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Re: Re: Design Review

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Posted by Rick Crockett on February 26, 2001 19:03:38 UTC

Hi Walter:
After arguing this point on the internet for a couple of years did S&T finally agree?

A f:5 system is fairly fast and is going to need a larger diagonal than say a f:8. One misconception is the obstruction. Contrast is reduced as a percentage of area and that is how a diagonal should be sized. It makes no sense to worry about an extra 2-5% by diameter obstruction when losing 90% of available light due to vignetting.

If your concern is planetary type viewing then you should have a longer focal ratio. For that purpose everything will perform better. If the tube is too long then a compound scope such as a Cassegrain is a solution. I use a 3.1" diagonal on an 8" f:4 for wide rich fields and then exchange secondaries for a much smaller Cassegrain 4x multiplier for planetary viewing.

By area which is the correct way to calculate obstruction, 6.4% is what you have with a 6" primary and 1.52" secondary.

Contrary to popular misconception, increasing the size of the secondary does not cause more light to be scattered to the rings of the airy disk. It does subtract light all the way across. I find this far less objectionable than as typical, having less than 10% of the field of view 100% illuminated.

Have Fun,

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