Flat Field & Reducing Advice

I have recently started deep sky imaging with my C11 & Nikon DSLR. At prime focus, the collected light was too dim so I decided to buy a 0.5x reducer that threads into the end of my 2" camera adapter. This works well for the very center of the image but tends to bend the light at the edges and completely cuts off at the corners. It seems like the 0.5x is too much for my setup and I was thinking of trying a .7 or .8x. What you you guys recommend for an SCT?

I have attached my flat field. With the corners being dark they blow out on the final image. Shouldn't the flat field cover the entire image evenly? I suspect this is due to the excessive reducing from the 0.5x. Also, how bright should the "ideal" flat field be, histogram-wise?

Any advice would be greatly appreciated.

Regards,
Steve

Attached Image:

Steve,
I'm not sure how a C11 compares with a C8 like I use, But I have used the Celestron 0.63 reducer with the minimum camera distance - works out to be about f/7.5. I am using a Canon 350D (22 mm CMOS width). Here are a few images I have done recently. Likely with an SCT you will suffer from some coma / flatness issues.

http://jthommes.com/Astro/DSLR/m3.htm

http://jthommes.com/Astro/DSLR/m13.htm

These images did have a central gradient removal performed - but I did not use flats.

Ideally, on flats I have done, I like to get the average level at about half well depth or for 16 bits , 32767 ADU. I suspect an average +/- 10,000 ADU from there will probably be OK. Don't let any of your pixels get near zero. Division by a very small number causes the blowout.