Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.7 Achieving Higher Signal-to-noise

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Special "central-wavelength dithers" (for STIS and GHRS known as FP-SPLITs) may be used to enhance signal-to-noise in spectroscopic data or to correct for fixed pattern detector features through a sequence of exposures taken at slight offsets in the dispersion direction. For COS, these motions are specified by the FP-POS Optional Parameter.

The default is to use 4 FP-POS positions: a nominal position, 1 position toward shorter wavelengths, and 2 positions toward longer wavelengths. The nominal position is executed first, then the longest-wavelength position, the remaining longer-wavelength position, and finally the shortest-wavelength position.

The number of steps to rotate the optical mechanisms is one for each adjacent FP-POS position. The amount that a particular wavelength moves in the dispersion direction on the detector due to one rotation step of the appropriate mechanism is 240 pixels for the FUV channel and 49 pixels for the NUV. The subsequent spectra will be aligned and co-added in pipeline processing. Wavelength calibration spectra will automatically be obtained for each FP-POS position.

FP-POS (=AUTO, 1, 2, 3(default), or 4) indicates at which position or positions to take separate exposures. Note that FP-POS indicates the relative position of an exposure, not the number of separate exposures. The one exception is FP-POS=AUTO, which takes four exposures in the order of 3, 1, 2, 4. FP-POS=4, for example, takes a single spectrum at position number 4. Obtaining exposures at small wavelength offsets from the specified central wavelength aids in the correction of the fixed-pattern defects of the detector. FP-POS=AUTO indicates that the specified exposure time will be divided evenly among four sub-exposures, and each sub-exposure will be obtained at a different predetermined offset from the specified central wavelength.

The values FP-POS=1, 2, 3, 4 or not specified on the proposal will result in the exposure being taken at an offset from the specified central wavelength determined by the value. The default value (FP-POS=3), or if FP-POS is not specified on the exposure, will result in the exposure being obtained at the nominal central wavelength (i.e., at a zero) offset and the exposure will be for the specified exposure duration. Note that utilization of FP-POS=AUTO at two consecutive central wavelength settings allows complete filling of the FUV detector gap.

To summarize, users may specify the full range of FP-POS sampling by using AUTO, or may design wavelength-dither pattern sequences of their choosing.

Wavelength calibrations will be obtained each time the FP-POS changes. For FLASH=YES exposures, the time-since-grating-move clock is not reset by an FP-POS movement, however there will always be at least one lamp flash during each individual FP-POS exposure. For FLASH=NO exposures, a separate wavelength calibration exposure will be taken for each FP-POS position change. Note for internal targets: FP-POS is not allowed for internal targets except Target=WAVE. Allowed values for exposures with Target=WAVE are FP-POS=1, 2, 3 (or not specified), or 4; FP-POS=AUTO is not allowed.