Spectra recorded with the cross-dispersed grisms in CASPIR have curved orders and the tilts of the slit images vary along each order. These distortions must be corrected in order to accurately align the dispersion direction with the image columns and the spatial direction with image lines. Files containing fits to these geometrical distortions for each grism are distributed along with the IRAF CASPIR package. However, it is advisable to define new transformations for each observing run.
Curvature in the spectral direction can be traced using a sky-subtracted image of a stellar spectrum. A single object spectrum near slit center is sufficient for cross-dispersed data where geometrical distortions along the short slits are minimal. Slit tilt is traced using spectral images of arc lamps. These also serve to establish the wavelength calibration.
First form a stellar curvature reference frame by subtracting suitable linearized star and sky images and xenon and/or argon arc lamp frames by subtracting lamp on and lamp off pairs, for example using cspflat:
cspflat ir011,ir014 ir012,ir013 star_jh cspflat ir005,ir006 ir007,ir008 xenon_jh cspflat ir001,ir002 ir003,ir004 argon_jh
Note that the geometrical transformation should be defined using frames that have not been flatfielded.
The geometrical transformation for a particular grism is defined using the cspxtrans task which has the parameters listed below.
I R A F
Image Reduction and Analysis Facility
PACKAGE = caspir
TASK = cspxtrans
curve = star_jh Curvature reference frame to use
xenon = xenon_jh Xenon lamp frame to use
argon = argon_jh Argon lamp frame to use
(verbose= yes) Verbose output?
continue= Process this order?
profile = Enter the profile section to use
(mode = ql)
The curve parameter defines the curvature reference frame name, the xenon parameter defines the xenon arc lamp filename, and the argon parameter defines the argon arc lamp filename. At least one arc lamp frame must be specified. Line lists for the xenon and argon arc lamps are distributed with the IRAF CASPIR package in the files xenon.dat and argon.dat.
The cspxtrans task treates each order of cross-dispersed spectral data separately by first extracting a subsection around the location of each order and then analysing that subsection. The curvature reference sub-image is displayed first, and an average profile through the first, middle, or last ten lines of the image, as specified by the profile parameter, is displayed in the graphics display using the noao.twodspec.identify task. Mark the position of the star by using the cursor and typing `m'. Enter the appropriate column number (1, 18, 26, 34, 51 for 0, 25, 50, 75, 100% positions across the order), and type `q' to exit this section. cspxtrans then uses noao.twodspec.reidentify to trace the spectrum. Answer `yes' to the question Write coordinate map to the database (yes)?. The cspxtrans task then displays the xenon arc lamp spectum sub-image (if one was specified) and uses noao.twodspec.identify to display an average profile through the central three columns (along the dispersion). Referring to the arc lamp spectra in Figs. 41-55 of Appendix J, type `m' to mark the locations of a few emission lines and enter their wavelengths in Angstroms. Then type `f' to obtain a preliminary fit to the wavelength calibration. Type `q' to exit the fit routine, and then `l' to automatically locate other arc lines. Delete erroneous identifications by typing `d'. Use the window commands `w t' and `w b' to change the top and bottom plot values, and `w a' to autoscale the plot. Fit the wavelength calibration again, and type `q' to exit the fit routine and another `q' to exit the identify routine. cspxtrans then uses noao.twodspec.reidentify to trace the slit images. The cspxtrans task then repeats the identification process for the argon arc lamp sub-image (if one was specified). Answer `yes' to Fit interactively (yes)?, and remove erroneous data points and points off the ends of the slit using the `d' key; typing `d' followed by a `p' deletes a single point, typing `d' followed by an `x' or a `y' deletes all points at that constant x or y. The fit is best displayed by looking at the residuals as a function of x position; type `x' followed by `x', `y' followed by `r', and then `r' to redisplay the plot. Redo the fit by typing `f'. Display the fit itself by typing `x' `x', `y' `y', and then `r' to redisplay the plot. When a satisfactory fit has been obtained, exit the fit routine by typing `q'. Then answer `yes' to Write coordinate map to the database (yes)?. This procedure is repeated for each order of the cross-dispersed grism.
The fit parameters are stored in a local subdirectory named database/ of the current directory, and in the database directory pointed to by the IRAF database$ environment variable. The x and y fits for a particular grism are stored in files with names like fcJH_grism_10x and fcJH_grism_10y in those directories.