Bright Region Reset: an on-detector strategy for minimizing the impacts of atmospheric emission lines on spectral observations

TL;DR

Near-infrared spectroscopy on large ground-based telescopes is hampered by bright OH lines that saturate detectors and vary on minute timescales. Bright Region Reset (BRR) is an on-detector method that periodically resets pixels containing bright lines while the rest of the detector continues integrating, using sub-window guide reads on HxRG detectors. The authors demonstrate BRR on the McKellar Spectrograph with arc-line proxies, showing no degradation of the overall spectrum and a marked increase in SNR within reset windows, along with higher-cadence monitoring of line variability. Limitations include nonlinearity and reset-anomaly effects observed in the test setup, but these can be mitigated with calibration and longer ramp sampling; on-sky demonstrations at 8–10 m class facilities are proposed to assess BRR’s practical impact for sky subtraction in premier spectrographs such as GIRMOS and HARMONI.

Abstract

Observations in the near-infrared using large ground-based telescopes are adversely impacted by bright atmospheric emission lines, particularly the OH Meinel bands. These lines can saturate a moderate-resolution spectrograph on the order of minutes, resulting in information loss at the wavelengths of the lines. OH lines also vary on similar timescales, requiring frequent sky exposures to be able to subtract the sky spectrum from that of the target. In this paper we present a new method, which we call bright region reset (BRR), to prevent the saturation of these lines in near-infrared spectra while simultaneously improving information about their variability. This is accomplished by periodically resetting pixels that contain bright lines on a detector capable of sub-window readout while the rest of the detector continues integrating. This method is demonstrated on the McKellar Spectrograph in the 1.2 m telescope at the Dominion Astrophysical Observatory in Victoria, Canada. Using a Teledyne H2RG detector, we reset the emission lines produced by an arc lamp while still recording their flux. We show no degradation in the resulting spectrum compared to a conventional observing mode. Unlike other OH line mitigation strategies, the BRR method not only avoids loss of information at wavelengths containing the lines, but also provides higher-cadence information on sky line variability, making it a promising technique for implementation at observatories. We advocate demonstrating this method on sky at existing 8--10 m class facilities with near-infrared spectrographs equipped with HxRG detectors in order to test its feasibility for use in sky subtraction schemes for premier modern spectrographs, including the upcoming generation of instruments for the Extremely Large Telescopes.

Figures (12)

• Figure 1: OH atmospheric emission lines, calculated with the ESO Skycalc modelNoll2012Jones2013. The top panel shows the wavelength ranges of selected NIR spectrographs on top of the Meinel bands (linear scale). The bottom panel shows the total sky background (log scale) as seen at the VLT along with the transmission of selected VLT HAWK-I filtersRodrigo2012Rodrigo2020 representative of typical NIR filters.
• Figure 2: The top image is a spectrum with emission lines observed using a normal, no-window scheme. The middle image shows the placement of a window (outlined in red) on the same spectrum which is periodically reset; the data from this window are then recombined back into the image to create the bottom image. In a working demonstration of the concept, the top and bottom images should be close to identical.
• Figure 3: Observed night sky spectrum at the McKellar Spectrograph, with known sky lines labelled with their wavelengths. The large spike around 927 nm is a bad row on the CCD. The 15-px binned spectrum is also overlaid. Low throughput of our system led us to use arc lamps instead of sky lines for our demonstration.
• Figure 4: Locations and relative strengths of OH and Ar emission lines, along with the FWHM range of our chosen narrowband filter. We use the 1108 and 1111 nm Ar lines which lie within the (5, 2) OH vibrational band.
• Figure 5: The entrance slit setup used for the light source. The PIE diffuser is on the left, covering the entrance slit. The clip-on incandescent lamp is in the center, and the arc lamp is in the top right. The arc light is redirected into the beam by a flat mirror. The optical path is indicated by the yellow dashed line.