Dark flat? Apartment with power cut?

[Demonperformer]

Apologies for my feeble attempt at humour, and for asking a question to which I'm sure there is already an answer somewhere on the forum but I have not been able to find it ...

What exactly is a dark flat? I understand dark frames (put lens cap on) and I understand flat frames (use even light source), but combining the two seems to be incompatible (use even light source with lens cap on:icon_scratch:). So I guess it must be something different. The answer I got from google ("The dark flats is just the flats made from the same setup") does not help me very much. I'm sure someone on SGL can do better than that.

Thanks for your help.

[Ant]

Your "flat" will have an exposure duration of X amount - depending on the sensitivity of the camera and how bright the light source is.

The "Dark Flat" is a "Dark" of the same time duration as the "Flat", giving a "dark flat".

HTH

Ant

[Demonperformer]

I thought my flats needed to use the same settings as my lights?

Specifically, on my setup, using the mintron, I generally use 128x sensitivity, giving a 2.56sec exposure time. I then use the same levels to take the darks and then move to a nearby, clear patch of sky, unfocus, and use the same settings to take what I have been calling 'flats' (I don't have a light box yet). So should I be loading these images into DSS as 'dark flats' rather than 'flats'?

Thanks.

[Ant]

It is absolutely vital that the camera orientation / focus / filter (if any) are not altered in ANY way.

You do not need a light box - I often use a clean white tshirt, strapped to the front of the scope, this gives an even flat field. I then expose to about a third of saturated (about 25,000).

There is a technical description for this value, but I cannot remember what it's called (ADU I think).

Flats will remove vignetting and also dust bunnies etc - it will NOT work at all if you adjust the focus, spin the camera etc etc etc.

Cheers

Ant

[roundycat]

Just a remark about convention. When you use darks to calibrate your flats they are known as flat darks, not dark flats. If your flat exposures are short and there is no real problem with thermal noise you do not need flat darks. Just subtract a master bias from each flat and then combine using median for the master flat.

Dennis

[RobH]

Here's a quick summary that I did a while back for someone....

Lights are your images....the shots the camera takes.

These must then be calbrated, and calibration frames are used.

These are Bias, Darks and flats, and if you are really picky, flat darks.

An individual exposure is known as a sub.

Bias frames are a record of your system's inherent noise with zero exposure.

They are of the minimum exposure length you can get, if possible, zero.

They are taken with no light entering the scope.

Darks are a record of the noise when exposing and are very temperature dependent.

They need to be of the same length and the same temperature as the lights.

They are taken with no light entering the scope.

Flats are a record of all the optical irregularities in the system, such as vignetting, dust particles etc etc in the optical train.

They are taken using an evenly illuminated lightsource, commonly a lightbox, or electroluminescent (EL) panel.

Flats need to be bias subtracted in order for the correct subtraction calculation to be made. If they aren't, they will either over, or under compensate.

There are also flat darks. These are dark frames of the same exposure length as the flats.

I find that the flat exposures are normally quick enough not to worry about noise buildup, and bias frames handle the rest, so I don't do flat darks.

I apply bias subtracted flats, bias, and temperature matched darks to my individual subs. The master bias, dark and flat are comprised of more than 20 individual sub exposures. If you don't do enough calibration frames you will end up adding noise to your images.

Flats, darks, bias and flat darks are all known as 'calibration frames'

I combine my calibration frames into master darks, flats and bias using an 'average' combine algorithm and do not use any alignment routine.

Cheers

Rob

[Demonperformer]

Thanks, guys. It would appear that what I have been calling 'flats' are nothing in particular, and have therefore maybe been doing more harm than good. OK, stopping doing them will not be a problem.

What exactly is 'a third of saturated'? Have a horrible feeling I am getting into deep dedicated ccd territory here and finding a suitable means of doing them with the mintron may be a problem, but worth asking.

Thanks.

[Ant]

Somebody may need follow this with a correction...

Each pixel can have an ADU count of 0 (nothing) to 65,000 (fully saturated).

When I take a FLAT I look at the settings in Maxim and have an average of around a third (actually I do 25,000).

Ant

[Demonperformer]

Thanks for this, Ant.

I am using sharpcap, so I will have a look at the settings and see what I can find.

[roundycat]

the adu range for a pixel depends on the cameras bit depth. If 16 bit then the max is 65,536adu. If the bit depth is less the max adu count is correspondingly less. 8bit we know is 256 from PS, 10 bit would be 1024 and 12 bit 4096 and so on. If your camera has the facility to change the ISO setting that changes the gain internally and affects the dynamic range so you may not know what your max adu is. You need software that can measure 16 bit values, shoot a well exposed flat and see what the maximum (saturation) value is. Then shoot flats that have an average of half that max value.

Dennis