGES_WFC (see also Procedure 02WH (G. Meurer) for a further description). Again, the correct integration times will need to be determined on-site.
|
HRC : FRINGES_HRC
|
WFC : FRINGES_WFC
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TOTAL TIME :
The number of images generated by each procedure is :
The overhead is dominated by the manual tuning of the monochromator and the read-out time of the CCDs. Assuming 5 minutes of overhead to tune each wavelength and 8 min and 2 min for read-out and dump of each WFC and HRC frame, respectively, then the execution time of MONO_HRC is ~2.0 hours, MONO_WFC, ~6.6 hours, FRINGES_HRC, ~4.0 hours, and FRINGES_WFC, ~12.8 hours, for a grand total of ~1.1 days. These are minimum time estimates; overheads to configure the monochromator, test exposures to determine the correct integration times, and contingencies will likely add up significantly. Realistically, it may be necessary to shorten the list of wavelengths in the fringe flat campaign.
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 ECF team for modeling.
REFERENCES :
