COS Dispersers

Choosing a Grating/Central Wavelength

The grating you choose is the main optical element that determines the spectral resolution of your observations. The tables linked below give the details of the various COS gratings, the corresponding wavelength coverage, spectral resolution, etc.

COS Dark current

The FUV dark rates reflect the fact that target acquisitions do not use any pulse height filtering. As a consequence, the background in target acquisition mode is slightly higher than that in spectroscopic mode.

The FUV dark rates appear to be correlated with solar activity. While the median dark rate shows a very slow increase with time, the average dark rate increases more steeply with time due to the increasing number of high dark rate frequency caused by the increasing solar activity. For the ETC, we took a conservative value that accounts for those events.


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 you can take 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. The default behavior for most COS observations is to take spectra at each FP-POS.

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. The default behavior for most COS observations is to take spectra at each FP-POS. Since each FP-POS has a slightly different wavelength range whereas the throughput calculated for the ETC assumes a fixed wavelength, calculations with the ETC do not account well for the edges of each segment. For G130M and G160M modes this is relevant only for 10-20 Angstroms on each end of a segment. For G140L this can impact 100-200 Angstroms from the edges of each segment.

The exposure time given by the ETC is the TOTAL exposure time needed. To implement this you would set your exposure time in APT to the ETC result, in the purple box, divided by 4 and set FP-POS=ALL.

Calculations with BOA at wavelengths shorter than 1200 Å

The transmission of the BOA aperture at the short wavelengths seen by the G130M 1055 and 1096 central wavelength and G140L 1105 and 1280 central wavelengths, while not fully characterized, is expected to be close to zero (due to MgF 2 cutoff for wavelengths shorter than 1200 Å). The values reported by the Spectroscopic COS ETC, for calculations using the BOA at wavelengths shorter than 1200 Å, are not accurate and should not be used for planning any COS observations.

Wavelength dependence on the Exposure Times and S/N calculated by the ETC for the G130M/1055 and G130M/1096 Settings due to changing resel size.

The resolution, and hence the resolution element (resel) size of the G130M/1055 and G130M/1096 settings is wavelength dependent. The ETC assumes resel sizes of 8 pixels for G130M/1055 and 9 pixels for G130M/1096, which corresponds to the region of highest resolution for each setting. For G130M/1055 this is roughly around 950 Å, and for G130M/1096 this corresponds to roughly around 1050 Å. ETC calculations away from these wavelengths will continue to use the fixed 8 or 9 resel width for the S/N calculation even though the actual resel becomes larger. If you instead wish to calculate the S/N for the appropriate resel size at each wavelength, results will differ by as much as a factor of five in exposure time and a factor of two in S/N. Exposure times scale as Resel_{nominal}/Resel_\lambda, while S/N scales as \sqrt{Resel_\lambda/Resel_{nominal}}. The table below gives detailed corrections for these two settings. The correction is multiplicative, that is t_{new}=t_{etc} \times correction or S/N_{new}=S/N_{etc} \times correction.

Exposure time corrections:

Cenwave Segment \lambda Resel Size Correction Factor
Angstroms Pixels
1055 B 900 10.6 0.75
950 8.3 0.97
1040 19.0 0.42
A 1050 21.5 0.37
1120 32.1 0.25
1190 42.6 0.19
1096 B 940 11.9 0.75
1000 8.4 1.07
1050 8.6 1.04
1080 11.9 0.76
A 1100 15.3 0.59
1170 23.9 0.38
1230 31.6 0.28

S/N corrections:

Cenwave Segment \lambda Resel Size Correction Factor
Angstroms Pixels
1055 B 900 10.6 1.15
950 8.3 1.02
1040 19.0 1.54
A 1050 21.5 1.64
1120 32.1 2.00
1190 42.6 2.31
1096 B 940 11.9 1.15
1000 8.4 0.97
1050 8.6 0.98
1080 11.9 1.15
A 1100 15.3 1.30
1170 23.9 1.63
1230 31.6 1.87