Space Telescope Science Institute
COS Instrument Handbook
help@stsci.edu
Table of Contents Previous Next Index Print


Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 5: Spectroscopy with COS > 5.8 Fixed-Pattern Noise

5.8
The S/N of COS observations is improved through two techniques, flat fielding and coadding spectra taken at different central wavelengths or FP-POS settings. Flat fielding removes the high-frequency, pixel-to-pixel detector variations by dividing the data by a high S/N flat-field response image. FP-POS exposures smooth out the detector variations by combining in wavelength space data taken at different positions on the detector.
5.8.1 COS Flat Fielding
The internal flat-field calibration system consists of two deuterium lamps and the flat-field calibration aperture (FCA). The system was designed such that light from the lamps follows nearly the same optical path as that from an external target. The FCA is placed near the usual location of the PSA, and the lamp beam illuminates the gratings and mirrors from this slightly offset position.
The deuterium lamps are not bright enough to map out the flat field at FUV wavelengths, so the FUV flats are constructed from on-orbit observations of bright white dwarfs. An image of the FUV detector using the deuterium lamp is shown in Figure 5.13. The light, vertical stripe is a shadow cast by a grid wire in front of the detector (Section 4.1.1). A detector dead spot and the hexagonal pattern of the fiber bundles in the micro-channel plate are also visible. Although significant structure is present in the FUV flats, it is reproducible and can be removed during data reduction.
Figure 5.13: Section of a Flat-Field Image for the FUV XDL
A section of the FUV flat field showing representative detector features and a grid wire (the light vertical stripe at 4720 ).
Grid-wire shadows are the greatest source of fixed-pattern noise. In the past calcos flagged these regions and eliminated their contributions to the final, summed spectra. During Cycle 18 a grid-wire flat-field calibration file was developed for the G130M and G160M gratings, and it was updated in Cycle 21 for all FUV gratings. Pixels affected by grid wires are still flagged by calcos, but their corrected values are included in the summed spectra. Figure 5.14 shows the effect of correcting the grid-wire shadows on a single G130M FUVB exposure of the white dwarf WD0320-539 obtained at the original lifetime position. This star has a relatively smooth continuum, making the corrections obvious. The upper (blue) spectrum contains grid-wire shadows (indicated by the vertical lines), which are corrected in the lower (green) spectrum. The affected regions are clearly improved, but residual structure, much of it fixed-pattern noise in the FUV detector, remains. This structure can be reduced through the use of multiple FP-POS settings (Section 5.8.2).
Figure 5.14: Correcting Grid-Wire Shadows with Flat-Field Reference File
G130M FUVB exposure of the white dwarf WD0320-539 obtained at the original lifetime position. The upper (blue) spectrum contains grid-wire shadows (indicated by the vertical lines), which are corrected in the lower (green) spectrum.
Table 5.6 gives the limiting S/N for the G130M and G160M gratings when the grid-wire flat field is used, both with and without multiple FP-POS settings. To attain higher S/N ratios special analysis procedures, such as those described in the January 2011 COS STAN, are required.
Table 5.6: Limiting S/N for FUV Spectroscopy
Single FP-POS Exposure
Combining 4 FP-POS Exposures
Because the grid wires are oriented perpendicular to the spectrum, their effect on the data is relatively insensitive to the location of the spectrum in the cross-dispersion direction. Much of the remaining fixed-pattern noise depends strongly on the spectrum location, and will require considerably more effort to characterize and correct. The grid-wire flats have been shown to be independent of the y position on the detector, so they can be applied at both lifetime positions.
In addition to grid-wire shadows “grid-wire impostors” were recently discovered in COS FUV Data. The morphology of the impostor in the extracted spectra is similar overall to that of the grid-wire shadows. However, unlike the grid-wire shadows the impostor features depend strongly on the y position on the detector. From the appearance of these features in the dark exposures it had been determined that the impostors are artifacts introduced by the geometric distortion correction. However, due to the way that the geometric distortion corrections are integrated into nearly every other aspect of calibration changing this reference file cannot be done quickly. Therefore, a 2D correction for the “impostors” has been incorporated into the FLATFIELD reference file until a fix to the geometric distortion correction can be fully investigated and tested. As a result calcos now corrects these features through the flat field correction.
Comparison of extracted spectra at difference FP-POS and CENWAVE settings has revealed y-independent illumination variations on each detector segment. These variations are now being corrected in calcos by applying a low-order flat-field correction (L-flat) that has been incorporated into the FLATFIELD reference file. The L-flat is shown in Figure 5.15 and an example of the improvement obtained using the L-flat is shown in Figure 5.16.
Figure 5.15: L-Flat Amplitudes for the COS FUV
The amplitudes of the L-flats for the COS FUV A and B Segments.
Figure 5.16: A Comparison of COS Spectra with and without the L-Flat
Correction.
The top panel shows two COS spectra without the L-flat correction. The bottom panel shows the same two spectra with the L-flat correction.
The NUV flat field used by calcos was built from a combination of external PSA deuterium lamp exposures taken on the ground and internal FCA observations taken on the ground and on orbit. Figure 5.17 presents a comparison between two NUV flat-field frames, one obtained on orbit and one on the ground. Each image was divided by a low-order polynomial to isolate the high-order fringe pattern characteristic of the NUV detector. Their ratio is consistent with the noise in the on-orbit image, confirming that the two flat fields may be safely combined. Pre-flight ground tests with COS show that the NUV MAMA can deliver S/N up to about 50 without using a flat field. Using a flat field it should be possible to routinely achieve S/N of 100 or more per resolution element.
Figure 5.17: Flat-Field Exposures for the NUV MAMA
5.8.2 Use of Multiple FP-POS Settings
Fixed-pattern noise in the COS detectors limits the S/N that can be achieved in a single exposure to 1525 per resolution element for the FUV and 50 for the NUV. To achieve higher S/N ratios one can obtain a series of exposures, each slightly offset in the dispersion direction, causing spectral features to fall on a different part of the detector. For STIS and GHRS these motions are known as FP-SPLITs. For COS these motions are specified by the FP-POS optional parameter.
Four FP-POS offset positions are available: a nominal position (0), two positions toward longer wavelengths (2 and 1), and one position toward shorter wavelengths (+1). Positions 2, 1, 0, and +1 are designated respectively as FP-POS=1, 2, 3, and 4. The nominal position, FP-POS=3, is the setting used to define the wavelength range associated with the grating central wavelengths (Table 5.3 and Table 5.4). In pipeline processing calcos creates individual calibrated spectra for each FP-POS position, then aligns and combines them into a merged spectral product, using only good-quality data at each wavelength.
The optical mechanism on which the grating is mounted is rotated by one step for each adjacent FP-POS position. The amount that a particular spectral feature moves in the dispersion direction on the detector is approximately 250 pixels per step for the FUV channel and 52 pixels for the NUV. The corresponding wavelength shifts for each grating are given in Chapter 13. There is a preferred direction for moving the grating mechanism. Overheads are reduced if FP-POS exposures are obtained in increasing order (see Section 9.5). When moving to a new grating or central-wavelength setting you may select any FP-POS position without paying an additional overhead penalty. Thus, the most efficient order is FP-POS=1, 2, 3, 4, as it requires no backward motion of the grating mechanism.
A wavelength calibration exposure will be obtained each time the FP-POS changes. For FLASH=YES exposures the time-since-last-grating-motion 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.
Requirements for Use of Multiple FP-POS Settings
The use of multiple FP-POS positions for each CENWAVE setting of the COS FUV detector is required unless a strong scientific justification to do otherwise is provided in Phase I. Using multiple FP-POS positions improves the limiting S/N and minimizes the effects of flat-field artifacts. The use of multiple FP-POS positions is especially important for G130M observations as, over time, exposure to the bright geocoronal Lyman-α emission causes localized degradation of the COS FUV detector. Each FP-POS position of each CENWAVE setting projects geocoronal Lyman-α light onto a different part of the detector. Spreading out this wear will extend the useful lifetime of the COS FUV channel. For the G140L/1280 setting the Lyman α light falls in the gap between Segments A and B, so the use of FP-POS=ALL is not essential. However, even for this mode FP-POS=ALL is still needed for flat-fields and to spread light over the detector. Proposers using the FUV channel of COS, but who do not intend to use all four FP-POS settings for each CENWAVE setting, must justify their observing strategy in their Phase I proposals. A modest reduction in observational overheads will not normally be considered sufficient justification for not using all four FP-POS settings.

Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 5: Spectroscopy with COS > 5.8 Fixed-Pattern Noise

Table of Contents Previous Next Index Print