Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.8 Achieving Higher Signal-to-noise using FP-POS

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5.8.1 Use of Optional Parameter FP-POS

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 full automatic wavelength dithering pattern uses four FP-POS positions: a nominal position ("0"), 2 positions toward shorter wavelengths (-2 and -1), and 1 position toward longer wavelengths (+1). The ordering of the four when FP-POS=AUTO is used is -2, -1, 0, and +1; i.e., in order of increasing wavelength. These four positions are designated respectively as FP-POS=1, FP-POS=2, FP-POS=3, or FP-POS=4 if a specific setting is desired. Note that FP-POS=3 is the default if no specific value is chosen.

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 by calcos in pipeline processing. Wavelength calibration spectra will automatically be obtained for each FP-POS position.

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 1, 2, 3, 4. FP-POS=4, for example, takes a single spectrum at position number 4 for the specified exposure time. 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. Note that there is a preferred direction to move the grating mechanism and so overheads are reduced in some FP-POS scenarios compared to others (see Section 9.3). We ordinarily recommend use of FP-POS=AUTO. 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, but that FP-POS=AUTO by itself at a single wavelength setting is not sufficient to cover the gap.

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.

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. Note that an explicit specification of exposure sequences FP-POS=1, FP-POS=2, FP-POS=3, and FP-POS=4 is marginally more efficient (by a few seconds) than using FP-POS=AUTO, but the explicit specification allows for greater flexibility in using your orbits in Phase II.

5.8.2 FUV Signal-to-noise

The FUV XDL detector has been shown to achieve S/N up to about 18 based just on photon statistics, without use of a flat field. By using FP-POS as well it is possible to reach S/N = 35. By also using a flat-field exposure, S/N up to 62 has been demonstrated during pre-flight ground tests. The best achievable S/N will be determined once COS is on orbit and it is possible to use astrophysical objects as flat-field sources. It is believed that S/N up to 100 is achievable in the FUV channel.

An example of an FUV flat-field exposure is shown in Figure 5.5. The regularly-spaced features are artifacts of the shadows of the wire grid over the detector. Although significant structure is present in this exposure, it is reproducible and therefore can be calibrated.

5.8.3 NUV Signal-to-noise

The NUV MAMA in COS is expected to behave very much like its STIS cousin, and observers may wish to consult STIS documents to see how its MAMA has performed in orbit. Pre-flight ground tests with COS show that the NUV MAMA can deliver S/N up to about 50 without using a flat field, just based on photon statistics. By using flat-field exposures, S/N up to 100 or more per resolution element should be routinely achievable. An example of a NUV flat-field exposure is shown in Figure 5.6.

Figure 5.5: Example of a flat-field exposure for the FUV XDL.


 
The regularly-spaced features are due to grid wires in front of the detector that cast shadows.
 
Figure 5.6: Example of a flat-field exposure for the NUV MAMA.