Deep Sky Tracker - Light, Dark, Flat, Bias

I believe I understand the Light, Dark, Bias frames, but I am struggling with how to capture the Flat.

I am referencing Deep Sky Stacker here (http://deepskystacker.​free.fr …q.htm#lightdark​flatoffset) and here are the instructions:
You can use a lot of different methods (including using a flatbox) but I found that the simplest way is to put a white T shirt in front of your telescope and smooth out the folds. Then shoot something luminous (a flash, a bright white light, the sky at dawn...) and let the camera decide of the exposure time (Av mode),


What I am puzzled with is exactly what they are saying to do for Flat frames. DSS indicates to put a white T shirt in front of your telescope and smooth out the folds....but can someone explain exactly what this means. How far does the t-shirt have to be from the lens? It mentions also to then shoot something luminous, this part is confusing to me as well.


NOTE: I am an amateur at this so I want to begin with the basics from this software/tool AND I will be shooting this with my T5i and 18-55 kit lens. (Its a start!)

The flat is taken to compensate for vignetting. You want something that is uniformly bright. The T-shirt idea works best when the shirt is out of focus. The program can then examine the flat frame and check how dark the corners are in comparison to the rest of the frame. It can then brighten the corners of your final stacked image by the right amount to compensate.

Thanks!

So, just for clarification. I put up a white t-shirt, but make sure there is enough light to pass through that shirt, then take a shot of the illuminated material?

kezug wrote in post #16737005
Thanks!

So, just for clarification. I put up a white t-shirt, but make sure there is enough light to pass through that shirt, then take a shot of the illuminated material?

You want to stretch the shirt over the front of the lens and use a fairly bight light.