COS Gratings
All of the dispersive elements in COS are holographically-ruled, ion-etched diffraction
gratings with excellent scattered light properties and reflectivities. The gratings
have Al+MgF2 coatings, except the G225M and G285M gratings, which have
bare Al coatings.
Further details can be obtained in sections 3.2.3,
5.11, and 13.3 of the COS Instrument Handbook.
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Grating
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Useful wavelength range (Å)1
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Bandpass per exposure (Å)
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Dispersion
(mÅ pixel-1)
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FUV Channel
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G130M
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1150 -1450
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2923
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NUV Channel
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1The useful wavelength range is the expected usable range realized in each grating mode.
Note that G140L is set so that Lyman-alpha falls in the gap between the two micro-channel plates to minimize the effects of geocoronal glow.
With G140L, one half records 1230-2050 Å. The other half records whatever spectrum is detected below 1100 Å,
but that is expected to be very little in most cases, hence the 820 Å nominal bandpass. The response of COS below
Lyman-alpha will be evaluated after launch.
2The lesser value of R
is realized for the low-wavelength end of the useful range, and R increases roughly linearly with wavelength.
3The inter-segment gap misses 14.3 Å.
4The inter-segment gap misses 18.1 Å.
5Some shorter wavelengths are recorded in second-order light.
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Wavelength Ranges for FUV Gratings
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Grating
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Nominal wavelength setting (Å)1
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Recorded wavelengths
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Segment B
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Segment A
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1The nominal wavelength setting has been chosen to be the shortest wavelength that is adjacent to the gap on segment A so that the indicated wavelength is an actually recorded one.
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Wavelength ranges for NUV gratings
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Grating
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Nominal wavelength setting (Å)
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Recorded wavelengths
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Stripe A
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Stripe B
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Stripe C
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1The wavelengths listed for central wavelength 2635 Å in stripe A are listed for completeness only and also in case a bright emission line falls onto the detector. Note that the NUV detector's sensitivity at these wavelengths is extremely low. To obtain a low-resolution spectrum at wavelengths below about 1700 Å we recommend G140L and the FUV channel.
2The values in shaded cells are wavelength ranges as seen in second-order light. In these cases the achieved dispersion is twice that for first-order mode.
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