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Old 19-07-2010, 01:03 PM
astrospotter (Mark)
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Dark frame ambient temp science experiment on Qsi583

Dark frame ambient temp science experiment on Qsi583

The Question: How bad is it to take darks later and perhaps at ambient temperatures that are as much as 10 degrees C hotter than the field? This of course saves valuable field time or we would be happy to take all darks right after our light frames.

Conclusion First: It seems acceptable to take dark frames for a Qsi583 cooled CCD at an ambient temperature of 11 degrees C warmer and still get a dark that is fairly close to the one that may have been taken in the field.

This seems logical but I had to try. This study cannot pass a judgement on this technique when large differences in ambient temp are involved but I sure would like to find that out sometime. I bet somebody knows. No speculation, only fact please. Thanks for any valid info.

Details Follow:

Last night and today I decided to try an experiment to see if I could take dark frames by day at higher temperatures than nighttime darks but at the same CCD temperature used for the shots in the field. I focused on 3 specific pixels where one was fairly bad leak, middle was medium leak and the third was leaky (shows as gray) but was a low leakage pixel. Maxim LE was used to look specifically at the hot values of these 3 mono pixels for each exposure. The max white pixel value is 16 bit, about 65k.

I would appreciate any other findings on this as my experiment is rather limited but I felt it best to take a peek before opening up a wide-open question of this technical a topic.

My sort-of-quick test results indicate that this is ok as long as your camera can get to the same stable CCD temperature and perhaps still be under 80% load. I did not test for maxed out 100% load on camera cooler. This test is only for a delta of 11 degrees C ambiant so is not a very broad range.

Measurements were done outside in back yard with ambiant temp at 15.5deg C and then again today at 26.7 degree C. Both sets of measurements were with a stable CCD temp at -15 deg C. 300, 600, and 1000 second exposures were done for the darks (because those are what I used in the field shots last weekend).

My very rough conclusion is that there is not much difference in the 3 pixels I studied that represent a mild, poor or really bad hot pixel.

Le Data: All for CCD temp of -15C. Temps in deg C and exposure in sec

Amb Exposure DimPixel MidPixel HotPixel
+15.5 300 1768 4306 7600
+26.6 300 1671 4183 7252
+15.5 600 3200 7800 14500
+26.6 600 3173 7480 14139
+15.5 1000 4900 11600 23170
+26.6 1000 4885 11370 22812

I took other data like difference in CCD temp only and that showed what one would expect as a lowering of the hot level of each 'class' of pixels as the temp went down with -20C being best. I felt no need to think a lot about that aspect which is well known. I have all the darks and could look later if desired.

Some thoughts on this data.
1) It is curious that the hotter ambient temps yielded lower pixel counts.
2) This test in the winter where the inside temps may be 30 deg C difference I suspect may show that the values are as close as they are here.
3) The test is in bad need of better statistical dataset but I am out of gas. For a minimum, the use of 10 measurements minimum what the spread is at any given condition would help us understand just how significant the differences I saw with 1 set of measurements really are.
4) Some numbers may be mildly rounded in the final digit so account for that in the overall lack of statistical accuracy. Never rounded more than about 9 counts so I think no big deal.

Ok, I got a bit out of control but it was perhaps of some value. I bet QSI knows all this stuff and has documented it but I like to mess with stuff. Think I may ask them for their studies and thoughts on this as they have an online yahoo group I joined.


Last edited by astrospotter; 19-07-2010 at 05:13 PM. Reason: Removed mention of filters which are not involved in darks. (I had been shooting flats with those)
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Old 19-07-2010, 05:18 PM
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mill (Martin)
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One simple thing you could do, but only if you want an comparable temp as outside at the moment, is to place the camera in the fridge
Simple and easy (just watch out that the wife/girlfriend isn't going to coock the cam ).
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Old 19-07-2010, 05:41 PM
astrospotter (Mark)
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Not so funny as it may sound.

I picked up a very little refrigeration unit that plugs into 12 volts partly to keep things kool on trip but also with the thought of putting in a proper temperature controller so I could actually set it. It goes right on down to about 2 deg C but has no regulator to set it in between. 'someday' I hope to put in a regulator (home brew) so I can set it and put a camera in it. The unit could hold my Canon 40D or this CCD unit easily OR a six-pack

Why go to all that trouble? Well the wife factor is one very good reason. I am a bit forgetful and just KNOW I would leave the fridge at say 15 deg C and once this is discovered. Well lets just say there would not be 'a good reaction' from my generally wonderful wife
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Old 25-07-2010, 12:15 AM
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mjc (Mark)
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I'm not sure what the relationship between temperature and mean pixel value in a dark frame for a given exposure but there is a linear relationship between dark count and exposure time.

So I'd suggest the taking and averaging of some bias frames (zero exposure time darks). They can be done only occassionaly if you want to save time. Then take some darks of resonable duration (whatever you determine is tolerable) - these have to be done in the field for this method. Subtract the bias and scale the darks to the required exposure.

So if you had a one hour exposure you could take a ten minute [bias corrected] dark and multiply all pixel values by 6. Maybe not all S/W packages allows for this but some certainly do - eg AIP4WIN.

If you can measure the temperature fairly consistantly then you can also build up a library of bias-corrected darks and scale from those - thus saving more time - but there will always be some tempeature variation between temperature that any given library frame was taken at and actual temperature in the field.

Mark C.

Last edited by mjc; 25-07-2010 at 12:57 AM. Reason: [biased corrected] dark
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