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how to do dark frames


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Hi all,

i`ve just purchased a meade dsi mono pro 2, and i want to take some darks so i can build up my library of them, trouble is i`m not 100% sure of what i`m doing as i`ve no experiance of this at all, so can some kind sole please give simple, and i mean simple, instructions on how to acheive these.

i know that i have to take a dark frame at a certain length of time and at certain temperatures to match my data images, but how many darks do i need at each temperature ? i also read on the net that i need to delete the master frame, what does this mean ?

many thanks Rob

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Hi Rob

I'm not familiar with your camera so can't comment specifically on its functions.

However, subtracting a dark frame from your lights will improve matters by removing some of the 'noise' which is generated by all CCDs. In short, the heat generated by the CCD is stored in the pixels and needs removing. Just one dark taken at the same temperature for the same duration as the light frame and deducted during your processing will show some improvement to your image. Creating a library is a good idea and should cover the range of temperatures and exposure times you'd normally use. What image processing software are you using? Some already have a means of creating and managing such a library

You can go on to the next stage and create 'averaged' darks where you'd take a set of darks all at the same temperature and exposure and then process them to create the average. This is usually what is referred to as the 'master' dark. Personally, I wouldn't start there. I'd concentrate on doing simple darks - one dark frame, same temperature, same exposure - until you're more familiar with the technique

Sorry but I've no idea what 'deleting' a master frame means.;)

HTH

Steve

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I always thought you took the same number of darks as you do lights so say you took...

10x60 secs exposures you take 10x60 second darks

Edited by Doc
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Rob

ps the easiest way to take the dark frame (assuming your camera doesn't have an in-built shutter) is to cover the scope with the lens cap to avoid any light getting into the image train so that the only current which the pixels are seeing is the so called dark current generated by the camera itself

Steve

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cheers chaps, i`m using the meade software, envisage i think it`s called, the camera is rigged up to the scope and lense cap is on, but i`m just unsure about how many darks to take, do i just take one dark for 1 minute at say 10 degrees then one at 2 minutes for the same temp and so on, could be here for weeks

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Doc

You can do that if you want to and indeed some imaging software eg CCDSoft has that very facility. But it's more time consuming. In your example, if the 10 exposures are all of 60 secs and are all taken at the same temperature (easier to manage of course with a CCD which is temperature regulated!) then the dark current is exactly the same for all 10 exposures. So what you can do is take one dark and deduct it from each of the 10 exposures. That's what some other imaging software does

Steve

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do i just take one dark for 1 minute at say 10 degrees then one at 2 minutes for the same temp and so on, could be here for weeks

Yes - you gradually build up a matrix of darks covering the temperature range you'd normally experience and the exposure times you'd normally use. You can always do it on cold cloudy nights when there's nothing else to do except go to the pub.

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Your darks need to be averages, about 10 exposures is a good starting point.

So if your lights are to be taken at 30s exposure, create a dark that consists of 10 averaged 30s exposures with the lens cap on. The best bet is to take them with the camera off the telescope with the nose piece covered. If you cover the telescope you can still get light bleeding in. Also, the filter tray on the DSI Pro bleeds light around its edges.

If your darks just consist of single exposures you will probably add more noise to the light when you use them than you take away.

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The Envisage program stamps each dark with a file name containing exposure and temperature information when it takes them.

By default, if you select dark subtract it finds a dark with the same exposure time and a temperature within 5 centigrade and subtracts it for you automatically as you subsequently capture your lights.

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There are some weird ideas about how many darks you take. If you go into the maths you can show that darks do not subtract noise so you end up with 'quieter' frames, although it does take out thermally 'hot' pixels. Dark subtraction by its very nature adds noise. If you only have one dark the added noise will be 41.4%, take twenty darks and that comes down to under 2%. The idea is to reduce the thermal noise contribution to below the level of the read noise or the shot noise.

The essence of light frame calibration is to remove camera and optical system artefacts so as to deliver the cleanest possible signal. Darks take away thermal noise and flats take away fixed pattern noise and optical system problems such as dust shadows.

For darks to work properly they must be taken at the same temperature as the light frames or they will not cancel out the right amount of thermal pixels, they will be left either 'hot' or 'cold' and the light frame will look a mess. If you cannot control the chip temperature then the darks are doomed to failure. It may be better to use a hot pixel filter prior to combining and rely on that.

Dennis

Edited by roundycat
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