Re: [ATM] two mirror folded Newtonian

["David Harbour" <scarab2@cox.net>]

Hey, guys-

I've been lurking for a while, and if I am not mistaken (you guys correct me 
if  I am; I do not want to appear stupid) I have the idea that you want to 
fold this newt to keep a certain focal length, but have the tube shorter. 
Why not just start with a very fast, short focus primary (whatever size) and 
not fold it, and then, when you want the option of a high EFFECTIVE focal 
length, use one of those new 5X telenegative amplifiers? I have not followed 
long enough to know exactly what the numbers involved in this project of the 
folded Newtonian are, but let us hypothesize a 20"  f/4 Newtonian; tube is 
approximately 80 inches long (Tube of  Newt is usually about as long as the 
focal length of the mirror, in my experience). Then, with a one inch focal 
length eyepiece, you get 80X. Slip in one of those telenegative amplifiers, 
use the same one inch focal length lens, and get 40X. Slip in a half inch 
focal length eyepiece, get 800X.

One time, I talked to Thomas Cave about a Classical Cassegrain; he said 
there was no justification in building a Cassegrain, which is more twitchy 
with the amplification requuired by a convex secondary; the object of the 
game is of course to get a high effective focal length. He told me: "You can 
do that just as easily by slipping in a 2X Barlow lens, without the 
twitchiness , the sensitivity to everthing that  a
Cassegrain entails". I think it would be safe to assume that his conclusion 
about the whole matter might equally apply to a folded Newt. And besides, it 
is not even necessary to get one of those expensive 5X amplifiers (Barlow 
lenses)- one can stack two 2X Barlow lenses together; even the apochromatic 
Barlows are no longer expensive. But I might have missed something about all 
of this, and therefore my comments are inappropriate. Dismiss me, guys and 
gals, if I haven't understood the thread.

I have something big in the works for you guys, something fine. For free. 
Stay tuned-

Davey

----- Original Message ----- 
From: "Richard" <cnc@cncservo.co.uk>
To: "Polaraligned" <atm@atmlist.net>
Sent: Tuesday, March 07, 2006 6:38 AM
Subject: Re: [ATM] two mirror folded Newtonian


> Hi Polaraligned,
>
> Tuesday, March 7, 2006, 11:28:07 AM, you wrote:
>
> P> The primary wavefront error is multiplied by (1/cos *  secondary
> P> angle).  This results in a smaller error introduced by the secondary
> P> for such a "folded" system.
>
> That is not correct. A perfect secondary will not change any
> defects to the wavefornt caused by the primary, whatever its
> angle. Wavefront distaortion caused by a defective secondary
> will change with the secondary angle though.
>
> -- 
>
> Best regards,
>  Richard in the UK
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


_______________________________________________
ATM mailing list http://www.atmlist.net/