Front Aperature Calculations

I'm building a 8" f6 travel scope and used Kriege and Berry's formulas from their book to calculate the diameter of the secondary cage as 8.48". So this is what I've based my design on and have already cut a lot of the wood.

Today I heard about the Newt program and decided to check my numbers with it. Newt says the Front Aperature Diameter needs to be 9.3"

This reminds me alot of the old saying "Man with one watch knows what time it is, man with two watches is never sure"

So my question is, which is correct? If I hadn't already cut my wood I would just go with the 9.3" and I may still make modifications to do so if it is truly necessary.

Thanks for your help,
-- Craig

Craig,

I would trust Newt's calculation. Without being too specific, I talked to a noteworthy Dob manufacturer last year and he said that the calcs in K & B's book are too aggressive and can lead to vignetting. But, the whole optical path comes into play--secondary size, lateral distance to the focal plane, baffling, etc, so YMMV on this.

I used Newt to design my recently completed 14-1/4" f7 Newtonian, and the results were excellent.

Regards,

Matt L
Glendale, AZ

The Newt number (9.3") is undoubtably the correct one. Just 0.48" of clearance isn't going to allow much of an entrance angle before vignetting from the cage/tube occurs.
--
Mike

See the disscusion between myself and Vladimir on 9/9/03, subject Diagonal for Photography, 5 pages back. Your present aperture will act as a baffle, and may be a good thing. Our discussion was not limited to photo-only scopes. Ron