Important notes for users

The program needs bias-subtracted, flat-fielded, unskysubtracted images. Flatfielding is usually not critical, bias subtraction is! If the sky was subtracted from an image, the program will add the sky back in if skyval>0.

For long slit data the program offers the option of subtracting the sky and/or the objects before identifying cosmic rays. These fits are strongly recommended - in particular fitting the sky lines. The order of the sky line fit depends on the size of the image; typical values are yorder=2 for y < 50, yorder=3 for 50 < y < 80, and yorder=5 for y > 80. The sky subtraction is time consuming: if only part of the spectrum is of interest it will help to cut out the relevant image section.

The processing time is approximately proportional to the number of pixels; scale from 1 minute per iteration for an 800x800 image on an Ultra1 (200 MHz).

IRAF primer

For users with no access to IDL and no experience with IRAF (assumes image is in fits format):
- make sure iraf is installed and you have a directory with a file
- if not, type 'mkiraf' and/or ask system management
- edit the file: add/uncomment line "set imtype=fits"
- make sure this line does NOT read "set imtype=fits,noinherit" (is the case in some IRAF installations)
- start iraf by typing 'cl' in the directory where the resides
- type 'stsdas'
- if error message: ask system management to install the stsdas package
- type 'task lacos_im = path/' where path/ is the path to the script you downloaded
- edit the parameters by typing 'epar lacos_im'
- get out of the editing mode using CTRL-D
- start lacosmic by typing 'lacos_im input output outmask'