We describe how we measured the OSELOTS absolute throughput in the OSELOTS paper draft (see: https://www.overleaf.com/project/5d0804b6f70d77533f15bbc6). We don't repeat that information here.
What we do describe here is an apparent inconsistency between the measured throughput and the data taken on sight.
Inconsistency between lab-measured throughput and observations on-site
The reduced night sky spectrum observed with OSELOTS at AuxTel look generally like the following plot of the data taken on 2022/06/29 (averaged over the whole night):
Note that the lines on the red end of the spectrum (longer wavelengths than about 930 nm) quickly trail off in height. Why is this a problem? Let's compare this night sky spectrum to that observed at La Palma, and reported on this (old) website:
https://www.ing.iac.es/astronomy/observing/conditions/skybr/skybr.html
(we chose this example because their spectral resolution is close to ours, making a by-eye comparison easier).
Note how well our measured spectrum conforms to this spectrum, up to about 950 nm. The O3 band at 8646 nm, for example, sits right between the thickets of OH lines centered at about 850 nm and 890 nm. This seems to indicate that the wavelength solution of the spectrum is not TOO far off - the bands are roughly where we expect them to be. Certainly, they cannot viably be shifted by more than ~ 10nm.
But the subsequent thicket of OH lines starting around 920 nm is much reduced in our spectrum, relative to the La Palma results. The lines are there, and are detected above the background, but are very weak, relative to the main spectrum.
This suggests that the throughput of OSELOTS has been mismeasured. Specifically, it suggests that we have OVERESTIMATED the instrument's throughput in the NIR (this may also be true in the 'blue' end of our spectrum, though there are fewer strong emission structures at that end of the the spectrum for us to make such claims with confidence).
Another possible explanation is that the wavelength solution (pixel to wavelength) for this data is incorrect. But the good (by-eye) correspondence of our spectrum that the La Palma reference spectrum makes that hypothesis seem unlikely.
So let's walk through the OSELOTS throughput measurement, and double check the calculation. Specifically, let's compare the measurements at 800 nm and 1000 nm.
Double Checking Throughput Calculation
The method we used to determine the OSELOTS throughput (described in https://www.overleaf.com/project/5d0804b6f70d77533f15bbc6) was to illuminate a sphere with a monochromator at various wavelengths. Here are the two processed (bias-subtracted) fits images (shown as pngs) for the monochromator set to 800 nm and 1000 nm:
