Rogue Pixel Masks
A rogue pixel is a pixel with abnormally high dark current and/or photon responsivity (a "hot" pixel) that manifests as pattern noise in an IRS BCD image. We identify rogue pixels using the dark current measurements taken for calibration of the IRS arrays (observations of a very dark spot in the sky). The rogues are chosen by thresholding a high spatial frequency "component" of the dark current image. All pixels in the high frequency image with dark current greater than 4-sigma are labeled rogue.
A rogue mask is a fits file containing a 128 x 128 image with all detected rogue pixels set to 1, and all other pixels set to 0. Since rogue pixels are a time-dependent phenomenon, rogue masks are available for every IRS channel and campaign starting from IRS nominal operations campaign 1 (IRS1) and extending to the end of the Spitzer cold mission. Observers who need to correct for the effects of rogue pixels on their data should use a rogue mask developed as close in time to the date of observation as possible. Campaign-dependent rogue masks are available from the table below. Alternatively, all masks from all campaigns are available as a single tar file (TGZ, 363 KB). The mask file names are of the format b?_rmask_IRS?[?].fits, where the first ? is the channel number (SL=0, SH=1, LL=2, LH=3) and the second ?[?] is the campaign number.
The file campaigns.txt (Plain Text, 1 KB) campaigns.txt provides the translation between IRS campaign number (starting at 1) and the SSC campaign label given in the FITS header keyword CAMPAIGN in your data (starting at IRSX002500).
After downloading the appropriate masks, users should consult the Data Analysis and Tools section of the Spitzer Heritage Archive for advice on how to use these rogue files. In particular, see the IRSCLEAN_MASK software page and the Data Analysis Cookbook.
1 This campaign was split into a number of mini-campaigns. We derived a single rogue mask per channel using dark calibration data for the entire campaign, and then copied the masks into multiple files (one per mini-camaign) per channel. All sub-campaign rogue masks are identical, but are associated with a different campaign label. Campaign IRS21 was split into four mini-campaigns due to intervening short IRAC and MIPS campaigns for Target of Opportunity observations. IRS23 was split into two campaigns due to an intervening short MIPS campaign. IRS51 was split into two campaigns to observe a gamma ray burst afterglow with IRAC. IRS57 was split into two campaigns to execute an 8-hour mini campaign with IRAC. IRS58 was split into three campaigns to execute two mini campaigns with IRAC. IRS59 was interrupted by an error causing the electronics to reset. IRS61 was split into two parts due to an IRAC 1-day mini campaign.
2The number of Long-High rogue pixels suddenly dropped in campaign 25. The bias voltage on the Long High array was reduced from 2.0 to 1.6 volts in IRS25, reducing the dark current and photon sensitivity of the array. The main reason for this change was to reduce the strength and number of rogue pixels and improve the signal to noise in observations of faint sources. For many purposes, the data and rogue pixel masks before and after the bias change should be treated as coming from two different instruments.
3The number of Long-Low rogue pixels suddenly dropped in campaign 45. For the same reason the bias was changed in Long High, in IRS45 the bias voltage on the Long Low array was reduced from 1.8 to 1.6 volts and the array temperature was reduced from 4.4 to 4.1 K. As for LH, this reduced the dark current and photon sensitivity of the array. In the case of Long Low, the number of detected rogue pixels decreased to 1/3 of its previous value. This combination of effects increased the overall signal-to-noise ratio in the LL array by up to 60%, depending on wavelength.
4The campaign 25-27 Long High rogue masks changed on 08 February 2006. All IRS data are processed such that certain "permanently bad" pixels are automatically excluded from consideration and set to the value NaN (not a number). Prior to February 8, the Long High dark measurements for all campaigns were processed using an old "permanently bad" mask, or "pmask", derived at the old Long High bias setting. Thus, many pixels that were actually well-behaved at the new bias were excluded from consideration. Using a new pmask, we were able to open up many pixels that had previously been designated NaN. Unfortunately, we also opened up a set of pixels that are rogues at the new bias, about 200 of the excluded pixels.