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

Requested vs acquired exposure times

ShuttersPosted by Peter Thejll Aug 18, 2013 08:22AM
We had a method to measure exposure times - but the method depended on an LED inside the telescope system, and it appears that the system scattered that interfered with exposures. So we had it turned off. From data collected before the device was turned off it is possible to look at the relationship between requested and supposedly actual exposure times.
For requested exposure times below 10 ms it appears we did not get anything useful. For requested exposure times above that there is a bias which decreases with the increasing duration requested. It is simple enough to fit a regression line to the above, it is:


and use this relationship to correct all exposure times. We have tried this.

On the left, results without correction of exposure time. To the right with.

We have extracted B and V magnitudes from the set of 'good images'. We have converted to magnitudes using Chris' NGC6633 and M22 calibrations. For B and V images taken within ½ and hour of each other we have calculated the mean B and V values for the BS and then B-V. We plot these values against lunar phase as well as the time of day, and we see that there is a phase- and time-of-day-dependence in B-V. When we compare B-V from images where the exposure times have NOT been bias-corrected with B-V from images where the bias HAS been corrected we realize that the problems are much worse for the bias corrected images, suggesting that the bias-correction is not valid.


That in turns suggests that the method for measuring exposure time is faulty. As far as we know the method was based on passing IR-light through the shutter aperture to a detector and then calculating the opening time of the exposure from data collected. We are not sure if this was based on a timing mechanism or a flux-collected method.

From our own images of a constant light source we ought to test the stability of the shutter. From the lab we have Ahmad's laser images, and from MLO we have many stellar images, Jupiter images and also hohlraum images.

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Posted by Peter Mar 18, 2017 01:52PM

Gowd, we've been through a lot of analysis! I do not think the exposure-time correction ever made it into our reduction of data, but also doe snot matter as long as we use BS and DS from the same images. Would have been very important if DS and BS had been obtained in separate frames.

Posted by Peter Thejll Aug 19, 2013 09:04AM

The regression I find is:


So I have an offset of 4.5 msec, and then a slope. The fit is for exposure times below 1 second and above 10 msec. Yes, there is an almost 50% correction for small exposure times. If you found 2.5 msec it seems to fit in a general sort of way.

Posted by Chris Aug 18, 2013 03:26PM

Crikey! This sounds very sub-optimal. One thing I do in the photometric calibration is an offset to the exposure time, which we have referred to as "shutter bounce". It was about 2.5 ms I think from memory. Do you include anything like that?