Re: [ATM] Secondary Cage Sizing?

[Mike Lockwood <melockwo@uiuc.edu>]

Matt,

I'm currently using a 2.6" diagonal in a 16" F/4 with great results, so 
I think you could easily get away with this size or even smaller.  With 
a fast mirror for visual use, you only have to illuminate a small 
diameter since the falloff in brightness is difficult to notice.  (I 
have never noticed it with this scope.)

Mel Bartels has posted a number of messages on this topic if you want to 
search the archives.  Mel, if you're reading this, I know you posted (on 
this list or somewhere on your web site) diagonal sizes for some F/4ish 
scopes a while back, but I can't find them.  They were pushing the 
envelope of small diagonal size, and I think it would be great to see 
those posted again.

Also (having no idea where the 13.1" mirror came from), I can say that 
if the mirror has a less than perfect edge, a little less light from the 
edge of it certainly will not hurt the image!

Spend the money you saved by going with a smaller diagonal and get a 
diagonal heater to keep it dew free.  :)

	Mike Lockwood


MATTHEW SIMMONS wrote:
> Thanks Mike. By 1.55" I was referring to was the hieght of the focuser. 
> And it is a Discovery optical set from their Web special they held last 
> year, so it is indeed a 2.6" mirror. Thanks for the info on NEWT though, 
> I will read it more carefully. In my novice mind VIGNETTING=BAD!
> 
> Matt Simmons
> 
>> From: Michael Lindner <mikell@optonline.net>
>> To: MATTHEW SIMMONS <msimmonsmcse@msn.com>, atm@atmlist.net
>> Subject: Re: [ATM] Secondary Cage Sizing?
>> Date: Mon, 19 Apr 2004 11:37:05 -0400
>>
>> On Saturday 17 April 2004 11:49 pm, MATTHEW SIMMONS wrote:
>> > So I have a design conundrum here....My specs are as follows:
>> >
>> > 13.1" f4.5
>> > 2.6" (?)
>> > .25" focuser board
>> > 1.55" Moonlite Crayford +.25" racked out
>>
>> Read the newt documentation. Vignetting the 75% ray is GOOD, not bad. 
>> It means
>> the edge of your field is more than 75% illuminated. A couple of other
>> things. If this is a Coulter optical set, the diagonal is probably 3" not
>> 2.6" (at least the few I've measured were all 3"), and the moonlite 
>> crayford
>> is a 2" focuser (I don't think they ever made a 1.25" crayford).
>>
>> IMHO anything from 13 to 15" diameter will wind up producing a fine 
>> scope. The
>> vignetting will be negligible at the eyepiece.  Worry less, build and 
>> observe
>> more!
>>
>> Clear skies.
>> -- 
>> Michael Lindner

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