Difficulties Of Persistence

NIRC2 is an imager and spectrometer for Keck Il's adaptive optics system that uses an Aladdin-3 array, a 1024^1024 InSb developed by Ratheyon Vision Systems, RVS [3]. NIRC2 is both an imager and a spectrometer, and since there is no slit viewing camera the images of the science target must be used to align the spectrometer slit. The difficulties of persistence in NIRC2 can be significant because the higher fluxes from direct imaging create persistence effects that are a problem for the subsequent spectroscopic integrations. Since persistence is a current source that gradually leaks out, longer spectroscopic integrations are particularly susceptible to this effect. The problem is illustrated in Fig. 1, which is a 1200 second exposure of a crowded field near the galactic center. The image in Fig. 1 contains many features, including horizontal continuum spectra from several stellar sources, emission features from hot gases, residual absorption and emission telluric features from materials in the earth's atmosphere, and artifacts from bad pixels, hot pixels, and cosmic rays. However, the source of the largest signal in this 1200 second integration is charge persistence from the previous image. This is in spite of care taken to protect the array from bright sources which could create the trapped charge. The region of interest on the array where the spectra were scientifically interesting was protected as observers went to extraordinary lengths to keep the array quiet prior to obtaining the spectra. This included observing blank-sky spectra prior to the science field, shuttering the camera when slewing, pre-imaging at very short exposure times with a narrow band filter to not saturate, and using nod patterns that moved the spectra of interest away from regions that are contaminated with persistence. Even with these precautions one can see that data quality can easily be degraded if a careful observing plan is not followed. Persistence is a major problem for imaging spectrographs (like NIRC2), when using the instrument as an imager prior to spectroscopic observations. However, observing bright sources like standard stars or the high flux background in the thermal infrared will still cause problems for subsequent faint-object spectroscopy in pure spectrographs.

high flux background in the thermal infrared will still cause problems for subsequent faint-object spectroscopy in pure spectrographs.

Figure 1. An example of persistence contamination: NIRC2 K-band (2.0-2.5^m) long-slit spectra from a crowded region near the Galactic Center (photo credit A. M. Ghez). The image includes continuum spectra from several point sources and emission features from hot ionized gases. There are persistence features from a previous direct image evident in the array. These features have a strong signal and can easily contaminate the data if not carefully accounted for. These data were acquired with a 1200 second integration. Since charge persistence is effectively a current source, longer integrations used for acquiring spectra are very susceptible to persistence effects.

Figure 1. An example of persistence contamination: NIRC2 K-band (2.0-2.5^m) long-slit spectra from a crowded region near the Galactic Center (photo credit A. M. Ghez). The image includes continuum spectra from several point sources and emission features from hot ionized gases. There are persistence features from a previous direct image evident in the array. These features have a strong signal and can easily contaminate the data if not carefully accounted for. These data were acquired with a 1200 second integration. Since charge persistence is effectively a current source, longer integrations used for acquiring spectra are very susceptible to persistence effects.

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