Note for coronagraphic observers.
Recommendations for ACQ exposure times can be found in Section 5.1.1 of the NICMOS Instrument Handbook. In particular, Figure 5.4 summarizes the exposure time recommendations for the F160W filter acquisitions.
If the NICMOS (imaging) ETC is used to this purpose, users should take into account that:
- When acquiring objects bright enough so that allocation of exposure time is not an issue, observers should plan target acquisition exposures to obtain ~ 77,000 e- in the central pixel of the PSF. This should not be exceeded for risk of a TA failure. For fainter targets, more shallow exposures will result in inferior target/hole positioning due to poorer target image centroiding, but is robust down to levels of several thousand e-.(see Instrument Handbook)
- All ETC calculations for NIC2 are relative to the NIC2 focus and not at the coronagraphic focus. The predicted count rate will need to be reduced by ~6% for the F160W filter at the coronagraphic focus. The above limit (i.e. 77000e-) already takes this correction into account. <For the ETC, saturation is said to be attained for the NICMOS #detectors when 81% of the well depth is reached (the point where the #median response deviates by ~2% from linear response). However, for #acquisition purposes the full well depth may be used (but not exceeded). #Actually, coronagraphic observers must avoid exceeding the full well #or the target acquisition will fail. However, they must expose deeply #enough (if necessary, even beyond saturation as defined above) to obtain #a robust target centroid by NICMOS flight software to obtain a proper #centering of the target in the coronagraphic hole.>