Quote:
Originally Posted by bmitchell82
wow looking at that rob your getting massive vingetting  . I recently looked at the raytrace programs and found that a increse in the secondary could virtually illuminate 100% the 40d sensor (58-70mm upgrade). but the MPCC causes some good vingetting.
In my flats now its kinda funny, i don't have a nice graduated gradient from the center to the outside. its a very abrupt line where it goes from 100% to 80% odd, this tells me that the MPCC is the culprit, i will do some tests removing the mpcc from the fray just to see what the difference is.
The only way around this is to go for a larger 2.5/3" corrector, they are made and they do cost! but they are quality objects.!
Myself i would like to go for a Wynne 3" 0.75x keller corrector they are about 1200 dollars but will take a F4.7 to F3.5 with a effective focal length of 895mm and effectively turning the scope into a newtonian apo with tight spot sizes! makes for some ultra wide field whooze action! |
I think the marvel is in how well flats normally do correct so well for vignetting. Its rare I have trouble with objects on the edge of my field unless data collection has been problematic for some reason (e.g. dewing). Its all relative to how hard you have to stretch your objects during processing I guess. I pushed a fair way beyond usual with the flat above.
Agreed wider corrector would be nice Brendan, but its hard to justify for an OTA that's really only worth a few hundred dollars. I'm still happy with what the 8" lets me explore. I suspect if I did enlarge my secondary I wouldn't gain to much more illumination because of MPCC as you say.
Will be interested to see how this next generation of short F scopes and correctors works out though - watching with great interest. Onwards and upwards!