how to properly take flats, dark flats and bias frames

[red dwalf]

been trying to read alot about taking light Flats, Dark flats and bias frames but i don`t think my brain can take anymore information, so was looking to confirm that what i`m doing is correct, i`m using a atik titan at the moment, mono version so have already got the lrgb subs as well as darks with the same exposure length and about the same temperature, now stuck alittle on the rest, as i understand it i can take flats with a suitable light source but exposure time is the thing that's confusing me, do i expose until the histogram gets up to about half the way accross, then take dark flats at the same exposure time as the light flats ?

then bias frames, are these just as quick a exposure rate as your camera will do ?

many thanks in advance for your help. 

[m37]

Hey Rob I can show you this tomorrow if you like!

[red dwalf]

Cheers Chris

[ollypenrice]

Expose your flats till the peak is about a third of the way to saturation. I find this best though higher will probably be fine.

Bias are just darks as short as the camera can take, probably 1/1000th sec.

You absolutely do not need dedicated darks for flats. Just use a master bias as a dark for flats. At very short exposure times there is no difference between a dark for flats and a master bias. Keep it simple!

Olly

[red dwalf]

thank you Olly, much appreciated 

[Dave_D]

been finally playing around trying to make flats using a led panel and APT, and got this... is the darkening along the top normal?

when i use liveview, i can clearly see the vignette which is pretty much central on the display

[ollypenrice]

Must admit, I'd be suspicious of that as a flat and wouldn't expect it to work. It looks more like top to bottom gradient. However, since I don't know your setup I can't say that it isn't correct. On mine I know that kind of result and I know it's wrong but that doesn't prove anything in your case. I get that kind of thing when I don't shoot flats in a dark environment.

Olly

[Uranium235]

Yeah, it doesnt look qutie right. Normally, there would be some darkening towards each corner - rather than a dark band across the top. With an LED panel its a good idea to use some A4 paper to diffuse the light a bit, and ensure the panel is flush against the telescope.

Not sure if this is of any merit for DSLR cameras, but ive noticed that the measured ADU of flats does change according to the temperature of the imaging chip (presumably because of noise). With an 8300 chip (Atik 383) the maximum and average ADU between -5c and -20c can be a difference of up to 5000 - so Ive tried to get the cooling down to as close as the imaging temperature as possible when taking flats.

[Dave_D]

Like a numpty, i hadn't correctly adjusted the secondary with the correct offset and distance from the primary lol.

Gonna try again later.

[ollypenrice]

Hang on, when shooting flats with a reflector you have to think about light leakage. Light can easily get in through the bottom of the tube. Be sure this can't happen.

Olly

[Dave_D]

so after much hair-pulling and leaping about i get this now after setting the secondary correctly. 

ran the image through ccdinspector flat analysis and it showed the vignette almost central, but now the scope 'appears' to be out of alignment according to my laser (which i spent 3 hours today getting the dot stationary at 20ft). i guess only a star test will show which is correct.

to be honest, i'm not 100% confident that this LED panel is doing the job correctly. unfortunately, A3 EL panels are well out of my reach at the moment.

[laser_jock99]

If you're using a light panel try to shoot in a darkened room to simulate shooting at night. The scope needs to collimated normally and the camera in the same focus position and orientation as it would be if you were shooting stars. Any coma correctors/reducers you use or other accessories that affect the light path or cause vignetting need to be in place- for instance I got a new OAG today and I'm interested to find how that affects the flats I take? Camera lens flat images need to be at the same f-stop and focus position as well.

I'm using an A2 sized white LED panel for my flats as it's large enough for a 12" scope and they are quite 'cheap', light & portable.

http://www.ebay.co.uk/itm/A2-LED-Ultra-Slim-Art-Craft-Photo-Design-Drawing-Light-Box-Board-Lightbox-Panel-/400467734123?pt=UK_Crafts_DrawingSupplies_EH&hash=item5d3dbcae6b

Final tip is to move either the LED panel or the scope (whichever is easiest) during the exposures to average/blur out the illumination source even more. If you use a slow shutter speed/low ISO setting then this movement will blur out unevenness in the light source- which is hopefully as evenly illuminated as you can get it.