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.
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. A preliminary FUV flat is shown in
Figure 5.12. The dark, 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.
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. Pixels affected by grid wires are still flagged by
calcos, but their corrected values are included in the summed spectra. For the G140L grating, for which no flat field is yet available, the regions affected by grid-wire shadows are masked and rejected by
calcos when it combines data obtained at different
FP-POS settings.
Figure 5.13 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).
Table 5.3 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.
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. Spectra taken at the new lifetime position (lifetime position 2) are calibrated with the grid-wire flat derived from data obtained at the original lifetime position. While we expect that the grid-wire flats will be similar, new observations are needed to determine if this is indeed the case. In the meantime observers are advised to check the
COS Web pages for the latest developments.
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 5.14 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, S/N of 100 or more per resolution element should be routinely achievable.
Fixed-pattern noise in the COS detectors limits the S/N that can be achieved in a single exposure to 15
−25 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 5.4 and
Table 5.5). 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.
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 and G140L/1280 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 Lyman
α onto a different part of the detector, and spreading out this wear will extend the useful lifetime of the COS FUV channel. 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.
