Flat Field question:

From what I've read about taking flat frames, is that the optical path must not be changed from imaging an object to taking a flat frame (focus, orientation of the camera).

So, how do I do that? It's 1:00am, I'm done taking my images, then I put a T-shirt over the end of the scope and point a lightsource at it then take the flat frames?

Should this be done at the end of every session?

How are these flat frames then applied to the actual astro images?

Take a look at the previous post:

http://cs.astronomy.com/asycs/forums/p/31823/372167.aspx#372167

You may also pick-up some ideas by looking at the lightbox photo, and description, 3rd row down at:

http://www.galacticphotography.com/astro_equipment_1.html

It doesn't matter whether it is 9pm, or 3AM, if you want to get rid of dust doughnuts and vignetting, take flats (and biases).

Take flats, and biases, after every filter during each session (if you are using a monchrome cam and filters). If you're using an RGB cam, just get the flats at the end of the session. If you made ANY focuser changes during the session, your flats won't be perfect, but...hopefully they work ok. If you are into a second session on a subject, as on the follwoing night, and there have been NO changes made since the previous session, you may take a short cut, and make do with the previous night's flats (they're not be precise, but sometimes good enough).

There are ways to approximate the effects of actual flats in your image processing software, at least if you use something like Photoshop or PixInsight, but the results are not as good as using well taken flats/biases.

Robert

 Thanks for all the info!

I see this is no small task.

Well, that's why it's such a great hobby. Always something new to learn. 

Thanks again!