Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

7.7 NUV Dispersed-Light Acquisitions

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The same methodology used with the FUV detector for dispersed-light acquisitions is available with the NUV channel of COS. The various parameters have the same range of available values and recommended defaults as for the FUV, and will not be repeated here. There are, however, several key differences to be aware of:

• With the NUV, you can use mode=ACQ with one of the mirrors (MIRRORA or MIRRORB) as well as a grating. As noted above, this enables you to execute a spiral search in integrated light, which is generally faster and more accurate than with dispersed light.
• On the NUV side, the optics produce three separate spectrum stripes on the MAMA detector, as compared to the single linear spectrum formed on the FUV detector. In order to reliably locate and center these spectra, it is necessary to extract a fairly large region of the MAMA detector. This fact, combined with a background count rate that is significantly higher than for the FUV detector, means that a dispersed-light acquisition with the NUV MAMA is more vulnerable to noise and is less accurate as a result.

As with the FUV, you can use ACQ/PEAKXD and ACQ/PEAKD procedures to refine the centering in the cross-dispersion and along-dispersion directions, respectively. One difference exists: with the NUV and PEAKXD, you can choose to extract just one of the three spectrum stripes. This is described in the Phase II Proposal Instructions.