Peter as I told you on your M33 thread, neither of the SCT correctors will work on your scope. They are made for F10 scopes with different optical errors than your scope has. The F3.3 only has a 9mm imaging circle whilst the F6.3 has about 12mm from memory. These numbers are only good for an F10 SCT. What you are seeing is savage vignetting so obviously the imaging circle from the F3.3 is even less on your F8 scope. In the M33 thread I recommened a AP CCDT67 corrector but on second thought this will not work as the correction would also be too strong as it is designed for scopes with F9 and above focal ratios. The only other one I can think of is the;
http://www.optecinc.com/astronomy/nguw_070.htm
(blurb from the site)
"This telecompressor was designed specifically for systems with well corrected optics and flat fields such as can be obtained from apochromatic refractors and the new coma free and correctly named LX400-ACF telescopes. The speed of a f/8 telescope would be increased to f/5.6 with this reducer and the optics would still be nearly diffraction limited".
The only thing is that these are limited to an 18mm imaging circle so if you want to get a larger chipped camera you are in trouble ( I sold mine as it would not cover a QHY8 chip).
Mark
