Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

7.4 COS Acquisitions and Coordinate Accuracy

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The strategy you choose for your COS acquisition will depend on the accuracy of your target coordinates. Unlike STIS, COS has only its small aperture to work with. Given uncertainties in the initial pointing of HST, errors in your coordinates should be under 1 arcsec if the target is going to reliably fall within the aperture. It is also necessary that target coordinates be compliant with the GSC2 system.

If you are less certain of coordinates, or wish to be more conservative, your acquisition should start with an acquisition in ACQ mode. In ACQ mode you can command the COS aperture to be swept in a square pattern up to 5 × 5 steps in size. With a STEP-SIZE of 1.767 arcsec (the recommended and default value), your target will be found if it is within about 5 arcsec of the initial pointing. There is more on this below.