Temperature Control for CCD

We first tested the lowest temperature we could achieve for the CCD, whilst keeping the cooler power at around 90%, so that small changes in atmospheric conditions could be accounted for by the cooler, so the temperature of the camera could remain as fixed as possible.

Ambient Temperature today: 12 degrees celsius, and tested in the range -10 to -20 degrees, the results of which can be seen in the table below:

From this we concluded that the optimum temperature for today's observations is around -17 degrees, and was used throughout.

Thermal Noise

We then set about determining the thermal noise created by the system. For this the lens cap was left on, as we were only interested in seeing what images where produced due to thermal noise, not from outside sources. We took images at different exposure times, as seen in the attached zipped file (Exposure_test_lens_cover_on.zip), containing the FITs files. A 200 second exposure can be seen annotated below.

Here A is most likely a 'hot' pixel, around 4 magnitudes brighter than the background of the image. B is another bright spot, but this time more spread out, and around 2 magnitudes brighter than the background. The interesting feature at C was first thought to be a cosmic ray, as it appears to have moved over the image during exposure, however it appears to have curved in a noticable way, over a small distance, perhaps identifying it as a low momentum, charged particle. We have yet to acertain whether this is a result of natural radioactivity however.

CCD Faults (?)

Then we removed the lens cover and took images of a smooth side of the inside of the dome, and from this we identified two interesting properties of the camera. First several ring structures are visible in such conditions, marring the observed image. These can be identified as dust on the optical components of the telescope, resulting in 3 distinct ring sizes, correpsonding to dust on one of the 3 components. A annotated picture showing these and related parameters can be seen below. Below saturation an overbright cirular line is visible on the images, this is identified as due to the cooling ring which cools down the CCD chip, which shows a sharp cut-off section around the edge of the image, uncharacteristic of any affect caused by the optics. However when saturation is reached (~40 seconds), the image is warped considerably, and this line is no longer present. The full set of files can be seen in the zipped file titled Exposure_test_lens_cover_off.zip

Here A shows the largest of the rings, with an outer ring diameter of around 340 pixels, and an inner ring diameter of 130. B is the medium ring, with outer and inner ring diameters of 180 and 75 pixels respectively. Seen very faintly at C is one of the small rings, which due to its size, only one ring can be measured for, at around 12 pixels. D hghlights the cooling ring of the camera, which although much brighter looking, has around 1000-1500 additional ADU's.

Analysis

The thermal noise image for 0.1 seconds was used for the following analysis. The pixels values were divided in to 50, 100, 300 bins as can be seen below.

For longer exposure times there were very bright collections of pixels that could be cosmic rays. This made it harder to plot a histogram since the pixel values for these unusual pixels were about 100 times bigger, so these pixels were removed by defining that anything above 600 ADU is to be removed. 32 of such pixels were removed. 1560568 pixels were left. The image below shows the histogram for 50s exposure time:

-- Public.DavidHadden - 13 Oct 2015