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Originally Posted by Octane
Greg,
I did say that the TOA would need a 4" focuser if the intent was to illuminate the entire 35mm frame. 
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Quote:
Originally Posted by cohiba
Well I took the plunge a purchased one from Midland Camera House got the kit for $4399 mind you its another $300 for a memory card, and $400 for additional battery and grip. The Tak only has a 2.7" focuser so I maybe up for another $2/3K when does it stop. I am no expert on imaging side with DSLR, but about 15 years ago when using 35mm film and processing all my own stuff I can never remember issues with croping,vingetting etc why is this the case with 35mm CCD Full frame
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Oh there is no problem with the camera but its the telescope that won't provide what is called a corrected image circle (where the stars are all in focus and no seagull stars out at the edges or elongated stars in corners). I don't think there is any refractor that does that due to the curvature of the lenses.
That is why you need a flattener which gives you pinpoint stars to the corners. APS sized sensors or approx 27 x 20mm work with 2.7 inch focusers. Full frame or 35mm sized sensors require a larger corrected field and light cone that the 2.7 doesn't give. It blocks out some of the light as it does not have enough aperture. That is why you need the 4 inch focuser to get enough aperture to allow the light through unvignetted. Unfortunate but true and just one of the costs of the game. Hence the value of some of these scopes and why they offer a 4 inch focuser version. The 2.7 is fine for visual or up to APS sized sensors.
Also generally speaking now, reducers also tend not to work on full frame sensors except the more expensive 4 inch ones from Tak for the TOA series and the AstroPhysics one for the AP155, 140, 160. These are unique.
Greg.