This is my setup for shooting "t-shirt flats" with the SkyWatcher Esprit 80. This really helped improve my photo of the North American Nebula. My setup for flats with my Nexstar 8se is very similar. I know that there are a lot of different opinions on this, but this approach has worked for me so far.

In Eastern Missouri, we have many more cloudy days than clear days (the same is true for our nights). So, even during the daytime, the level of light we receive from the sky often changes from minute to minute. This makes it tricky to get the exposure correct sometimes. I have also tried Av-Flat mode on the camera, which seems to help.

I use Backyard EOS to shoot all of my frames. I knew that my focuser was at 43.5 mm when I shot the NA Nebula last week. So, I just set the focuser to 43.5 mm for flats, and verified via a few preview shots that I could see the "dust donut" that lives somewhere in my camera. That confirmed my focus. I target 50% to 60% on the histogram for exposure of flats, and the best settings to get that so far were 0.4 to 0.5 sec at ISO 100. ISO 100 seems to work best.

Yesterday, was completely overcast but we had some variations in brightness. It took 3 t-shirts, folded in half rubber banded in 3 separate layers to get 50% exposure for 0.5 sec at ISO 100. It is very important to smooth out all of the wrinkles in each layer after applying the rubber band. Don't try to do more than 1 or 2 t shirts per layer - you won't be able to get the wrinkles out. That was the problem I had with the first set of flats shot 1 week ago that caused the brown streaks in rev A of my NA Nebula photo.

In my opinion, late afternoon is best for flats shooting. You don't want a lot of sun because more sun = more t-shirts to fold and attach to the scope. (For big aperture scopes like my 8se, I find I need more t-shirts anyway.) As the sunlight level yesterday changed over the 30 minutes of flats shooting, I had to adjust the exposure time between 0.3 sec and 0.5 sec to keep the histogram at 50% - 60%.

I hope this is helpful to anyone like me who has struggled with flats shooting. (I also write these descriptions to help myself remember what I did, and what actually worked ! )

Clear skies,

Pat