program names, file names
variable names
prompts, commands, program code

This section is currently appropriate only for data taken at the Mount Lemmon Observing Facility (MLOF). The optics used in the University of Rochester 3^rd Generation Infrared Camera when observing at MLOF introduce optical distortion. This sounds a bit worse than it really is, as there is an algorithm for removing this distortion. The program is called dist_astro:

IDLprompt> dist_astro, impackname

impackname is an impack which has been reduced up to and including background normalization. After runtime, both impackname image and weight planes will have been corrected for optical distortion, and the header will have been annotated to reflect this change---the distortion correction parameters are written to the header.

If you are dealing with a series of impacks in a single array, rather than with individiual impacks, use the following form:

IDLprompt> dist_astro, impackarrname, arr=IND

where IND is the array index of the image package you wish to alter.

This algorithm performs both the astrometric and the photometric corrections. This program runs rather slowly, as it must completely remap the image to accomplish the astrometric correction. I like to distortion correct several images at once using a batch file (or a for loop, using the arr keyword)---I go eat lunch or something while it happens.

The completion of this step in the data reduction process represents a very good time to save the work you've done up to this point.

Upon looking at the result, you'll likely notice three things, all of which are results of the image being remapped pixel-by-pixel: (1) some of the pixels along the image's border (either in the corners or along the edges away from the corners) will have values of zero, (2) much of the image will look somewhat blurred, and (3) concentric rings may be apparent around some point in the new image. This third item is explained by considering that in remapping the image, dist_astro moves pixels in toward, or away from, the optical center of the image. Some pixels are moved by fractional pixel amounts, which inherently blurs them. Other pixels are moved by nearly integer pixel amounts, which introduces less blurring. So, much like a diffraction pattern, there will be rings of increasing and decreasing blurring centered on the optical center.