Taking darks during an all nighter imaging session?

[Vega]

I was just wondering how many people do automatic all night imaging sessions and how they deal with taking darks. At the moment I have been setting up the kit, framing, focusing then punching in all the exposure settings etc. Then get some kip until the morning. Of course this causes a problem when taking darks due to the temperiture change in the morning. At the moment I can only think of one option which is to set the noise reduction in my 400D so that it takes a dark and subtracts it automatically from each exposure taken.

Does anyone have any other fancy whizzy method of automatically taking darks after an imaging session?

Matt

[themos]

It's about time they put a temperature sensor in the DSLRs and a temperature field in the EXIF.

I record the temperature and use a library of darks shot at the same temperature. Tonight, I'll be putting my EOS in the fridge.

[roundycat]

your camera should be roughly the same temp from one night to another so if using a DSLR which is not temp controlled rely on that fact and take your darks on a cloudy night so as not to waste time.

Dennis

[Vega]

Excellent. Something to do in cloud cover too. Will be starting my 'darks library' very soon . Its a bit warm tonight though (15deg) so will give it a miss tonight. Think through Autumn it might just be colder than that most nights!

Cheers all

Matt

[brianb]

your camera should be roughly the same temp from one night to another

Unfortunately not - when it's clear the temperature is usually several degrees lower than it would be at the same time of year with a cloudy sky. Especially if it's calm enough to do long exposure astrophotography without a really solid mount and a dome type observatory. 8 to 10 degrees C would be a typical cloudy/clear temp differential.

Darks need to be taken interleaved between the lights. IMHO the best way to do the job is to enable "long exposure noise reduction" and let the camera automatically take & subtract a dark after each light frame. The time "wastage" is not great, and DSOs don't change fast enough that you need to be in a hurry.

[Tim]

Brian is right, the in camera dark subtraction is the way to get the absolute best result, and will save you file space and processing time. However, it will also half your clear nights.

The 400D is pretty good as far as dark noise is concerned anyway, and mine has no amp glow effects up to 6mins (havent tested further).

Personally, i dont get too hung up about taking matching darks, i'd rather get the most out of the clear patch.

A good compromise would be to dither the frames (Maxim or PHD+Nebulosity will do this), and then the sigma stacking will get rid of any persistent hot pixels anyway.

Jerry Lodriguss (sp?) recommends a DC power source over batteries, as less internal heat is generated. I'd be interested to see some side by side results though.

Is your camera proving noisy then?

TJ

[Vega]

When I go up to 8-10 min subs yes (No amp glow though). I'd take my hat off to anyone who has a DSLR thats not noisy at those exposure lengths. On saying thart, I guess its all down to signal to noise ratio.. I do make a habit of stretching the image in curves as far as I can to get detail out of my target. My thinking is that using darks reduces the noise, allowing you to stretch the image a bit further before noise comes in (when compared to an image without darks subtracted).

Im still torn between using the auto noise reduction or creating a darks library...

[themos]

I find that field rotation due to polar alignment error does the dithering for me.

An oven digital thermometer helps you monitor the temperature of the camera body so you can match it against the darks library.

[brianb]

A good compromise would be to dither the frames (Maxim or PHD+Nebulosity will do this), and then the sigma stacking will get rid of any persistent hot pixels anyway.

Hmmm, if your tracking is good then dithering won't spread things around enough for sigma stacking to be any help. Dark frame subtraction is NECESSARY to remove hot pixels, which are always in the same place. Sigma stacking removes objects which are on only a small percentage of frames, perhaps one, like an unwanted aircraft trail or cosmic ray hit.

[themos]

The way I understand it is that dithering will force each frame to be repositioned on the final canvas, taking the hot pixels with it, end result being that the hot pixels will NOT be on the same pixel on the final canvas and hence will be sigma-stack rejected.