CCD temperature for darks

[carl20171]

I was thinking about ccd temp. I set my cam to -20. As far as I can tell that is -20 degrees from ambient temp. So if it is 70 deg F and I go minus 20 that would make the ccd chip 50. RIght?

So if I take my darks on a different night and it is 40 deg f and I set at minus 20 then the ccd would be 20?  If this is true then I should plan to take my darks by ccd temp not just saying I'm at -20 so they match. What do you think?

[PhotoGav]

When you set the CCD temp to -20 you are setting it to cool to -20° Celsius. The only effect that ambient temperature has is to dictate how much cooling is required to get there and it is possible that, if ambient temp is too high, the CCD won't reach the temp you have set. The max cooling of the Atik 490 is -25°C from ambient, so if ambient is warmer than 5°C, you won't get to a chip temp of -20°C.

Anyway, the result of that is that you can create a library of darks at a fixed chip temp and use them for all projects. They do have a limited shelf life, but I can't tell you what that is... Anyone?!

Good luck.

[carl20171]

So if I am understanding this right. If it was 26 C then I could achieve 1 C chip temp with the Atik 490 at 100% fan. So if I catalog my darks by the - number that may not be the actual chip temp. Then I would need to also know the ambient temp,   for example my ambient was 27 C And I was at -20 on the fan and it was not pegged at 100% so I know I am actually getting 7 C chip temp. I would save that file as a 7 C chip temp dark. RIght?

[carl20171]

I actually just rethought what I wrote.   If it was 30 C and I set for -25 and was at 100% I would save the file as a 5 C sample. 

[carl20171]

I got it.     The only time I need to know the ambient temp is if the fan is at 100 percent meaning I am not able to achieve the desired minus temp that I have set. If it is at 100% I need to lower the desired temp so the fan in less than 100% then I know the fan was able to achieve the desired chip temp and label the file at the - setting since I know that is the actual chip temp.   Yahoo.. 

[PhotoGav]

Your ccd control software should give you the chip temp, so you don't really ever 'need' to know ambient temp! The chip temp should also be detailed in the FITS header of each image file. I guess the trick is to choose a chip temp that you can be confident of always achieving. Or perhaps a summer chip temp and a colder winter chip temp and two associated dark libraries.

Sent from my iPhone using Tapatalk

[carl20171]

So one key is watching the fan percentage and the temp in the camera control window. In the end it looks like the camera control window telling me the actual chip temp would be the best. Since I dont use my photos for science I may test to see what temp difference makes any changes to the dark. Do you ever notice a difference visually in say 10 deg different chip temp.? Can you actually see a 10 deg difference, or 20 for that matter? What temp difference to you use for your summer vs. winter chip temps?

[PhotoGav]

Well, I'm no expert with the Atik cameras, but from things I have seen and read around here and online, the difference between the chip running at 0ºC and -20ºC is very significant - do a google search and you should find something. I use a QSI CCD which cools to 45ºC below ambient, so I am able to always image with the chip cooled to -20ºC. Unfortunately, here in the UK, it is very rare to see a summer night's temperature get up to 20ºC, let alone above that! Don't know what your summer night temps are like where you are?

[lensman57]

I was thinking about ccd temp. I set my cam to -20. As far as I can tell that is -20 degrees from ambient temp. So if it is 70 deg F and I go minus 20 that would make the ccd chip 50. RIght?

So if I take my darks on a different night and it is 40 deg f and I set at minus 20 then the ccd would be 20?  If this is true then I should plan to take my darks by ccd temp not just saying I'm at -20 so they match. What do you think?

First you have specify which CCD you are talking about as you seem to have a few from different manufacturers. The CCD temp specified is the max delta T so if the ambient is 20c and the max delta with the CCD is - 25C you will have the CCD at -5c if you are lucky. If you are using a Sony ICX sensor then for up to 1200s exposure darks do not matter and the there is very little benefit in cooling the sensor below -10c as the dark current changes very little. For Kodak sensors the delta needs to be at least -45c as these sensors are not only noisy but they need to be really cooled down to reduce the dark current noise and above 900s sub frame exposure these do need dark frame subtraction. I have an Atik 383 L+with the KAF 8300 sensor with a read noise that I have measured at just above 8e and this one really does need dark subtraction even though it can reach -20c sensor temp easily at UK temps.

Regards,

A.G

[strutsinaction]

...If you are using a Sony ICX sensor then for up to 1200s exposure darks do not matter and the there is very little benefit in cooling the sensor below -10c as the dark current changes very little.

Interesting. I have a QSI 690 (ICX814) which I cool to -10C. Are you saying that exposures <20 mins don't require dark frames, just bias (and flats)? I'm new to this game so welcome any advice on this topic.

Regards

John

[lensman57]

Interesting. I have a QSI 690 (ICX814) which I cool to -10C. Are you saying that exposures <20 mins don't require dark frames, just bias (and flats)? I'm new to this game so welcome any advice on this topic.

Regards

John

Yes John, a master bias made from 200 bias frames or one that is made using the super bias tool of Pixinsight  plus a cosmetic correction routine usually yields much better results. Kodak sensors do need dark subtraction but the newer Sony ICX sensors can happily live without them.

Regards,

A.G

[strutsinaction]

Thanks, A.G! I have PixInsight on trial at the moment so I'll make a note to investigate 'super bias tool' and 'cosmetic correction' further.

Regards

John