DSS; how do you all capture your flats?

Okay.. I'll admit it. I've shot star trails for QUITE a while.
2 shots have really wowed me and have pointed me towards a new quest:
First; this one from Tom:
https://photography-on-the.net/forum/showthre​ad.php?t=589469
and more recently this one:
http://www.dpreview.co​m/challenges/Entry.asp​x?ID=191689&View=Resul​ts&Rows=4

Now the question; when using DSS to process images, I read the stuff below about flats. Do people really fire a flash 10-20 times at the end of an imaging session??
If you don't use flash; what do you use to
a: illuminate the Tshirt
or
b: What do YOU use to get flats?
Thanks in advance.. this astro stuff is all new to me.

I think taking good flats is harder than taking the frames themselves. My method might not be perfect but I have yet to find a better one and I know of several people who do the same. Sky flats are fine but rely on you being setup and ficussed at dusk or dawn. That is rarely the case for me.

At the end of an imaging session I lock the focus on the scope and take the scope and attached camera inside without moving anything. I hang a sheet of white paper over the front of my computer screen and in photoshop create an image that is 20% grey and display it full size. I then point the scope (at a distance of a couple of feet) at the middle of the screen and adjust exposures so that the average exposure reading is around half of the saturation values. With a DSLR the Auto Exposure seems to work reasonably.

The computer screen may well not be perfectly illuminated but the scope is only looking at a fairly small part of it and the paper diffuser should help.

I usually take flats during twilight, typically 10 to 15 minutes after the sun sets, with the telescope pointed high in the southeastern sky. Just set the camera to AV mode and shoot. Typical exposure times are 1/10 to 1/20 seconds at ISO 800. As Chris said this requires being set up and ready to go in time.

If I fail to get the twilight flats, I leave everything set up and shoot t-shirt flats the next morning, using a double layer of white t-shirt, and pointing the telescope at an area of blue sky far from the sun's position. If it happens to be overcast in the morning, that's even better for t-shirt flats.

Some people use light boxes, either home made or purchased, but I suspect that getting the required perfectly even field illumination with a light box could be problematic.

I've even heard of people using a laptop PC screen, holding it up to the telescope with a solid white panel (e.g., an empty notepad document) displayed. I never tried that, but I suspect that getting the required even illumination could be a problem there too.

Whatever your method, you should take 15 or so flats, and either dark flats, bias frames, or both. I use both dark flats and bias frames. I have never had any trouble getting good flats, and they make a big difference in the final results.

Don

Thanks for the replies; I appreciate it.
Unfortunately, when I'll REALLY need them I'll be in the bottom of a side canyon attached to the Grand Canyon.. so no PC's avaliable. I'd appreciate any other ideas, as well as any other DSS suggestions you can give.
Thanks forum buddies! (Totally cheesey I know : )

The problem many people have with DeepSkyStacker is in trying to use it for final processing of the images. It is not intended to be used for that purpose, and the author addresses this on the DSS website and in the user manual. The stacked image from DeepSkyStacker should be saved as is, and then opened in Photoshop or some other processing software to finish it.

There's a setting in DSS that can, if misused, destroy your images. When saving the stacked image, a dialog box presents two options - "Apply settings to the saved picture" and "Embed settings in the saved picture but don't apply them". In just about all cases, the second option is appropriate. The image saved without applying the post-processing settings may be very dark when opened in an image processing application, but all of the data will be there, so it will be possible to produce a good final result. If the DSS processing settings are applied when the image is saved, the resulting file may contain an image that is best suited for the recycle bin.

Don