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Inconsistency between lab-measured throughput and observations on-site 

Summary of Problem

The brightness of the sky measured with OSELOTS falls off unphysically in the blue and red ends of the bandpass.  The issue seems to be a very small quantity of incident flux being detected by OSELOTS, relative to what our measured lab-measured throughput would suggest. 

Step by Step Demonstration

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): 

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Next Steps

I cannot tell where the issue lies.  The data going into the calculations appears correct, and appears to be used correctly in the analysis.  We should illuminate OSELOTS with a sources of known wavelengths (say at 1000 nm and at 800 nm) to see if the relative throughput appears more consistent with the lab-measured results or with the sky-measured results.  I suggest taking a range of LEDs in the OSELOTS bandpass (~600 - ~1100), illuminate the fiber tip with these, and take a series of OSELOTS exposures of these PDs.  Then, abut the slit and optical fiber pair to a reference PD, and measure the current from each LED.  If the PD is right next to the slit, it should capture all the light shining into OSELOTS, and you should get a good measurement of the flux these LEDs are shining into the spectrograph. 

This measurement will not provide a measurement of the absolute throughput - that requires using a surface of uniform surface brightness to simulate the sky.  But that should tell us if the relative throughput of the instrument is accurately measured.  And if the throughput appears to have shifted since the lab calibration, we can try to use these on-site measurements of relative throughput to correct our lab measurement of absolute throughput.