Theodore Grosson

Topic

Development and Demonstration of an On-Detector Technique to Limit the Impact of Atmospheric Emission Lines on Near-Infrared Spectra

Department of Physics and Astronomy

Date & location

• Monday, August 19, 2024
• 10:00 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Alan McConnachie, Department of Physics and Astronomy, University of Victoria (Co-Supervisor)
• Dr. Kim Venn, Department of Physics and Astronomy, UVic (Co-Supervisor)

External Examiner

• Prof. Suresh Sivanandam, Dunlap Institute for Astronomy and Astrophysics, University of Toronto

Chair of Oral Examination

• Dr. Peter Dietsch, Department of Philosophy, UVic

Abstract

Observations in the near-infrared using large ground-based telescopes are limited by bright atmospheric emission lines, particularly the OH Meinel bands. These lines can saturate a spectrograph on the order of minutes, resulting in the loss of information at wavelengths containing the lines. OH lines also vary on the scale of minutes, so observations longer than this timescale cannot capture this variability. Both of these properties necessitate the use of short exposure times in order to perform accurate sky subtraction. To observe faint science targets, several short exposures must be coadded instead of taking a single long exposure. Because each exposure includes its own independent read noise, this results in an increase in the total noise of the coadded image. In this thesis I present a new method to achieve longer exposure times in near-infrared spectra without the saturation of these lines, while still preserving information about their variability so that sky subtraction can still be applied. This is accomplished by periodically resetting the pixels on an H2RG detector that contain bright lines while the rest of the detector continues integrating. This method is demonstrated on the McKellar Spectrograph, where we reset the emission lines from an arc lamp while still recording their flux. I show that, when comparing the resulting spectrum and its signal-to-noise to a more conventional observing mode, there are no measurable systematic differences. This method does not have the drawbacks of other measures to mitigate the effects of OH lines, such as short exposure times or completely removing the information at the relevant wavelengths, and as such shows promise for potential future use at observatories. We advocate demonstrating this method on sky spectra in order to test its feasibility for use in sky subtraction schemes for premier modern spectrographs.