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Hubble Space Telescope
Gain Sagged Regions on the COS FUV Detector

As described in the November 2010 STAN, the gain sag on the COS FUV detector can lead to artifacts in the extracted spectra that look like absorption features. For TIME-TAG data, the lower pulse height threshold used by CALCOS has been changed from 4 to 2 in order to minimize this effect, but artificial absorption lines may appear in the extracted spectra until a more permanent solution is implemented. Until then we recommend that users carefully inspect their data from the most heavily sagged regions on the detector. The most seriously affected locations, starting with the worst one, are listed in the table below. The gain sag in a given detector area increases with the total exposure that area has seen. Artifacts in extracted spectra should be minimal for observations taken prior to August 2010 as long as the data have been calibrated using the lower pulse height threshold of 2.

Segment X Pixel Range
B 9049 - 9119
B 7121 - 7191
B 8087 - 8157
B 5231 - 5301

We are in the process of creating a list in wavelength coordinates, and we will post it on the COS web site when it is available.