Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

7.1 Planning a COS Acquisition

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This chapter describes how to plan COS acquisitions. COS offers several very different methods for locating your source on the sky and centering it in the aperture you'll use. The information here assumes that the object to be acquired is a point source. The COS acquisition software was written to work on point sources (objects less than about 0.1 arcsec in diameter). COS is not well-suited to observing extended objects, but if you need to acquire something other than a point source, then we recommend that you start by acquiring a point source that is near to your target and then offsetting, using a POSTARG.

The acquisition of your target has three goals:

• Isolation of the specific object of interest if there are other sources nearby.
• Accurate centering of the target to get the best overall throughput and adequate photometric accuracy.
• Precise centering of the target in the along-dispersion direction to ensure an accurate wavelength scale.

You will also need to ensure that your target is safe to acquire; i.e., that it itself does not produce too high a count rate for the COS detectors nor do any nearby objects.

As we show below, the quickest and simplest acquisitions use ACQ/IMAGE mode and the NUV detector, no matter which detector one wishes to obtain the science spectrum with. The COS acquisition ETC allows you to estimate accurately the acquisition count rates achieved for different kinds of sources. Acquisitions of faint sources are likely to use the PSA and MIRRORA to get the best throughput, but if the count rate is estimated to be above limits then one can choose to use MIRRORB, or the BOA, or both, to attenuate to various degrees.

An observer also has the choice of acquiring an object using its dispersed light. A dispersed-light acquisition is relatively slow and less precise than an acquisition done with ACQ/IMAGE, but can be done for any object that is itself safe to observe.