Purpose: corrects column pulldown in IRAC Warm Mission BCDs. The code computes the pulldown triggering threshold internally and applies this threshold to identify the affected columns within a given BCD. It can be used on individual BCDs as well as on large ensembles of images. A version of this code will be used in future processings of IRAC Warm Mission data by the online pipeline.

Author: Roberta Paladini (SSC) and Sean Carey (SSC)
Date Contributed: 14 February 2011
System Requirements: IDL v7 or higher; IDL astrolib library

irac_warm_correct_pulldown was written by R. Paladini (SSC) and Sean Carey (SSC) and is provided "as is" for testing and usage by other observers. This software is not considered part of the pipeline and no installation support from the SSC should be expected. We are however very interested in your feedback in order to improve or replace the algorithm and are glad to work with you in test cases and analysis. Please send your comments to paladini{@}ipac.caltech.edu (remove the {} from the email address first).

Information and Download

Download irac_warm_correct_pulldown.tar (Tar File, 170 KB)

A depression of the intensity of the pixels along a column in the detector array, caused by a high pixel value (usually a star or a cosmic ray), is called "column pulldown". Unlike during the cryogenic Spitzer mission, the pulldown is not constant along the column, but is instead better represented by an exponential function. Pixels below and above the location of the triggering pixel are affected in a different way, and need to be corrected independently.

The code performs an automatic flagging of the columns affected by pulldown. This is done in three steps: 1. for each column, the code calculates the median by applying an iterative 3-sigma clipping; 2. the code calculates the overall median of the columns by applying a new iterative 3-sigma clipping; 3. the code looks for outliers with respect to this global median by applying sigma-thresholds which have been empirically derived, and which depend on integration times and background levels. Once the affected columns have been identified, the parts of the affected column above and below the triggering pixel are fitted independently with an exponential function. The corrections are then added back into the original BCD.

The code has been tested successfully on a variety of Warm Mission data, with integration times ranging from 1.2s to 100s, and background levels from < 0.001 MJy/sr to > 0.1 MJy/sr. The "Warm-Mission Column Pulldown Corrector" by M. Ashby, also available as contributed software on this website, works best in the absence of, or with only little, diffuse emission. The "Warm Column Pulldown Corrector" (this code) appears to perform well regardless of the amount of extended diffuse emission in the images but the other code can give better results for some data with little structure in the background. We recommend that observers use the code that works the best for their particular data.

To compile and run the code, simply type:

IDL > .r irac_warm_correct_pulldown
IDL > irac_warm_correct_pulldown, 'bcd.fits', 'bunc.fits',
'bimsk.fits', 'outbcd.fits', 'outbunc.fits', 'outbimsk.fits',
/pthres

where 'bcd.fits', 'bunc.fits', and 'bimsk.fits' are the input BCD image and its corresponding uncertainty and imask files, while 'outbcd.fits', 'outbunc.fits', and 'outbimsk.fits' are the corresponding output files. If the output files are not provided, the routine overwrites the input BCD, BUNC and BIMSK files.

To apply the code to an ensemble of BCDs, first create, for each type of image (BCD, BUNC, BIMSK), an ASCII file. Then run the code using, as inputs, these lists. The example routine irac_warm_correct_pulldown_bcdlist.pro is included in the .tar file provided above.