x2d -- Perform 2-D rectification of calibrated STIS images.
x2d input output
- input [file name]
- Input calibrated image to be processed through x2d.
- output [file name]
- Name of output rectified image.
- (helcorr = "perform") [string, allowed values: perform | omit]
- Compute heliocentric correction factor.
- (fluxcorr = "perform") [string, allowed values: perform | omit]
- Convert output image to absolute flux units. The units for the output image will be ergs / (sec * cm**2 * Angstrom * arcsec**2). Note that these units are for extended targets, not point sources. See example 2 for a description of how to extract a 1-D spectrum if the target is a point source.
- (statflag = yes) [boolean]
- Compute statistics on the image and update keywords in the extension headers?
- (center = no) [boolean]
- For FUV G140L and G140M spectra,
the target has at different times been
offset to a location either above or below the middle of the detector,
to avoid the repeller wire.
Normally, the A2CENTER and CRPIX2 values in the SDC table
result in the middle line of the input image being copied to
the middle line of the 2-D rectified output image.
Setting center = yes will result in CRPIX2 being modified
so that the image will be shifted vertically
as it is copied into the output,
so that the target ends up in the middle of the output image.
This may be desirable because the target would then be at the same location
in the rectified image regardless of when the data were taken.
For an extended target or a wavecal, however, some of the data could be
shifted off the rectified image and be lost.
Regardless of how center is set, the location in the cross-dispersion direction of the target in the output, 2-D rectified image is given by the CRPIX2 keyword in the extension headers. The exception to this rule is that POSTARG is not taken into account, so if POSTARG2 is non-zero, the target will be offset from CRPIX2.
- (blazeshift = INDEF) [real]
- Blaze shift, in pixels. INDEF instructs the task to get the value from the photometry reference table.
- (err_alg = "default") [string, allowed values: default | wgt_err]
- This specifies which method should be used for
interpolating the error estimate array (the ERR extension).
default (weight the variance): sqrt (sum ( w[i] * err[i]^2 )) wgt_err (weight the error): sqrt (sum ( (w[i] * err[i])^2 ))
- (verbose = yes) [boolean]
- Print verbose time stamps.
The task x2d performs the 2-D rectification of calibrated observations (imaging or spectroscopic) taken with the CCD or MAMA detectors. For imaging data the task performs geometric correction. For spectrosocpic data, the task extracts a 2-D spectrum that is linear in both the wavelength and the spatial direction. Only the steps specified as "perform" in the input parameters are performed along with the 2-D rectification. If fluxcorr and helcorr are both set to "omit" then 2-D rectification will still be done, but fluxcorr and helcorr will be omitted; this would be appropriate for a wavecal, for example.
The input file may contain echelle data. In this case, the output file will have one image set (imset) for each spectral order in the input. The entire input image cannot be 2-D rectified at once because the dispersion relation is different for each spectral order, so each order is corrected and written separately. Currently there are two problems with x2d for echelle data, however. If the data were taken with a long slit (specifically the 6X0.2 aperture), the slit is tilted on the input image by an angle that can be from one to six degrees. This angle is much larger than the physical tilt as given in the aperture description table ("_apd.fits"), and it depends strongly on the particular echelle grating in use. The 2-D rectification only takes into account the angle as given in the aperture description table, which is the physical tilt, so the spectral lines in the output images will still be tilted. The second problem is that the 6X0.2 slit is so long that it overlaps several spectral orders, while the height of the output images is small enough that they cover only one spectral order per image. A workaround for the first problem is to edit (tedit) the aperture description table and change the value in the ANGLE column for the aperture in use to a value that agrees with the angle as seen in the input image. This is only a partial solution, however, because the angle varies by up to two degrees across the image. A workaround for the second problem is to edit (tedit again) the SDCTAB and change the values in the NPIX2 and CRPIX2 columns for the OPT_ELEM and CENWAVE that were used for the observation. More than one row may need to be changed, depending on which spectral orders (SPORDER) contain data for which a full slit width should be corrected. Set NPIX2 to a large enough value to cover as much of the slit as you need, and set CRPIX2 to (NPIX2 + 1) / 2 to center the spectrum.
INPUT AND OUTPUT IMAGES
The input files to this task should be fully calibrated files, i.e. flatfielded and cosmic-ray rejected if applicable. The pipeline files that would be suitable as input to x2d have suffix "_flt" for MAMA data or "_crj" for CCD data.
For accurate wavelength calibration, it is important that wavecal processing be done prior to running x2d. This is normally done in the pipeline, but it can be done independently by running the wavecal task. You can check that it was done successfully by examining the SHIFTA1 and SHIFTA2 keywords in the SCI extension header(s); if they are exactly zero then wavecal processing was not done (or was not successful) and you should run wavecal to update them.
REFERENCE FILES and PROCESSING STEPS
The names of the reference files (images and tables) to be used by x2d have to be specified in the primary header of the input file, under the section CALIBRATION REFRENCE FILES.
When an environment variable is used as part of a reference file name (e.g. "oref" in PFLTFILE = "oref$h230851ao_pfl.fits"), the variable must have been set (in Unix) before logging into IRAF, and the directory name must include the trailing "/". Setting an IRAF environment variable will not work, nor will using ! to escape from the cl. For example,
% setenv oref /data/reffiles/stdata/ # If the reference files are in the default directory, use: % setenv oref ./
The following files are needed to perform 2-D rectification:
- Aperture Description Table (APD)
- Contains the geometric description of the apertures and their offsets (in arcseconds) from a reference aperture. The name of the aperture description table must be provided in the keyword APDESTAB.
- Aperture Throughput Table (APT)
- Consists of wavelength dependent transmissions for each aperture with respect to a reference aperture. This table is used in conjunction with PHOTTAB to convert observed counts to absolute flux. The name of the aperture throughput table must be provided in the keyword APERTAB.
- Photometric Correction Table (PCT)
- This is used to correct the DIFF2PT keyword for the finite extraction height, when extracting a 1-D spectrum of a point source by summing over the spectrum in the cross-dispersion direction. The default extraction height for CCD data is 7 pixels, and for MAMA data it's 11 pixels. The PHOTTAB throughput tables are normalized for an "infinite" extraction height, and the PCTAB contains the correction to the throughput for various extraction heights.
- Spectrum Distortion Correction Table (SDC)
- Consists of a set of world coordinate information used to construct rectified, linearized 2-D spectra. One corrected image is produced per spectral order. The name of the spectrum distortion table must be provided in the keyword SDCTAB.
- One-D Spectrum Trace Table (1DT)
- Consists of displacements of spectra along Axis 2. This is used for correcting the distortion in the cross-dispersion direction. The name of the 1-D spectrum trace table must be provided in the keyword SPTRCTAB.
- Incidence Angle Correction Table (IAC)
- Contains coefficients of the change in dispersion coefficients as a function of angular offset from the reference position. These corrections are applied to the default dispersion coefficients. The name of the incidence angle table must be provided in the keyword INANGTAB.
- MAMA Offset Correction Table (MOC)
- Contains coefficients of the change in the dispersion coefficients as a function of commanded offset (MOFFSET1 & MOFFSET2) from the nominal position. These corrections are applied to the dispersion coefficients after applying the incidence angle correction. This is valid for MAMA observations only. Note: This is currently not applied; the zero point offset is accounted for by the SHIFTA1 and SHIFTA2 keywords.
x2d reads the aperture description table (APD), the aperture throughput table (APT), and the spectroscopic distortion table (SDC). When reading the SDC table, x2d reads into memory all rows that match the optical element (i.e. grating) and central wavelength of the input image (all spectral orders -- just one for long slit, but many for echelle). The range of spectral orders is used to set the limits on the loop over spectral order. A2CENTER is one of the columns to be read. In this table, A2CENTER is the line number on the detector corresponding to CRPIX2 of the output image.
The heliocentric correction factor is computed from the time of the observation and the direction toward the target.
Within the loop over spectral orders:
Extracts the information for the current order from the list read from the SDC table. The size for the output image and the coordinate parameters CRPIXi and CDELTi are then scaled depending on the binning of the input image, as specified by LTM1_1 and LTM2_2 for the input image.
Reads the dispersion coefficients table (DSP) to find the set of rows corresponding to the current spectral order (and optical element and central wavelength). For echelle, there may be just one matching row, but for first order there should be many rows, each for a different location A2CENTER on the detector in the cross-dispersion direction.
Reads the spectrum trace table (1DT) to find the set of rows with matching optical element, central wavelength, and spectral order. As with the DSP table, there can be many rows that match, each with a different A2CENTER.
Reads the incidence angle correction (IAC) table for the current grating, central wavelength, and spectral order to get terms used to correct the dispersion coefficients. For MAMA observations, read the similarly structured MAMA offset correction table.
Reads the wavecal offsets (SHIFTA1 and SHIFTA2) from the SCI extension header. These offsets are in reference pixel units, i.e. unbinned CCD pixels or low-res MAMA pixels.
Finally, reads the aperture description table to get the offset of the slit from the position reference slit, and also from the slit used to determine the dispersion relation.
1. To rectify a CCD observation in the data set with rootname "o3s4060do" that has been previously cosmic-ray rejected. Apply heliocentric correction and display the spectrum in flux units. The reference files are in the directory "/data/reffiles/stdata/". Note that we must have assigned oref as an environment variable in Unix before starting the cl, and the value must include the trailing "/".
cl> show oref /data/reffiles/stdata/ cl> lpar x2d input = "o3s4060do_crj.fits" input FITS file output = "o3s4060do_x2d.fits" output FITS file (helcorr = "perform") Convert to heliocentric wavelengths? (fluxcorr = "perform") Convert to absolute flux units? (statflag = yes) Compute statistics, update extension headers? (center = no) Center target in output? (verbose = yes) Print verbose time stamps? (Version = "5Apr2000") calstis version (mode = "al")
2. Suppose o3s4060do from the above example was an observation of a point source. The best way to extact a 1-D spectrum is to go back to "o3s4060do_crj.fits" and run x1d. However, for the purposes of an example, the following procedure can be used on the output from x2d.
Get the values of the CRPIX2 and DIFF2PT keywords; sum the image over 7 pixels (CCD) or 11 pixels (MAMA); and multiply the result by the value of the DIFF2PT keyword. In this example, CRPIX2 was 601, and DIFF2PT was 0.10308649. "oned.hhh" is the extracted 1-D spectrum.
cl> hedit o3s4060do_x2d.fits crpix2,diff2pt . cl> blkavg o3s4060do_x2d.fits[*,598:604] sum.hhh 1 7 option="sum" cl> imarith sum.hhh[*,1] * 0.10308649 oned.hhh
As described above in the DESCRIPTION section, when an echelle observation was taken with a long slit, the angle of the slit is not accounted for properly, and the height of the extracted image (for each spectral order) does not cover the full length of the slit.
P. Hodge, et al., STIS ISR 98-13. ICD47, R. Shaw, April 1997. Iraf task written by R. Katsanis. X2D routine written by P.Hodge.