Darks: Manual or automatic?

[TheCrazedLog]

Had a quick look for this, but difficult to figure out the search terms:

When image processing using shots from a DSLR, should I:
1) Turn on dark shots in the camera itself and let the camera do the darks internally
2) Turn off the dark shots in the camera and do it myself
3) Do both

Thoughts?

[rcheshire (Rowland)]

There are two schools of thought and some very adamant opinions.

1. Take your darks after imaging at a similar temperature and exposure time to the light frames - create a super dark/master dark, which is an average of typical dark noise over the imaging period. This method increases the time for obtaining signal.

2. Use in camera reduction which is more accurate.

This method should produce better sub frames than applying an averaged dark and possibly less subs are required because SNR per image is better. Half as many doesn't add up in my mind and this is something I intend experimenting with, with my cooled DSLR at the next astro camp.

Need to do the maths.

Mixing the methods would mean combining one set of in-camera dark subtracted subs with post imaging average dark subtracted subs. That's OK, I guess.

[andyc (Andy)]

That's a really good question, I'd be very interested to know if experienced imagers use in-camera darks at all? Or if the overall merits of a set of darks done after imaging makes it much more worthwhile?

[JB80 (Jarrod)]

I take the approach that in camera darks are wasting imaging time, if your subs are 2 minutes long then you have to spend another 2 minutes waiting for the dark.

I find it far easier to take my subs until the battery runs flat then replace the battery and lens cap and just let the camera click away whilst I fall asleep somewhere. Also I imagine it probably kills the battery quicker too.
I have found that the most common advice is to turn all in camera enhancement settings off so that's what I do and it seems to work well for me.

[raymo]

Just a thought from a digital newbie. If you spend 2 or 3 hrs taking subs, and then let the camera spend a similar amount of time taking darks,
you could get a significant change in ambient temp between taking the first sub, and the last dark. At least the darks match the subs when
using in camera noise reduction; obviating the need to spend time
creating a library of darks. I think it's entirely a matter of personal preference. As far as I can gather from everything I have heard or
read, there is no actual benefit to the image which ever way you go.
raymo

[Rigel003 (Graeme)]

I think most will want to take darks as at the end of an imaging session, as Jarrod has said. Otherwise you're wasting 50% of your imaging time where you should be catching valuable light frames while your object is high in the sky. You can take dark frames after it's dropped too low for decent definition or after it's set. Certainly you get better temperature matching of dark frames with in camera noise reduction, but at a considerable cost, and temperatures don't normally vary so significantly across a few hours of the night that the darks will be too inaccurate.

[JB80 (Jarrod)]

From what I have read slight changes of temp are negligible and I get the impression that most of the more serious dslr users build up and use a dark library, I don't I just run a set after imaging.

But even this I have seen argued against with merit as continuous shooting does heat the sensor up and it should be given a chance to cool down properly before taking the next dark.
It starts getting too complicated for me anyway.

This article also has something to add and may be of some use to people... http://www.cloudynights.com/item.php?item_id=2786

[rcheshire (Rowland)]

Depending on the camera, sensor temperature will stablise or suffer from more significant fluctuations between images. The first or the first few images, depending on the camera will be cooler.

If you image at a set rate and take your darks in the same way, temperature differences should be acceptably close, providing ambient air temperature does not change significantly.

Temperature matching is best achieved for an uncooled DSLR on the same night. Older APS-C Canon DSLRs don't get lividly hot.

The best advice is to keep taking darks immediately following imaging, if you do not use in-camera reduction (hot topic). Slip the lens cap on and keep going - keep the sensor as close to temperature as possible. Dither the lights to eliminate/reduce temperature mismatch artifacts.

Don't let the sensor cool between images too much - just enough to dither and lock the mirror up, before activating the shutter.