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Using Multiple FP-POS positions with COS observations

The use of multiple FP-POS positions for each CENWAVE position of the COS FUV detector was strongly encouraged in the Cycle 19 Call for Proposals, Primer, and COS Instrument Handbook, as this improves the limiting S/N and allows the effects of flat-field artifacts to be minimized. Where practical, observers should try to use all four FP-POS positions of a given CENWAVE, as the advantages of the resulting dithering in the dispersion direction will often outweigh any small reduction in exposure time required by the additional overheads. Even for very short exposures, observers should try to use at least two FP-POS settings for each CENWAVE.

Using multiple FP-POS positions is especially important for G130M observations as, over time, exposure to the bright geo-coronal Lyman alpha emission causes localized degradation of the COS FUV detector. Each FP-POS position of each CENWAVE setting projects Lyman-alpha onto a different part of the detector, and spreading out this wear will extend the useful lifetime of the COS FUV channel.

STScI will be monitoring the ongoing and planned usage of the different COS CENWAVE and FP-POS positions and may contact some observers after their phase 2 submissions to discuss whether changes in their planned usage in order to either improve the quality of their data or minimize the use of certain areas of the detector are compatible with their scientific goals.