How Big Should the Secondary Be?

I noticed that secondaries seem to come in restricted sizes and the cost jumps considerably. One in particular is from 2.1" to 2.6". What is the disadvantage in trying to use a 2.1" secondary in a 12" f/5 dob? It looks like the light cone would probably be about 1.8" as it hits the secondary. Has anyone tried this (successfully or unsuccessfully)?

I noticed that secondaries seem to come in restricted sizes and the cost jumps considerably. One in particular is from 2.1" to 2.6". What is the disadvantage in trying to use a 2.1" secondary in a 12" f/5 dob? It looks like the light cone would probably be about 1.8" as it hits the secondary. Has anyone tried this (successfully or unsuccessfully)?
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The disadvantage is that a smaller secondary reduces the size of the fully illuminated field of view, if you spend money on a 31mm Nagler, it is nice to be able to use that large field of view. Of course if the secondary is too small, then you may be effectively reducing the aperture below 12 inches.

If you want to see the effect of the secondary size, I suggest downloading Newtwin.

http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

It's freeware and pretty easy to use. It lets you analyze your scope, the illuminated field of view etc.

Jon

There is no one answer to this. Just gfo down to the ATM area and ask down there. You will get lots of opinion. It depends on how much of the field of view you want fully illuminated, which is a matter of taste, of vision, of type of eyepieces. Everything is a compromise, and you just have to make some choices.

Alex

Hi all --

Something may be missing in this discussion. During a phone chat with a very knowledgeable person at Protostar I was told that secondary size isn't an issue if a Barlow is part of the optical path. The Barlow greatly expands the view of the primary (this can be verified by looking down the empty focussing tube and then with just a Barlow inserted). This makes me wonder if the same isn't true for widefield eyepieces used without an added Barlow. I haven't tried to check this out.

Bob Polcyn