Dark frame library

[decoyp]

Hi.

Just started doing some imaging with my unmodified canon 300d. The results I've had so far haven't been too bad (as long as I dont look at the various astro picture of the week/month in any magazine I'm happy !! )

I just wondered if anyone could help me with a couple of questions.

Firstly I have heard meantion of compiling a dark Frame Library. I guess this would take the ball ache out of taking loads of dark frames after doing your imaging run - but would you not have to compile a set of dark frames for every reasonably forseable temperature ?? My understanding is that dark frames should be taken under the same conditions as the light frames. Does anyone use the libary method ?

Secondly and on a completely different topic. How do you do drift alignment and is it worth the effort ?

Thanks a lot in anticipation

Richard

[BlueAstra]

I've also produced photos that look OK to me but don't compare with the high quality stuff out there! Up the curve we go.

I've started a library of dark frames, each folder a particular combined equipment setup and exposure time (eg Cam, Scope, Focal reducer, Exposure time). I've not factored in temperature yet (always seems cold to me outside) but I suppose I should do, since a colder camera should have lower noise. I also have a library of flat frames, also categorised by equipment setup.

For drift alignment:

Drift Alignment - AstroPedia

Polar Alignment - WCS - Fast and accurate polaralignment for astronomical mounts, using a CCD / webcam and drift alignment

Andy's Shot Glass - Drift Alignment for Amateur astrophotography,ccd, Neutonians and Refractors, amateur astronomy

Drift Method of Polar Alignment

- Ian King Imaging

[George]

Only thing that matters is temperature & time duration. What equipment your using i.e focal reducer etc is immaterial because you are covering the end of the scope.

A dark's bank is something I have not got round to doing......yet

[greenkat]

If you want to practice drift alignment, try this site, it has a software simulator

Classic Astrophotography - Photographing the Cosmos with ordinary film

[sgazer]

I've thought about this too. The equipment doesn't matter, you can simply stick the cover on the camera without any scope or lens to do darks, however the temperature is important. I guess you could do darks spaced at every 5 degrees or so from -10 to +20 but that's a lot of effort.

The other factor I've found is that when doing a lot of lights one after the other, the camera and sensor warms up, so it seems to work best to do the darks after the lights have been taken so the sensor is at a similar temperature.

Just my opinion.

[jgs001]

One question along these lines, it's something I've been thinking about... How important is the accuracy of the temperature ? Can you get away with 5 degrees either side ? 2 degrees ? Anyone have a clue or is it going to be dependant on sensor and camera design etc what you can get away with ?

[decoyp]

Sounds like you guys think the dark frame library is a good idea. I must admit it sounds attractive to me and it's something that can be done when the conditions are no good for imaging. i.e full mood (unless you are into lunar imaging) or cloud or even rain as long as you can keep the camera dry there's no reason you can't use this "dead time" to compile a darks library. That means you get more imaging time on the night. I agree with the temparature and time dependant parameters also, but I have no clue about exactly how acurate the temp should be.

Thanks for the links to drift alignment instruction. I'll dry and suss it out im theory at least, while the weather is rubbish.

Cheers

Richard

[decoyp]

maybee I'll take some typing lessons too !!

[roundycat]

I would say you can possibly get away without a dark frame library. If you can dither the mount via it's software or via the camera control software (Maxim) the hot pixels will not overlay each other in successive frames. Simple outlier rejection combining algorithms will eradicate most, if not all of the thermal noise. I have done this on numerous occasions with the ST10, Artemis 11002 and the STL 11000k. The cameras were cooled to between 0 and -20 but that only controls the amount of noise, not the underlying principle. If you do this you must remember to subtract Bias from each sub frame before combining.

A Flat library is pointless as the dirt and dust will move around between set ups and will not divide out.

Learn drift aligning until you can do it in your sleep, it will be well worthwhile.

Dennis

[Darth Takahashi]

A point to keep in mind when using DSLR is that the maximum exposure time should be limited to around 3-5mins. This is because the cameras sensor isn't cooled. Over time the sensor will warm and start to produce / record more noise.

With my cameras D1 MIII I can go to around 6-7mins but the Kiss 2X (D450) can only achieve 3-5mins max. I guess this is because it retains more heat inside the plastic body.

Still you can achieve quite a lot with lots of 3-5min exposures.

Good Luck and clear sky's

Neil