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Issues and Problems
The following are known problems or unwanted "features" associated with GNIRS
A. Characterization of Persistence
- Persistence appears following any single exposure resulting in signal of ~ 1000 ADU or more. It increases linearly with signal above 1000 ADU. E.g., the persistence in a 120 second exposure immediately following a “flat” of 2000 ADU is ~4 ADU, for a “flat” of 3000 ADU the persistence is ~8 ADU, for a flat of 4000 ADU it is ~12 ADU. For a single flat of 1000 ADU it is ~0.
- Repeated exposures result in a somewhat larger persistence (by ~30 percent) in subsequent exposures. Repeated exposures at or slightly below the above threshold also create persistence.
- The “persistence current” decreases with time as ~ 1/t whether or not dark exposures (flushes) are taken. The persistence in a frame is then proportional to log (t2/t1), where t1 and t2 are the start and end times of the frame, both times measured from the end of the last exposure to cause persistence.
B. Effects of persistence on data and possible remedies/workarounds
- In the 1-2.5um region observations of bright standard stars (or bright targets) result in their spectra being imprinted on subsequent frames of faint science targets. The remedy for this is to observe the science target and standard star in different locations along the slit. The OT library templates are set up in this manner.
- In the L and M bands the sky background is bright enough to result in persistence, which is probably unimportant for L and M band science frames but could affect subsequent JHK faint source science spectroscopy frames. This can be mitigated by appropriate scheduling.
- Emission lines such as strong sky OH lines in the H and K bands and strong arc lamp lines can leave persistence on subsequent frames such as flats. This is difficult to avoid. The observing templates in the OT library are set up so as to obtain flats prior to arcs. The template flats are designed to produce large signals in order that the relative effect of the sky-line persistence is minimized.
- Observations of spectral flats result in persistence in subsequent frames. Because flats are "flat" in the spatial direction in most cases the persistence will subtract in subsequent A-B subtracted pairs. Problems may arise when a XD program is followed by a long slit program; this is a scheduling issue.
- The sky background in faint source acquisition frames can leave persistence in the form of an imprint of the keyhole on the first few science frames. Over much of the persistence region the effect goes away when one subtracts the negative spectrum from the positive spectrum (in A-B subtracted frames); but on the curved edges of the keyhole it does not. The acquisition exposure times in the OT library have been shortened to reduce this effect.
Several types of observations result in signal levels that cause persistence and can deleteriously affect subsequent science frames.
While the GNIRS team is making efforts to understand and avoid persistence as far as possible, it is an intrinsic property of the detector and cannot be avoided in all circumstances. The practical impact of low-level persistence on many science programmes may also be minimal. Users are encouraged to consider the possible effects on their science data and use the above information to plan accordingly.