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Calibration Database System

ACS Reference File Tables

Reference files used by the ACS calibration pipeline (CALACS and PyDrizzle/MultiDrizzle) are updated on a regular basis. Current reference files files can be retrieved from the HST archive, or downloaded directly from the jref directory. If you choose to access the files via this ftp server, keep in mind that there is a known bug in trying to access files via ftp server using Safari. The recommended browser is Firefox. See the logsheets for a detailed history of all ACS reference file deliveries and installations. The pixels affected by these reference files are generally identified by data quality flags in the [DQ] extension of calibrated FITS data.

ACS Bias Reference Images (BIASFILE)

Superbias images for subtracting bias structure.

See ACS ISR 2004-07 for details on the production of ACS superbias and superdark reference files.

Since launch (March 2002), a WFC and HRC superbias has typically been produced for every week or biweek. The "best" superbias is typically available for use in the pipeline 2-3 weeks after an observation date. ACS users are advised to retrieve (or re-retrieve) their data via OTFR after the best superbias is available.


Figure: A typical WFC superbias. Click to enlarge image.

ACS Cosmic Ray Rejection (CRREJTAB)

Parameters for rejecting cosmic rays from CR-SPLIT images.

These tables contain the parameters used by CALACS/ACSREJ in the pipeline, to reject cosmic rays from CR-SPLIT images only. Note that with MultiDrizzle added to the pipeline (~Sep 2004), image combination (including cosmic ray rejection) will be performed for any type of associated dataset, using the parameters contained in the MDRIZTAB reference files.

ACSREJ uses the table to determine the number of iterations for cosmic-ray rejection, the sigma levels to use for each iteration, and the spill radius to use during detection. This allows the rejection process to be tuned to each detector, with suitable defaults being applied during pipeline processing. Users may fine-tune the cosmic-ray rejection parameters when manually reprocessing data with ACSREJ by editing the CRREJTAB.


Converts commanded properties to calibrated values for each CCD amplifier.

The CCD tables define the key characteristics of the ACS CCDs (WFC and HRC) for the CALACS pipeline. They convert the commanded values to calibrated values for each CCD amp. The proper table row is selected by the values of DETECTOR, CCDCHIP, CCDAMP and CCDGAIN.

ACS Distortion Correction Tables (IDCTAB)

Parameters for geometric distortion correction.

The Science Instrument Aperture File (SIAF) contains position and scale information for every aperture of each of HST’s science instruments. Thus, it supports accurate target acquisitions, image processing, and photometry. The conversion between distorted and undistorted positions in the SIAF is controlled by a polynomial expansion which is derived by the individual instrument teams. A simple, common reference file can therefore be created for each instrument which describes the geometric distortion of each detector.

For ACS, the coefficients to the polynomial fit are found in the IDCTAB reference file. The distortion correction is not a part of CALACS, but is instead used as input to MultiDrizzle where it is used to convert the distorted image into a geometrically corrected image.

SBC Linearity Table (MLINTAB)

Parameters for determining linearity in MAMA images.

The MAMA Linearity Table, MLINTAB, provides the basic parameters for determining linearity. The global limit (GLOBAL_LIMIT) column from this table refers to the total count rate at which the data are affected by greater than 10% non-linearity across the entire detector.

CALACS will attempt to correct for non-linearity up to the global limit using the non-linearity time constant in the column TAU. The global linearity correction is computed for every pixel below the global linearity limit specified by iteratively solving the equation to get the true count rate N.

The LOCAL_LIMIT, which refers to an individual detector pixel, can actually be much higher than the global limit and is difficult to correct using a simple algorithm. Each pixel found to exceed this limit will simply be marked as non-linear in the DQ file. This DQ flag will be extended by a fixed radius from the original pixel, given in the EXPAND column and is currently set to 2 pixels.

ACS Residual Geometric Distortion Images (DGEOFILE)

Corrects residual geometric distortion.

The solutions described by the IDC tables are correct to about 0.1 pixels. However the distortion can be specified to 0.01 pixels by the inclusion of extra corrections (such as those shown in Figure 4.6 of the ACS Data Handbook) which can not be expressed by the fourth order polynomials. These further corrections are available in the form of two images for each detector for the x and y directions. These DGEO correction images have the extension ‘_dxy.fits’ and have been incorporated into the pipeline processing as of September 2004. For filter combinations for which these images do not exist, the final DGEO image correction step is omitted. Note, however, that for most purposes the polynomial solutions provide adequate precision.

ACS Bad Pixel Table (BPIXTAB)

Flag "permanently" bad pixels in the data quality (DQ) arrays of ACS images.

These tables are used by the DQCORR step in CALACS to populate the data quality (DQ) arrays of ACS images with flags for known (quasi-permanent) bad pixels. See the flag definitions for more details. The correct row in the table is selected by DETECTOR, CCDCHIP, CCDAMP, and CCDGAIN.

ACS MultiDrizzle Parameter Tables (MDRIZTAB)

Parameters for combining and cleaning associated datasets.

The same default parameters used for combining and cleaning observations within MultiDrizzle are recorded in the MDRIZTAB reference files that are delivered with the data. These parameters were chosen to avoid introducing any scale changes, shifts, or rotations relative to the original pointing, aside from those corrections incorporated in the distortion model itself.

ACS CCD Overscan Table (OSCNTAB)

Describes CCD overscan regions used to measure the bias level.

These tables describe the CCD overscan regions of the WFC and HRC, which are used CALACS to measure and subtract the bias level of each image.

ACS A-to-D Tables (ATODTAB)

Analog to Digital conversion information.

This table contains information that would be used in converting pixel values in the raw input image from units of DN (16-bit integer) to counts (32-bit float).

ACS Flat Field Files

Pixel-to-pixel correction file containing small scale variations.

The flat field image used to correct the data is created using up to four flat field reference files: the pixel-to-pixel file (PFLTFILE), the low-order flat (LFLTFILE), the delta flat (DFLTFILE), and the coronagraphic spot flat (CFLTFILE). The PFLTFILE is a pixel-to-pixel flat field correction file containing the small scale flat field variations. The LFLTFILE is a low-order flat which corrects for any large-scale flat field variations across each detector. This file is stored as a binned image which is expanded when being applied by CALACS. The DFLTFILE is a delta-flat containing any needed changes to the small-scale PFLTFILE. The CFLTFILE is a spot mask which contains the vignetting patterns of the occulting spots and which is applied to coronagraphic observations only.

If the LFLTFILE, DFLTFILE, or CFLTFILE are not specified in the SCI header, only the PFLTFILE will be used for the flat field correction. If all four reference files are specified, they will be read in line-by-line and multiplied together to form a complete flat field correction image. Currently, the LFLTFILE and DFLTFILE flats are not used for ACS data. The PFLTFILE reference flat in the pipeline is actually a combination of the pixel-to-pixel flats taken during the ground calibration and the low-order flat correction derived in-flight. The CFLTFILE will be applied only when the OBSTYPE is equal to CORONAGRAPHIC.

All flat field reference images will be chosen based on the detector, amplifier, and filters used for the observation. Any sub-array science image will use the same reference file as a full-size image. CALACS will extract the appropriate region from the reference file and apply it to the sub-array input image.

ACS HRC Coronagraphic Spot Table (SPOTTAB)

Position of the coronagraphic spot versus time.

CALACS uses this table to determine the position of the coronagraphic spot nearest in time to the observation date. It (CALACS) then shifts the spot flat by the required offset, multiplies the spot flat and pflat together and divides the data by the result.

ACS HRC Coronagraphic Spot Flat Files (CFLTFILE)

Vignetting patterns of the occulting spots.

ACS coronagraphic flat fields differ from the standard flats because of the presence of the occulting spots and alteration of the field vignetting by the Lyot stop. The large angular size of the aberrated PSF causes vignetting beyond one arcsecond of the spot edge, which can be corrected by dividing the image by the spot pattern. To facilitate this correction, separate flat fields have been derived that contain just the spot patterns (spot flats) and the remaining static features (P-flats). For a full discussion see ACS ISR 2004-16.

ACS Dark Reference Images (DARKFILE)

Reference images for dark current subtraction, including hot pixel correction and flagging.

A unique dark reference file is created (approximately) daily using four 1000 sec frames for both the WFC and HRC. The four darks are combined n = Ne(–τN) (with cosmic ray rejection) and bias-corrected to form a “daydark”. Every two weeks, a high signal-to-noise “basedark” is created from all the frames (~56) in that period. It does not contain the newest hot pixels present in the daydarks since they are rejected with the cosmic rays during image combination. A copy of the basedark is made for each day in the two-week period, and daily hot pixels are skimmed from the respective daydarks and added to the basedark copies. Only the hot pixels above 0.08 e-/sec are identified with flag 16 in the data quality (DQ) array of WFC and HRC reference darks, which propagates to the DQ array of the calibrated science data. The flagged hot pixels are typically excluded in subsequent processing steps, as they are likely too noisy to be correctable. But the many “warm” pixels below this threshold are assumed to be adequately corrected by the dark calibration. This produces a reference file with high signal-to-noise, which accurately reflects (and corrects/flags) the hot pixels present for a given observation date. The “best” dark file is typically not available in the pipeline until 2-3 weeks after the date of observation, because it takes a few weeks to collect enough frames to make a basedark (see ACS ISR 04-07 for more information).

The reference file for dark subtraction, DARKFILE, is selected based on the values of the keywords DETECTOR, CCDAMP, and CCDGAIN in the image header.


Figure: A typical WFC superdark. This image has been smoothed to enhance the subtle features (other than hot pixels). Click to enlarge image.

ACS Photometry Keyword Tables (IMPHTTAB)

Replaces calls to SYNPHOT for generating photometry keywords with pre-computed values.

These tables will replace calls to SYNPHOT in determining such photometric parameters as PHOTFLAM, PHOTBW, and PHOTPLAM with pre-computed values. The use of this table is triggered by the PHOTCORR keyword switch.

ACS Pixel-Based CTE Correction Table (PCTETAB)

Parameters used in generating the CTE corrections in WFC Data.

This table is used in CALACS to generate the pixel-based, time-dependent CTE corrections for full-frame ACS WFC data.

ACS CTE Corrected Superdarks (DRKCFILE)

De-trailed reference images used for dark current subtraction and hot pixel flagging and correction for use with the pixel-based CTE correction step within CALACS.

These darks are generated by applying the CTE correction to a standard dark by use of a Python script; the CTE correction removes the hot pixel trails seen in ACS standard dark reference files. The original superdark is mulitplied by the average exposure time of the raw dark files, and some of the header information is changed to prepare the file for the CTE correction code. CteCorr (the stand alone Python version of the CTE correcton) is run and the superdark is divided by the exposure time so that the final corrected superdark is in electrons per second. The header keywords are then replaced with the values from the original superdark file.

ACS Column Correction File (D2IMFILE)

File which describes the column/row correction as a 1-D correction.

The D2IMFILE reference file contains column (or row) width corrections for the ACS WFC chips. Like the NPOLFILE, it is an optional correction which is only added to the FITS and used in coordinate transformations if the filename is specified in the header. It contains a one-dimensional array of column width corrections. In the pipeline, this correction table gets attached to the image as a FITS extension of type ‘D2IMARR.’ Each element in the D2IMARR array specifies the correction (in units of pixels) for every pixel in the column of both science image extensions. It’s applied at the very start of the distortion correction process so that column width-corrected coordinates are used as input to the polynomial and non-polynomial distortion corrections.

ACS Non-polynomial Offset Files (NPOLFILE)

Files which, along with D2IMFILE, replace DGEOFILE with 65x33 table of non-polynomial corrections for X and Y for each chip.

The NPOLFILE contains tabular information about residual distortion corrections for some instrument modes. It's an optional correction and is only added to the FITS file and used in coordinate transformations if the filename is specified in the header. When used with CALACS, these files create image extensions (WCSDVARR) in the calibrated data which contain the non-polynomial distortion corrections. MultiDrizzle uses the DGEOFILE reference files for applying non-polynomial distortion corrections. There are two full-chip size image extensions in the DGEOFILE reference file for each instrument detector chip; one extension contains residuals of the polynomial fit in x, and the other contains residuals of the polynomial fit in y.

As outlined in Anderson, 2002, residuals of the polynomial fit were originally modeled by sampling every 64 pixels in the WFC, creating a 65 x 33 table for each chip. For the HRC, it was every 16 pixels sampled and modeled to create a 65 x 65 table. The value of a residual distortion at any point on the chip is determined by interpolation. These tables were used to create a full-chip distortion image, the DGEOFILE reference image, for each camera.

Because of the coordinate transformations, and the many steps involved in creating DGEOFILE files, it was not possible to start with the original tables. Therefore, the NPOLFILES were created for use by AstroDrizzle, done by subtracting the column-width corrections from the DGEOFILE file, then sampling each full chip size DGEOFILE file extension to re-create the look-up tables. Tests have shown a near 1-to-1 match with the original images.