Cosmic Origins Spectrograph Far-Ultraviolet Flat Fields
Abstract
The COS FUV detector is composed of two micro-channel plate (MCP) segments with cross delay line anodes. The detector displays a variety of localized, non-uniformities in its response, including hexagonal and moire patterns, dead spots, gain variations, and shadows from the wire grid used to increase MCPs quantum efficiency. These features produce fixed-pattern noise in COS FUV spectra. Typically, such irregularities are eliminated by dividing the raw data by a flat field image. Ground testing of the prelaunch flats revealed that the S/N did not improve and consequently an attempt was made during SMOV to construct a flat field image from external targets. However, it did not result in a flat field of sufficient quality to be used for standard CALCOS processing. In contrast, it has been demonstrated that the fixed pattern features can be largely eliminated in high S/N data by using the iterative FP-SPLIT algorithm (similar to the one used for GHRS and FOS data) to obtain COS spectra with a S/N > 100. A product of this approach is a "1-D flat" which characterizes the contribution of fixed pattern noise to the spectrum. We have begun to explore various methods for utilizing these 1-D flats to correct unrelated, individual exposures. Initial results have been encouraging, and the results the our study will be discussed.
- Publication:
-
American Astronomical Society Meeting Abstracts #217
- Pub Date:
- January 2011
- Bibcode:
- 2011AAS...21725403W