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Issue with R150 grating at GMOS-N

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R150 grating at GMOS-N


A new R150 grating (R150_G5308) was installed in GMOS-N in December 2016 (semester 2016B). This grating replaces the previous R150 grating (R150_G5306) which had developed an issue with the coating causing decreasing throughput in the blue part of the spectrum (see details below). The old R150 grating was available until June 2016.

The improved sensitivity of the new R150 grating is evident from the figure below, which shows a spectrum of the standard star Feige 34 taken with the new R150 grating on 2016 December 22 in comparison to a spectrum observed with the old R150 grating on 2015 December 31. Both spectra were taken using the 5.0" slit, 2x2 binning, and a central wavelength of 710 nm.

Comparison of Feige 34 spectra with the old and new R150 grating


Details on the degradation of the previous R150 grating

During the GMOS North performance monitoring program in 2014B, the GMOS team noticed a feature appearing in the spectra of standard stars taken with the R150 grating, used for throughput determinations. Inspection of the grating showed that the coating looked cloudy. Below are two R-150 spectra from 2012 and 2014, for the standard star Feige 34, using the same configuration (R150/700nm/5.0'' slit/2x2 binning) and exposure time. With the grating positioned at 700nm central wavelength, the feature causes an absorption located between 480-620nm. (Data of Feige 34, taken on 2015 Dec 31, showed that there was no significant change in throughput compared to the 2014 data shown in the plot below.)




The next plot shows a comparison between two spectra for the same object, taken in 2010 and 2011. Note that these regard to the previous CCD installed at GMOS-N, which was replaced in late 2011. 


Users of the old R150 grating (R150_G5306) should be aware that there may have been a loss in sensitivity of about 0.5-1.0% per month in the affected region. Spectrophotometric standard stars can be used to calibrate the flux.