ISPI Data Reduction

• WCSTOOLS. These are C libraries and programs by Douglas Mink at the SAO. These routines update the headers of ISPI images with FITS keywords describing the image coordinate system, its projection on the sky, etc. Install the WCSTOOLS software, and read the information about imwcs which is the program used to compute a coordinate solution and update the image headers.
  
• Catalogs. WCSTOOLS needs an astrometric reference catalog. These can be existing catalogs like 2MASS or user catalogs. The best way to proceed is to download and install one or more catalogs locally. At CTIO, we have so far experimented with 2MASS, since this is very useful for near infrared observations in the Galactic plane. Other catalogs will work well for regions where the optical extinction is not high. To install the 2MASS catalog, follow the instructions given by the WCSTOOLS pages. The disk space requirement is about 40 GB.
  
• A program to find image centroids in the ISPI images. This can be accomplished in IRAF (for example daofind). Other well known programs are DOPHOT, DAOPHOT, and SExtractor. Finding accurate centroids is necessary for certain options using WCSTOOLS.
• A program to resample or interpolate images. We have used SWARP, a program by Emmanuel  Bertin.

• The images should be preprocessed to have a near zero rotation from some known position. For ISPI, the usual N up E left projection is easily accomplished by choosing the "flipy" flag in the osiris.cl IRAF meta script. ISPI raw images have N down and East left when displayed in IRAF. An EPOCH keyword should also be added to the header which matches the value of the ISPI EQUINOX keyword (the default is 2000.0).
  

• The simplest task needed for raw ISPI images is to add appropriate world coordinate system (WCS) keywords. The program imwcs should be used to do this. There are a variety of options, but a simple call would be the following:

• The result of IMWCS is an image updated (if desired) with a WCS which is a standard tangent plane projection on the sky. In the above example, a new image t150sw.fits was created from t150s.fits. The pixel data are not changed, only the header.
  
• Once this step has been performed, use DS9 (SAO image) to display the updated image. The WCS coordinates should appear in the upper left of the image display. WCS specific IRAF scripts or any other program which is based on the FITS standard should also now be able to read and use the WCS information stored in the updated ISPI header.
  

• The above call to imwcs results in a new image header, but it is often desired to actually resample or interpolate the image to a new projection. This could just be to have the pixel to sky projection modified or to allow for correcting higher order distortions. The latter is the case for ISPI since these distortions will affect the combination of dithered images. ISPI exhibits distortion, so combining dithered images without correcting for this will result in a degraded PSF at positions away from the center of the image. The effect can be significant (of order several pixels change in FWHM.
• The key to creating a plate solution which accounts for the ISPI distortions is to have an image or images with many stars across the field of view and a set of accurate catalog positions.
  
• Once a matched catalog of pixel and sky coordinates is in hand, any program which fits the latter to the former and writes an appropriate solution can be used to further update the image header. In practice, our solution relies on the IRAF task "ccmap" because this task will do the fit and add the appropriate keywords to the header. This is done in the IRAF TNX system. The advantage is that the SWARP program used to resample the dithered ISPI images automatically handles the TNX system (which is not officially part of the FITS standard). TNX is a combination of the linear WCS solution plus higher order terms. The fit is a general polynomial in x and y with cross terms. Initial testing with ISPI suggests ccmap fits of 4th-6th order (in IRAF speak) with full cross terms do well.

 

In this image, the residuals to a linear fit are shown. The peak errors in position would be about three pixels if the distortion were not accounted for. In the next image, the residuals for a 4th order fit are shown. The RMS is less than 0.1'' which is about 1/3 of an ISPI pixel.

End to End Processing of ISPI Images

1. Make flats: Use med.cl to combine lights on and off images into a domeon and domeff image, respectively. Subtract the off image from the on.
   
2. Make bad pixel masks: Use maskbad.cl to create bad pixel masks.
3. Fix flats: Use fixfits.f to fix the flat images. The resulting images will have bad pixels interpolated over. If the images are named "dFilterNamef.fits", then the osiris.cl script will apply the right flat according to the FITS header filter name where FilterName is th evalue of this header parameter (e.g. KA for the ISPI Ks filter).
   
4. Run osiris.cl: The meta script which will preprocess images, divide by a flat, and add appropriate head key words.
5. Sky subtract the images: Use sky_sub.cl to make and subtract sky frames.
   
6. Add a WCS: Use do_wcs.cl to add a World Coordiante System to the images. (Preliminary script included in cirred package. Requires WCSTOOLS. Requires a catalog implementation for the WCSTOOLS.).
7. Compute high order distortions: Use do_ccmap.cl to compute the high order distortion coefficients and add them to the image header (Preliminary script included in cirred package. Requires WCSTOOLS. Need to incorporate TNX keywords manually as described above).
8. Shift/Combine images: Use SWARP (by Emmanuel Bertin) to reinterpolate the images and optionally shift and combine them.
9. Make a color image: SWARP can be used to shift and orient the final J, H, and K images so they can be combined in a false color image.