Capturing 'Darks' the next day

[feilimb]

Just a quick question on the capturing of darks as I have only recently started using calibration frames.

If I don't get a chance to capture darks on the same night as imaging the lights, could I do the following in theory on the next day:

If the outside temperature is similar to the previous night when lights were captured:

• set up the DSLR body outside (not attached to telescope) with clip in cover over the lens adapter part of DSLR (or does it need to be attached to telescope?)
• capture 'N' dark frames of the same exposure length, and ISO speed as the lights from previous night

It seems like the most important thing is that the ambient temperature is similar to the previous night, and capturing darks the next day has the benefit of being able to take them even if the weather is poor (with some rain I could put the camera in a shed which has no insulation and would be similar temperature to outside).

Another mad thought - could the camera even be placed inside the refrigerator for the capturing of darks, or maybe this would be too cold.

[spillage]

If you are using phd for guiding then you should be able to dither. This will allow you to ditch the darks and just use bias and flats.

[almcl]

It isn't the ambient temperature so much as the sensor temperature that you want to match.  The EXIF data for some of the Canon cameras records this and APT actually records it in the file name.  While my tests with a remote IR thermometer suggest that the EXIF recorded values may not be exactly what the sensor is at, they are a much more reliable guide than the ambient air temperature.

You can build a 'dark library' with the camera off the scope, lens cap on and some like to put it in a light proof box as well, although I am not sure how necessary that is.  I find doing this that the temperatures increase steadily during a dark capture session, so several runs are necessary to get enough say 10°C darks to match my lights from a previous night.

If you don't have software to read the EXIF data, Michael Covington has a small program here that makes a neat job of presenting the info.

[feilimb]

Thanks for the tips, interesting about the temperature of the sensor as opposed to ambient temperature.  I had a look using the 'FITS Header' utility within PixInsight at some of my light frames to see if any temperature was recorded, but there does not appear to be any (it's a fairly old Canon 450D).  I guess I'll just experiment and try to eventually build up darks library.

I have been trying to dither between frames, but my auto-guiding software keeps failing saying 'lost track of guide star' so I have just gone without dither for the last few sessions.

[Stub Mandrel]

I found that sorting my darks into medium, cold and freezing nights worked well, having lots of darks seems to be more important than exact temperature match (with is impossible using exif data anyway). You can build up a dark library this way. I have got darks by putting the camera in the fridge before now...

 

[spillage]

If phd keeps loosing your guide star you may have dithering set too high. What program are you using to dither?

[feilimb]

6 hours ago, spillage said:

If phd keeps loosing your guide star you may have dithering set too high. What program are you using to dither?

Thanks spillage, I have actually been using the guiding software which comes built into Ekos/Indi (guide module is described here: http://indilib.org/about/ekos/guide-module.html).  The software is running on a Raspberry Pi 3 which is connected directly to the HEQ5 mount (as well as the DSLR, and guiding camera - QHY5LII Mono).  I think it is possible to run PHD2 on the Raspberry Pi also and use it as an alternative to the one built into 'Ekos' but I haven't managed to figure that out yet.

[spillage]

sorry I have not tried indi much to my shame. I would suggest turning down the dithering settings if possible.

[feilimb]

It is a pretty amazing piece of free software and has really good support via its own forum. I have found it a bit finicky at times though, sometimes leading to frustrating sessions - but I guess you can have that with other competing tools also. The ability to run it entirely on a Raspberry Pi 3 (running Ubuntu Mate) makes it very appealing and portable.