[ATM] 8" f/3.5 !

[Thomas Janstrom]

Hi Francis,

I have two F3.3 8" mirrors both have now been figured to -0.7ish and as both
had been previously cored (2" perforation) I  am building a pair of 8"
F14ish Dahl-Kirkham Cassigrainian scopes.

Now if your's isn't perforated you could always build the same, but with a
Neighsmith (sp?) focus through one of the fork mounting truion (sp?)
bearings.

Just a thought.

BTW If you're not keen on figuring such a steep curve, you could leave it
spherical and make an ellipsoidal secondary such as found in a
Richie-Cretian Cassigrainian..... A bit harder to do, but not a bad scope as
things go if you keep the system F ratio in the teens, this goes for the DK
too. 

Cheers, Thomas.

-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Francis J. O'Reilly
Sent: Monday, 15 January 2007 9:57 AM
To: atm at atmlist.net
Subject: [ATM] 8" f/3.5 !


Many years ago, probably the mid to late 90's, I ordered a full thickness
8" pyrex pre generated blank from Newport glass. I wanted to build an 8"
f/6 reflector. Unfortunately, I was very tired at the time and asked for a
radius of curvature of 48 inches instead of 96 inches. I ground the blank
out anyway and polished it. The radius of curvature is now 57 inches, focal
length is roughly 28.5 inches. 

I put the glass under the knife tonight, a skill that I never really
mastered, however, I try hard at it, and I determined that the mirror is
most likely spherical, good for a point source, but definately not for
light rays coming from many light years away so as to be nearly parallel
when they strike my mirror. (Isn't space curved anyway Prof. Einstein?)

Any thoughts on the usefulness of this mirror, or am I wasting my time with
something that will never give me much utility?

Best regards to all.

Francis J. O'Reilly 
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