Hubble Space Telescope
COS Gratings

The COS FUV gratings are concave and have holographically-generated grooves to provide dispersion and correct for astigmatism. The gratings have aspherical surfaces to correct for HST’s spherical aberration. The FUV “M” gratings have been ion etched to produce triangular groove profiles for better efficiency. The G140L grating has grooves with a laminar profile. All FUV gratings are coated with MgF₂ over aluminum.

The NUV gratings are flat and were not constructed holographically. The NUV MAMA has low but measurable sensitivity at FUV wavelengths, and with some gratings second-order light could contaminate the spectrum. To minimize this effect, the optics are optimized for wavelengths above 1600 Å. Even so, it is possible for second-order light to appear in G230L spectra, especially in the long-wavelength stripe.

Further details can be obtained in Chapter 5 of the COS Instrument Handbook.

COS Grating Parameters

1. Normal wavelength ranges are for the primary (default) central wavelength setting.
2. Modes covering wavelengths below 1130 Å are expected to have a much reduced resolving power of about 2000.
3. Second order contamination possible longward of 2150 Å.

Wavelength Ranges for FUV Gratings

1. These are new modes that are currently being characterized, so these numbers are preliminary. 1055 and 1096 central wavelength settings have been offered starting from Cycle 19 and 1222 central wavelength mode starting from Cycle 20.
2. It is not yet clear how much of the G140L segment B short wavelength (< 900 Å) ranges will be available due to uncertainties in the HST OTA throughput. This is currently being investigated.
3. The G140L grating and 1105 central wavelength setting moves the zero-order image onto segment B. Therefore, only segment A is available for this setting.
4. Beginning in Cycle 18, the G140L 1230 setting was replaced by the 1280 setting.

Wavelength Ranges for NUV Gratings

1. The wavelengths listed for central wavelength 2635 Å in stripe A are 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.
2. The 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. Note, however, that some first order light may contaminate the spectrum depending on the SED of the target.
3. The spectrum longward of 3400 Å may be contaminated by second order spectrum, depending on the SED of the target.