Calibration reference files are specific to each instrument configuration and observing mode; in addition, monitoring has shown that some instrumental properties change with time. Therefore, calibration reference files can be time-tagged, indicating that they should be used with data taken within a specific range of dates. The best set of reference files for calibrating the observation (according to the pipeline) will therefore depend on the instrument configuration and observing mode that was used and the date when the observation was taken.
StarView, the interface to the HST catalog and Archive, can be used to determine the current best available reference files for any dataset and to retrieve those files from the HST Archive (Chapter 1 describes how to use StarView to access the HST catalog and archive). The Calibration Reference screens (one per instrument) in StarView can be used to obtain a listing of the current best reference files for any dataset and a listing of the files that were originally used to calibrate the dataset by the RSDP pipeline. Figure 45.4 shows StarView being used to determine the best and used calibration reference files for one particular WF/PC-1 observation. Either the used or the recommended (or both) reference files can be marked for retrieval and fetched from the Archive using FTP, as shown in the StarView tutorial in Chapter 1.
45.6.1 When is "Best" Not the Best?
The current limitation of StarView, and the reference files chosen automatically by the pipeline, is that only one "best" reference file can be specified. Frequently, there are several possible reference files that are appropriate and the files used may not be the optimal calibration files for some purposes, depending upon the scientific objectives.
http://www.stsci.edu/ftp/instrument_news/WFPC/This memo suggest possible alternate reference files that can be used for recalibration. These memos summarize the file names and histories of available reference files, as well provide more detailed information about the nature and quality of each reference file. For high accuracy in the science analysis, it is best to try using several different reference files to calibrate the data, to see which ones give the best results and to empirically determine the degree of uncertainty in the data attributable to the choice of reference file.
wfpc1_memos.html
A history of all flatfield reference files generated and delivered to the HST Archive is maintained in the Reference File memo on WWW, at:
http://www.stsci.edu/ftp/instrument_news/WFPC/This file is updated as new files are added. The flatfields listed in the memo include those used in the pipeline as well as some alternate flatfields, which were never used in the pipeline but which may be of use in recalibration. The Reference File memo provides a single one-line history comment for all files; for example, some reference file changes only involved keyword changes to the header and do not affect the data. On the other hand, some flatfield reference files were mere placeholders until data could be obtained and a flatfield generated and delivered. These placeholder flatfield files contained only values of 1 in order to prevent the routine pipeline processing from crashing yet avoid applying an improper correction. The same wfpc1_memos WWW site also provides links to more detailed memos concerning specific reference files listed in the Reference File memo, such as the Closure Flatfields, the Super-Sky (MDS) Flatfields, and the Delta Flat Corrections.
wfpc1_memos.html
See Chapter 1 for information about how to retrieve the flats (and other reference files) from the HST Archive. See Chapter 46 for more information on improving the calibration of WF/PC-1 data.
SV -Flatfield -Camera and Filter Combinations
Non-SV Flatfield Camera and Filter -Combinations
Earthcal observations for the final Cycle 2 and 3 closure flatfield set were taken after August 2, 1993. The closure flats were generated in the same manner as previous cycles, using the IRAF/STSDAS tasks streakflat and normclip. In addition, the STSDAS wfixup task was used to interpolate across known bad pixels (the fixed pixels are flagged in the flatfield DQF file, suffix b6h/b6d). As for all reference files, users are encouraged to refer to the HISTORY records in the file headers for details about the generation of the file. The closure flats were installed in CDBS, the Reference File Memo updated, and a detailed Closure Flats Memo posted on WWW; filter and camera combinations are given in Table 45.5.
Closure flatfields are not only useful for 1993 WF/PC-1 data, but along with an
appropriate delta flat, are useful for recalibrating older data as well.
Closure (Cycle 3) Flatfield Camera and Filter Combinations
Particularly for science data taken in the same epoch as the streakflats (August 2, 1993 to December 1993), the closure flats will generally be the best choice for flatfielding. For data taken between the August 7, 1992 and August 2, 1993 decontaminations, the closure flatfields may be the better choice, as long as an appropriate delta flat is used as well. The older Cycle 0 and 1 flatfields may be most appropriate for data taken before February 1992, before the appearance of the "permanent measles", since the closure flats do contain the measle features. Delta flats, discussed in the next section, may be used to correct any epoch of flatfield to any science data epoch.
http://www.stsci.edu/ftp/instrument_news/WFPC/wfpc1_memos.html
Camera |
Filters | ||||||
---|---|---|---|---|---|---|---|
PC
|
F336W
|
F368M
|
F413M
|
F439W
|
F492M
|
F517N
|
F547M
|
|
F555W
|
F569W
|
F606W
|
F622W
|
F648M
|
F664N
|
F673N
|
|
F675W
|
F702W
|
F718M
|
F725LP
|
F785LP
|
F791W
|
F814W
|
|
F875M
|
|
|
|
|
|
|
45.6.3 Choosing and Generating Delta Flats
Observations taken during one epoch (time period between decontamination events) and processed with flatfield reference files created from data taken during a different epoch may require a delta flat correction to correct for QE changes that occurred during the decontaminations1. A delta flat, generated from ratios of INTFLATs, can also be used to reduce, although not eliminate, the effects of the measles contamination. In practice, since the pipeline flatfields are inverse flats (calwfp multiplies the science data by the FLATFILE specified in the .d0h header), the delta flat correction is the ratio of an INTFLAT of the same epoch as the flatfield calibration file divided by an INTFLAT taken close in time and wavelength to the science observation to be corrected. The calibrated pipeline image is multiplied by the appropriate delta flat: F = CxD where F is the final image, C is the calibrated pipeline product image (.c0h) which has had the flatfield reference file applied, and D is the delta flat.
The INTFLATs were taken by illuminating the backside of one of the two shutter blades (A or B) with internal lamps. Due to variations in the shutter blade reflectivities, all INTFLATs used in a delta flat must have been taken with the same shutter blade (given by SHUTTER keyword). Since the flatfields and measles features have been found to depend on wavelength, the delta flat (or INTFLATs) closest in filter to the observation will provide the best possible correction. Note that prior to the development of the persistent measles in February 1992, regular INTFLATs were being obtained using a relatively small number of filters; therefore, INTFLATs for the baseline epoch, and thus the resulting delta flats, are available for only a small number of filters. However, after February 1992, the delta flat proposals were greatly expanded in order to obtain INTFLATs in nearly 20 filters in each camera; these may allow the correction of the closure flatfields to data taken prior to August 1993.
http://www.stsci.edu/ftp/instrument_news/WFPC/For some epochs, there are two types of delta flats available for use, average delta and individual delta flats. The average delta flats were created by averaging all the individual delta flats taken between decontaminations (dates of all decontaminations are listed in the delta flat file). These average delta flats are partially corrected for cosmic rays, and therefore, should be used whenever possible. The closure delta flats, generated from INTFLATs taken August 8,1992 through November 1993, have all been cosmic ray corrected using the STSDAS crrej task and wfixup to average across questionable pixels. Note that a subset of the closure flats already include the appropriate delta flat correction; the Closure Flat Memo flags these appropriately.
wfpc1_memos.html
Custom delta flats can be generated from internal flats observations retrieved from the HST Archive using StarView.
Hi-Fi (Cycle 1) = Hi-Fi (Cycle 3) * (clean Cycle 3 intflat) / (clean Cycle 1 intflat)
stevens@stsci.edu Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 01/14/98 16:04:33