[William Optics] Re: Flat box
--- In William-Optics@yahoogroups.com, Perry Holcomb <hpholcomb@...> wrote:
First, you don't need to quote all the message -- just the part you're referring to. Thank you.
> 1.ï¿½ How long do I expose on the "white screen"?ï¿½ Three mins. sounds awfully long, but that's what the subs were.
Expose till your sensor captures 50-75% of its maximum.
You have to experiment here. Shots many flats at different times and check the histogram your DSLR surely can display. Peak should be between 50-75% for x axis.
DON'T USE INFINITY FOCUS but the focus you're currently using the shot an image. If you change focus, take another set of flats.
> 2.ï¿½ Is one flat enough, or if not, how many do I take??ï¿½ I guess if there's more than one, then I combine and stack as I would "darks".
I take at least 9 shots, plus at least 9 darks using the same setup of time/ISO/whatever. Because flat fields are "light" images that captures noise, you have to subtract it from your images. And because flats are usually fast (1/60s or so), it doesn't take much time to take darks, too.
I usually take 18 flats/darks.
With my Megrez 90 is very simple to shot some flats and darks. Put on the box i built, I setup my camera, switch the light box on, take 18 flats, switch the light off, take another 18 shots. (sometimes, setup 1/4000s or faster and capture bias frames) That's all. It takes one minute.