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Old 13-03-2011, 02:35 PM
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NightCal
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DSLR darks and temperature

I'm still trying to learn the techniques involved in capturing DSLR astro images. I known that I need dark frames to calibrate my image by taking out the thermal signal created by the camera. I also know that I need to have a set of darks of the same exposure length, ISO setting and temperature as the light frames. I'm planning to build up a 'bank' of dark frames of different ISO settings,temperatures and exposures. Now here is my question - the temperature of what? Should it be the outside temperature, the camera body temperature or the camera sensor temperature? I can get the outisde temp from a sensor/thermometer and the sensor temperature from the image EXIF file. For the camera body temperature I guess I would have to get a temperature sensor and tape it to the camera body.
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Old 13-03-2011, 03:15 PM
adman (Adam)
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As far as I know it should be the sensor temperature as that is where the thermal noise is generated. If you are using one of the dslr astro programs (BackyardEOS or APT) then you can have the sensor temp inserted as part of the filename which is very handy. They also support a USB humidity/temp device called TemperHum which will give (obviously) temp and humidity and calculate the dewpoint for you.

Adam
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Old 13-03-2011, 04:29 PM
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With out being to technical, if you can, the temp of the camera overall which is usually about the same as the outside temp if it is pre cooled before imaging outside.

The sensor temp changes temp with the images it takes so to get a dark at each temp of the sensor is probably unrealistic.

I usually image with the camera in the ICNR on No. 2 and although your imaging run is doubled in time it seems to work really well.

Also I suggest you stick to one ISO setting from 200 to 400, and do all your imaging with the same ISO setting, that is only IMHO.

Leon
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Old 13-03-2011, 06:54 PM
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wasyoungonce (Brendan)
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I use dark library to match darks to lights, download here. In this they say that the EXIF is not necessilary the best match to the actual sensor temp but probably the best that can be had.

The other issue is that EXIF temp is generated at the start of the frame so a long frame may see it heat somewhat and EXIF may not be assurate.

I find EXIF temp to be a reasonable match and temps don't rise that much during subs unless the ambient changes somewhat.
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Old 21-03-2011, 09:20 AM
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I don't think you have to worry too much about this. If your dark frame is recorded at an outside temp of (say) 10 degrees and later you take a light frame at the same outside temperature, then presumably the sensor temp (whatever it may be) will be the same in both cases.
Geoff
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Old 21-03-2011, 09:49 AM
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wasyoungonce (Brendan)
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Well that's true but in general most people take their light frames then when finished their darks.

I found it gets quite a bit cooler in the wee morning hours, a 5degree C drop is common. Thus very easy for darks to be at a different temp.

I took a series of darks on different nights, to make a "dark library" but found matching these to light frames impossible unless I inspected the temp on every exif.
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