Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

9.4 Acquisition Overheads

---

An on-board target acquisition is required only once for a series of observations in contiguous orbits (i.e., once per visit). The drift rate in pointing induced by the observatory is less than 10 milliarcseconds per hour. Thermal drifts internal to COS are expected to be even less. The various types of on-board target acquisitions exposures are described in detail in Chapter 7, "Target Acquisitions" on page 83. The exposure overheads associated with each are given below:

NUV ACQ/IMAGE: If this type of acquisition exposure is performed as the first exposure of a visit, the associated overhead is 7 minutes plus twice the exposure time, which includes OSM1 and OSM2 movements. If this exposure is performed subsequent to the first exposure of a visit (it may often follow an ACQ/SEARCH) with no OSM2 movement necessary, the associated overhead is 2 minutes plus twice the exposure time. The reason for doubling the exposure time in calculating overheads is that after the computed centering slew of HST is performed by the acquisition procedure, a final confirmation image is automatically taken by the on-board flight software.

NUV ACQ/SEARCH: Multiply the number of dwell points by (20 seconds + exposure time at each dwell) to account for slewing and exposure time overheads. Add the grating change overheads from Table 9.3 and Table 9.5.

NUV ACQ/PEAKXD: The overhead is 70 seconds plus exposure time. Add the grating change overhead from Table 9.3 and Table 9.5.

NUV ACQ/PEAKD: Multiply the number of dwell points by (20 seconds + exposure time at each dwell) to account for slewing and exposure time overheads. Add the grating change overhead from Table 9.3 and Table 9.5.

FUV ACQ/SEARCH: Multiply the number of dwell points by (20 seconds + exposure time at each dwell) to account for slewing and exposure time overheads. Add grating change overhead from Tables 10.2 and 10.3.

FUV ACQ/PEAKXD: Overhead is 80 seconds plus exposure time. Add the grating change overhead from Table 9.3 and Table 9.5.

FUV ACQ/PEAKD: Multiply the number of dwell points by (20 seconds + exposure time at each dwell) to account for slewing and exposure time overheads. Add grating change overhead from Table 9.3 and Table 9.5.