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A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Filter Wheel and/or Shutter annoy, again.

Mechanical designPosted by Daddy-o Jul 03, 2012 09:37AM

The above is the result of testing flux constancy in images taken of the Hohlraum lamp on a given night, through all filters. Since the lamp is constant and the telescope is not moved then, provided that the FW and shutter works, it should be possible to get constant fluxes for each filter. The above shows that this is hardly the case.

In the first five panels (left to right, top to bottom) we see histograms of the fluxes (cts/s) derived by opening images, subtracting the bias, and extracting the total flux as well as the exposure time and filter name from the headers. We see that in no filter is there a prevalent flux - that is, either the lamp altered its brightness or the FW never acquired the requested filter or the shutter was miles off (so that the requested exposure time was nowhere near the one we got). The bottom right plot shows the fluxes plotted in order of acquisition - since we see straight sequences I think the lamp is not the problem - but the shutter and or the FW is.

Near the top we see a special pattern - this is understood when inspecting the list of of filters and fluxes:
6712986.9 _VE2_
6714074.6 _VE2_
103523071.0 _VE2_
103529310.0 _IRCUT_
18201912.2 _IRCUT_
18202890.8 _IRCUT_
18206373.8 _IRCUT_
18204287.7 _IRCUT_
18206416.7 _IRCUT_
18205757.8 _IRCUT_
103526333.0 _IRCUT_
103539529.0 _V_
8245622.5 _V_
8245177.7 _V_
8244685.3 _V_
8247179.9 _V_
8246506.9 _V_
8247198.6 _V_
103533820.0 _V_
103570347.0 _B_
6290177.6 _B_
6288536.8 _B_

We see that when the filter supposedly changes, the flux is all strange given the otherwise regular sequence.

So: never use the last or first image in a sequence from a stack! Whether this has been the case throughout the 1.5 years of data we soon have, is to be revealed by further analysis. For now, let us simply reject all first and last images in all sequences.

Added later:

A quick look at the NGC6633 images shows the ABSENCE of the above problem:
8660536.8 _B_
8661084.9 _B_
8659963.1 _B_
8629954.5 _B_
8661203.9 _B_
8630249.1 _B_
34529656.0 _VE1_
34784961.7 _VE1_
34790632.0 _VE1_
34526064.7 _VE1_
20713556.2 _VE2_
20713411.2 _VE2_
20713019.0 _VE2_
51780586.5 _IRCUT_
51774002.0 _IRCUT_
52019051.0 _IRCUT_
51776315.5 _IRCUT_
52013538.0 _IRCUT_
51770409.5 _IRCUT_
52012276.5 _IRCUT_

The filters did NOT act strange when changed. So - more intermittency, but also a lesson: check all sequences of images for the above problem!

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