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Old 24-03-2018, 04:06 PM
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CCD dark current vs temperature

I have been trying to measure the dark current dependance on the temperature.

I used the PI script BasicCCDParameters, loaded it with two flats and two biases and then with two darks taken at the same CCD temperature with 30s and 300s exposures. Amongst the other things the script calculated the dark current for that particular CCD temperature.

Then I took two sets of darks (30s & 300s) at several different temperatures and tried to plot the dependence of the dark current vs temperature. The outcome is attached.

The graph surprised me as it is the opposite from what I expected. I thought that the dark current would be going down with the decreasing temperature. Clearly I must be doing something wrong.

Self-made CCD, similar to QHY8 Pro, details are in my signature.

Any thoughts.
Is there a better way to measure the dark current?

Thanks in advance
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Old 24-03-2018, 07:45 PM
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I agree this seems reversed.
How is the dark current actually being measured?
If you pick a medium level hot pixel and measure the same pixel level at different temperatures do you get the same result?
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Old 24-03-2018, 07:58 PM
JA
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Quote:
Originally Posted by luka View Post

The graph surprised me as it is the opposite from what I expected. I thought that the dark current would be going down with the decreasing temperature. Clearly I must be doing something wrong.

Self-made CCD, similar to QHY8 Pro, details are in my signature.

Any thoughts.
Is there a better way to measure the dark current?

Thanks in advance
Yes that definitely seems odd. Can you look at the images for say -10deg C compared with the images for +5 deg C. Even post the images for comparison in this thread. On viewing - Do the -10 deg C images really suggest that they have a 10 times greater dark current (higher number and/or intensity of energised pixels)

Best
JA
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Old 24-03-2018, 09:15 PM
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Thank you for the replies, greatly appreciated.

The images actually look OK. The "colder" images have less hot pixels and the hot pixels are also less bright. They also seems less noisy compared to the "warmer" images. The average deviation of the "colder" images is also smaller, indicating less noise. Nothing weird there I would say.

So it must be my way of calculating the dark current. I am still relatively new with PixInsight but the use of BasicCCDParameters script seems quite straightforward. I was following this guide.
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Old 27-03-2018, 01:19 AM
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billdan (Bill)
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Hi Luka,

Bit of a long shot here, could it be amp glow that's overriding the dark current?

Cheers
Bill
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Old 27-03-2018, 08:25 AM
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the bias will also vary with temperature. suggest you take both bias and dark at each temp, and subtract the right bias to get the dark current on its own at each temp.

cheers ray
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Old 27-03-2018, 06:18 PM
ErwinL (Erwin)
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Hi Luka,

some camaras (e.g. Canons) subtract the dark current internally, even for RAWs. Then, the better way to determine the dark current is to calculate it from the dark current noise (which cannot be removed if you switch off smoothing).

Erwin
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