2.4 COS Observations Below 1150 Angstroms: Resolution and Wavelength Calibration Issues

Starting in Cycle 21 the spectral resolutions of the FUV G130M 1055 and 1096 central wavelengths settings were substantially increased above the values offered during earlier cycles and, when combined with the 1222 setting first offered in Cycle 20, this now allows resolutions of 7300 to 11,000 to be obtained at any wavelength between 900 and 1150 Å (see Figure 5.5). Users should note, however, that for each of these modes the focus values have been set to optimize the resolution over a limited part of their wavelength range, and it will be necessary to use multiple settings to get the maximum resolution over this full range. At longer wavelengths the resolutions offered by any of these settings will be inferior to those available with the original complement of G130M central wavelength settings (1291, 1300, 1309, 1318, and 1327).

Users should also note that targets that are too bright to observe at longer wavelengths with the COS G130M grating may be observable on Segment B with the 1055 and 1096 settings by turning off Segment A, which covers longer wavelengths. However, in this case there is no usable TAGFLASH wavelength calibration lamp spectrum recorded because there are no visible wavelength calibration lines that fall on Segment B. As a result, the spectrum observed on Segment B cannot be corrected for mechanism drift or zero-point offsets. In such cases, for each CENWAVE and FP-POS position, a separate GO-wavecal exposure should be taken with Segment A “on” immediately adjacent to the Segment B science observations. This Segment A wavecal will allow the zero-point offset of the wavelength scale of Segment B to be corrected to within 10 pixels. Several minutes are required to reconfigure the COS FUV detector each time the segments are powered on or off. However, with careful planning much of this overhead can often be hidden in the occultations or other overheads. Additional details and examples of observing scenarios can be found in Chapter 5 and in Section 9.7 "Examples of Orbit Estimates."

The 1280 setting of G140L also covers from below 900–1185 Å on Segment B of the FUV detector with resolution of 900–1800 over this range. In principle, the spectral format of this setting may allow observations at even shorter wavelengths; however, the throughput and resolution between 500 and 900 Å are poorly characterized. The new 800 setting of G140L places the entire wavelength range from 800 to 1950 Å on Segment A of the FUV detector, and it is optimized to reduce the astigmatic height of the spectrum below about 1100 Å. This allows for decreased detector noise and correspondingly better S/N at these wavelengths compared to that attainable on segment B with the 1280 setting.

Limited amounts of data were taken using the G130M 1055 and 1096 central wavelength settings at the first lifetime position (before July 23, 2012). Calibrated data from these observations can be obtained by contacting the HST Help Desk.