Digital cameras

Re: Creating Flat Frames

galactic_photog — Thu, 13 Mar 2008 04:56:15 GMT

I use a monochrome cam + filter wheel, flats should be taken for every filter used for lights, especially if you don't keep things VERY clean (gets rid of dust doughnuts).

I suppose if you are using a cooled one shot color, or a DSLR, you'd just shoot to the desired exposure, then convert the flat exposures to greyscale. You should convert the RGB light exposures into and be working with individual (greyscale) RGB layers, so use the greyscale flats and biases on the individual (greyscale) RGB lights

Maybe someone who does proper flats/biases/darks witha DSLR will chime in.

Robert

Re: Creating Flat Frames

Boise_Dob — Wed, 12 Mar 2008 19:32:36 GMT

I don’t know that I’m ready to build the light box that you described here (I can tell you are a true enthusiast), but I am going to try some of the exposure specific stuff that you mentioned. From what you’ve stated here, I know that my flat was too dark. I will shoot for ¾ full well on my next attempt. Also, should flats be in color or B/W. I noticed that my flat was shifted slightly to the blue side of the spectrum and I was wondering if that would have an effect on the final image.

Re: Creating Flat Frames

galactic_photog — Tue, 11 Mar 2008 17:10:21 GMT

You are beginning to understand how difficult it is to take a good flat, but you can get one. The twilight flats taken by experienced shooters offer the best potential, but are the most difficult to successfully get. I'd suggest that you consider other types of flats initially.

I use a "light box" when using my 10" newt or cassegrain. The box is about 14"X14"X9", with a circular hole that fits over the business end of the scope. The box uses six 12" long 12V light tubes (bought at Home Depot), separated from the scope by 4 layers of 1/8" thick light diffusers, each separated by 1/2" of air to create a relatively flat (uniformly illuminated) field at the scope aperture. I use a 6"X8" GEPE slide film viewing "table" (a 1/2" thick uniformly illuminated "white light" slide viewer) for the "flat" source with my smaller refractor. GEPE makes these in sizes up to about 12"X15", but they get expensive as the size goes up.

I use 20 flat exposures each @ about 3/4 full well of the camera, 20 bias exposures each @ minimum exposure (~50ms). Subtract the median combined (master) bias from both the flat exposures (each one) and the light exposures (each one). Then form a median combined (master) flat, and divide each bias corrected light by the master flat, and renormalize before combining your lights. A good imaging program like MaximDL will do all of this for you.

Robert

galacticphotography.com

Creating Flat Frames

Boise_Dob — Mon, 10 Mar 2008 19:17:26 GMT

My current imaging setup consists of a Canon 350D and Celestron C8-SGT with the Celestron F/6.3 focal reducer. The setup creates quite a bit of vignetting, which to my understanding can be corrected with a Flat frame. I am looking for some tips and tricks for creating good flats. I’ve heard of people using the twilight sky during each photo session to create a flat that is calibrated to your current setup, but most of my imaging sessions start well after dark and well before dawn, so there is no twilight to work with. I tried coving the end of my telescope with a white sheet and taking a couple short exposures in the middle of the day. This created a nice flat that did seem to get rid of most of the vignetting, but when I “stretched” the image using either levels or curves, I started getting some strange color variations on the outside edges of the image.