Observations in the near-infrared using large ground-based telescopes are limited by bright atmospheric emission lines, particularly OH lines, which can saturate a spectrograph on the order of minutes. Longer exposures will not contain useful information about the emission lines and also run the risk of detector effects such as bleeding and persistence. By using guide windows on a HAWAII-2RG infrared detector, we demonstrate on-detector suppression of these bright lines in long exposures. This is achieved by periodically resetting detector regions which contain bright emission lines before they have the chance to saturate, while the rest of the detector continues integrating. Used with extended exposure lengths, this could allow for significant reduction of the read noise overhead required for stacking shorter exposures. In addition, through non-destructive reading we are able to monitor the lines which are being reset, allowing us to retain information about the characteristics and variability of these lines. We present the results of a first demonstration of this technique using controlled observations of arc lamps with the 1.2-m McKellar Spectrograph at the Dominion Astrophysical Observatory in Victoria, Canada. We find promising results for the potential future use of this technique.
Persistence effects in HgCdTe infrared detectors cause significant artifacts that can impact the quality of science observations for up to many hours after exposure to bright/saturating sources. This problem will have a substantially greater impact on viable observing modes for infrared cameras on future ELTs due to the leap in sensitivities that is expected. In this paper we present new results from an updated test system that was previously used to prototype “on-detector guide windows” to provide fast T/T feedback to AO systems, interleaved with simultaneous (slow) full-frame readouts for science. We now explore the possibility of continuously resetting these small regions of the detector that are illuminated with a compact source as a strategy for mitigating persistence, using two different detectors. While our results generally show promise for this observing strategy, we found for one of our detectors that the combination of fast localized resets with intense illumination can introduce a potentially problematic persistent change in local reset levels.
Cosmic rays are particles from the upper atmosphere, which often leave bright spots and trails in images from telescope charge-coupled devices (CCDs). We investigate so-called “fat” cosmic rays seen in images from Vera C. Rubin Observatory and the Subaru Telescope. These tracks are much wider and brighter than typical cosmic ray tracks and therefore are more capable of obscuring data in science images. By understanding the origins of these tracks, we can better ensure that they do not interfere with on-sky data. We compare the properties of these tracks to simulated and theoretical models to identify both the particles causing these tracks as well as the reason for their excess spread. We propose that the origin of these tracks is cosmic ray protons, which deposit much greater charge in the CCDs than typical cosmic rays due to their lower velocities. The generated charges then repel each other while drifting through the detector, resulting in a track that is much wider than typical tracks.
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