Initial Investigations

Whilst the camera was cooling we attempted to take light frames (pointed at a roughly uniform section of the inside of the dome) in order to see the receeding cooling ring, which we could not see previously as we cooled too much. Initial readings were unsuccessful during initial cooling phase, as saturations were quickly met and not enough data was collected. Future investigations will be conducted to better understand the phenomena of the expanding cooling ring and whether or not this is an affect we need to be wary of.

Next we investigated the so called bias level in ADU, needed for working out an accurate number for the amount of photon events compared to the number of ADU's outputted by the CCD. This was achieved by taking five zero second exposure dark frames, which will then be averaged over their arrays, and between each image to find the bias.

In addendum to dark frames taken previously, It was questioned whether or not the 'hot' pixels appearing in longer exposures were a fault of the camera, and appeared in the same places, or whether this was due to other effects. Looking into the dark frames, it was seen that the 'hot' pixels appeared to be randomly distributed over the images for several exposures, appearing both as 'dots' and as 'streaks'. Therefore we conclude that these must be from a foreign influence, not from the CCD or optics of the system. Examples of these can be seen below.


Additionally, looking at higher exposure times, there appears to be an increase in the amount of 'dots' and 'streaks', however as of yet this has not yet been thoroughly tested. If this is indeed the case, then this would be consistent with a flux of incident particles decaying at a constant rate, and could be explained by naturally occuring cosmic rays. Upon taking an hour long exposure (significantly longer than previous max of 300 seconds), only very few of these effects were seen, converse to the trend seen for lower exposures. A possible reason could be that over such an exposure, the 'hot' pixels are masked as the backgrond around them is increased, making them less visible, so the chance of seeing any easily detectable events is reduced.

These 'cosmic ray tests' were first done with the camera placed in the back of the telescope, at around 45 degrees from the horizontal. Cosmic rays are generally expected to fall directly downards, so if the camera was facing upwards, then cosmic rays would register on the CCD as a bright dots, whereas if at an angle, these could register as streaks. For the data taken for an upwards facing camera [no] streaks were seen, but as seen before, some streaks can be seen for the angled position. Therefore these may well be due to cosmic rays, or decay related radiation produced in the atmosphere. Below is seen a comparison of two hour long exposures, one at an angle, and one facing upwards:

-side by side, streaks, no streaks, up or angle-



In the first image, a full comparison of the 1 hour exposures for different angles are seen, and within many bright patches are seen in identical pixel positions for both. Although they are hard to see looking at a static image, so a zoomed in region is shown below, showing an almost identical, intensely triggered group of pixels in the same position. Many of these are seen, and are now shown without a doubt to be due to faults in the CCD chip, although it is not currently understood whether or not this is a prominent affect for stellar observations.

Another feature no longer seen is the streaks seen before, which is perhaps because they are very infrequent events, which are covered up by the background for exposures of this magnitude. This would perhaps point to them being random cosmic rays, or radiative decays, which strike the CCD chip at random positions, so do not create a large effect for longer exposures.

-- DavidHadden - 27 Oct 2015

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Topic revision: r7 - 27 Oct 2015 - DavidHadden

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