Taking Flats and Darks

[dmill1220]

Hello!
how many people that have joined this Forum have a true Astronomical CCD Camera, by that I mean, not a DSLR ?
I was wondering how many different methods people are using for their Flats.
Since my Camera control software automatically selects the correct dark frame for me, I, at least dont have to worry about dark frame subtraction.
I normally do my flats first, during the 30 minutes or so of twilight, then after its dark, and my camera has reached thermal equilibrium, I'll run my dark frame series at the begining, and the middle of the night, especially if the temp drops.
I have heard of other folks constructing a light box for flats

I usually try for a gray image then I do an average combine.

Dennis

[stuart]

Yeah, I do twilight flats because the roof doesn't have a circle for dome flats.

And with my DSLR, I fit a 2-D polynomial to the image and divide by that as a flat to correct for vignetting.

[dmill1220]

Would it be to much to pick you brains some more Stuart?, and explain to me How a 2 D poly image can correct for Vignetting, You have me at a severe loss on how that works? I'm sorry I'm not looking for a free College Education here, but I hope you don't mind my ignorance on the methods I'm in my first year of Astrophotography With a CCD and I'm really hungry for some help.
Dennis

[stuart]

Basically, vignetting is when the corners are darker than the center of the frame. If you were to represent this with numbers, the brighter parts being greater in number, then it would look like two people holding down the four corners of a sheet and bringing it down to the ground, it bowing up in the middle. If you were to take any row or columns of pixels, then it would look like a simple (to first-order) 2nd-degree polynomial. Hence, the vignetting can be represented by a polynomial in x- and y- directions, or a 2-D 2nd-degree polynomial.

Thus, instead of flats, I fit the image with a 2-D 2nd-degree polynomial, and use that as my "flat," treating it as if I took that fit as a flat image, and so use that in standard reduction.

[dmill1220]

Hi Stuart!
Is this Vignetting a result of the larger area of a DSLR's Chip in realtion to the light cone path restricted by the Telescope design?
Since my chip is only a 1/2" large, and I have a large format Focuser tube I dont see this effect on my imaging system. am I right in this Reasoning?
Dennis

[stuart]

I'm not sure. Various cameras have various amounts of vignetting. If you think it's an issue for you, then you should try to correct it (such as by taking twilight flats, fitting a 2D polynomial, etc.). Another benefit of the 2-D fitting if you can't take flats is that even if vignetting isn't an issue, it will take out large-scale non-flatness from the image.

[dmill1220]

Thanks Stuart!
I Really appreciate the info!
What type of software are you using to create a 2D Polynomial Flat?
Or is it just a technique?
Here is what I have been playing around with for CCD processing software, I have Pix Insight LE, Auto star Envision IP, Registax, and a version of Photo Shop, that has a clone feature, to drop out those occasional gamma ray strikes.
I Don't want to spend the money for Maximum DL or Photoshop its very Pricey!
Do you have any other suggestions for image Processing?
Dennis

[stuart]

I use IDL, which is $15 for anyone at CU ... but retail is much more expensive. Or Igor, which academic student is $85, but more expensive. It's more of a theoretical process that you could use if you can find a program that will do image arithmetic. I actually think that the free software IRAF will do it, but it is semi-difficult to use.

[dmill1220]

Thanks Again Stuart!
I may just try a search for those that you mentioned, just to price them out!
Dennis