Spitzer Documentation & Tools

Purpose and Description

The IDL procedures coad.pro and doall_coads.pro both combine Spitzer IRS BCDs. Each procedure requires the user to supply a list of input BCD images to be combined. The difference between the two is that coad.pro combines all of the input images to produce a single coadded image, while doall_coads.pro makes one coadded image for each subset of the input list that shares the same module, aorkey, and expid (see the IRS Instrument Handbook for definitions). Each coadded image produced by either of these procedures is accompanied by an uncertainty image and a mask image.

The output combined image is computed as a trimmed mean of the input BCD images. NaN values and fatal bits (8, 12, 13, 14 by default) are not included in the mean. The IDL procedure "meanclip.pro" (found in the IDL Astronomy User's Library) is used with its default values.

The output uncertainty image is computed as the square root of the sum of the squares of the pixel values in the uncertainty files associated with the input BCDs, normalized by the number of valid BCD pixels that contribute to a coad pixel. NaN values and fatal bits (8, 12, 13, 14 by default) are not included. The procedure finds the appropriate uncertainty files by assuming that they are in the same directory as the input BCD files.

The output coad mask file keeps track of only two conditions: whether a coadded pixel was derived from input pixels with uncertain flatfields (bit 7) and whether a coadded pixel value could not be derived due to no invalid input pixels (bit 13). Both of these are considered fatal by default in SPICE.

The names of the output images are determined automatically, using the same root as the first input BCD image, but adding 'coa2d.fits', 'c2unc.fits', or 'c2msk.fits' to the end of this root.

The header information in the output combined images will be identical to the input images of the same type. All are appropriate for use with SPICE.

Downloading and Installing the software

Follow these instructions for downloading and installing the software.


All updates to coad will be announced on this page. Please check back periodically.

Getting Help

Support for coad is available from the Helpdesk. Before contacting us, please check the IRS Instrument Handbook for information.

If you contact the Helpdesk, please be sure to include the version of coad you are running, the operating system on which you are running it, the AORKEY you are working with, and as much detail about the problem as possible.


See Downloading coad for information on running the software.

IRS Instrument Handbook

Frequently Asked Questions