Re:Re: ATM Aplanatic 4-mirror telescope system /2

[Michael Peck <mpeck1@ix.netcom.com>]

At 05:18 12/4/2002 +0000, kwhitefire@juno.com wrote:

>I'm wondering if the 50% rule is carved in stone or not.  I have a half a 
>degree with my numbers

Nothing's carved in stone, but... The way you set this up you have a 
slightly diverging light cone coming from your secondary. If you want no 
vignetting from the primary over your half degree field the primary 
perforation has to be ~40% of its diameter. If you don't mind some 
vignetting at the primary perforation you might make it as small as ~30%. 
That's the minimum effective central obstruction. And this will still allow 
the tertiary to see some sky directly and form an additional image that in 
this case will come to a focus about 4" in front of the one you want.

>the help of a little cardboard, and compromise the huge FOV with a bigger 
>useful aperture?  I figure the opening in the primary will be about equal 
>to the diagonal.  I have an old 8"

That would make your central obstruction 100%, more or less. I don't think 
it's quite that bad though.

Some of your design choices are a little unusual, which doesn't mean they 
can't work. I'd suggest taking a look at the links I provided, or look up 
"Stevick-Paul" or "Paul-Baker" on the web for some additional design ideas.

I don't understand what Dwight Elvey is saying about exit pupils. In a 
galilean telescope the exit pupil of the objective + "eyepiece" is in front 
of the eye lens, which is inconvenient. In a Keplerian telescope the 
eyepiece is a positive lens and the exit pupil is behind it. In both cases 
the exit pupil of the objective alone is the objective itself. The same 
holds for a modern refractor or newtonian telescope. Add an eyepiece and 
the combination produces an exit pupil behind it, where your eye goes. I 
guess I'm just not getting some key point.

Mike Peck


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Michael Peck
mpeck1@ix.netcom.com