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Earthshine blog

"Earthshine blog"

A blog about a telescopic system at the Mauna Loa Observatory on Hawaii to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

No significant dark current

Bias and Flat fields Posted on Nov 20, 2011 02:05PM

I have plotted the mean dark value as function of exposure time from the dark frames obtained night JD2455883. The range of exposure times is 10-200 seconds and thus covers all higher exposure times we might be interested in. No dark current is observed, only the usual scatter due to the 20 minutes period. The period was clearly seen in a plot from the same data of dark count as function of time since first frame.

I think it is safe to say that we no longer have to make dark frames – only bias frames. This will save observing time, and therefore there should be time to ALWAYS obtain a bias frame before and after each science frame or flatfield. This will improve the scaling of the superbias.

The horizontal line in the plot is the mean value of the superbias. It can be seen that it is not in the middle of the scattered dark values. I have seen this before, as well as the opposite with dark values generally being higher than the mean of the supebias. However, in both cases values are within plus/minus 0.5 ADU and the scaling of the superbias means it is not a problem.



JD2455886

Observing log Posted on Nov 20, 2011 01:53PM

Clear, but 80% RH and T=5.6 C in dome.

Halo enormous – no sign of DS despite phase. Exposures of the BS only in the 1000’s – must update exposure time – heavy clouds??

Shutter clearly open during readout – dragging downwards.

Also sticking so that saturation in stripe downwards happens – i.e. shutter started to close, then stuck open for a long time. T now at 5.2 C.

T now at 4.9 C.

Experimenting with SETIMAGEREF and MOVEMOONTOREF because it seems the mount calibratio I did earlier tonight was not quite right. Noticing that when Moon is half out of the image frame downwards I get a reflecting coming in from the upper side of the frame – like was saw during the tau Tauri occultations in July – then it was left to right. So, something in the optical train is causing reflections onto the CCD when the source is not well centered.

More and more clouds now. T now 4.8 C.