How to take Flats?

[BlueAstra]

Can anyone give me some advice on flats. I basically have a plain wall, painted magnolia, that looks pretty evenly illuminated by daylight/room lights. Can I use this as 'flat' target? I assume you just set the DSLR to auto and let it take an exposure with auto settings. Is this right? Is it important to be able to actually see the vignetting in the resulting photo? Does the colour matter?

[jgs001]

The real important thing with flats, is to adjust nothing in the imaging train. Then make sure for flat and even illumination. Achieving this last bit is open to debate, but I use, and have had success with using notepad maximised on the laptop screen held in front of the objective. Set the SLR to Av (Aperture Priority mode) and let the camera pick the optimum exposure. This will capture in raw format.

[MishMich]

I tried notepad last night and it was not very successful - Fortunately I was working through the EOS 50mm lens just a tab back from infinity, so I was able to re-do them this afternoon. What had been quite bad vignetting ended up as two streaks after processing the flats. When I looked at the flats, the vignetting was there, but also a darker band the same intensity going across the middle of the screen, and this appeared to have moved for each frame, so I'm guessing that there is some issue with the LCD like you get with a CRT? I re-did the flats using a white photographic mask held in front of the lens, in daylight. The biggest problem with that was that I had to take the speed down to 1/4000 second, and even then frames taken when the sun was out were too bright, only the ones when the sun slipped behind cloud were good enough.

M.

[jgs001]

M, sounds like your shutter speed on the laptop was high enough that you were getting the refresh rate showing in the image. You need a slower shutter to not get that. Something around 1/8 or 1/10 is what I normally get and I don't see those effects, but then I've normally got my lappy on battery and therefore dimmer at that point, and at f/7.5 not the nifty.

[MishMich]

Oh right, thanks, never thought of that.

Exposure was 1/125 on the laptop - never thought of turning it down. Yes, at f1.8 it doesn't need a lot. I tried 90 seconds on Orion last night, totally overwhelmed by light pollution from the lower right. Didn't appreciate that these really wide shots fare far worse when there is light pollution coming from one direction. Have had real problems processing my 60 x 30 second frames.

Tell me (apologise for drifting off-topic) how do you ensure focus using a DSLR lens?

M.