Re: [ATM] New page - Newtonian secondary sizing

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

hi Mike,


> Here's the link:
>   http://bi-staff.beckman.uiuc.edu/~melockwo/diagonals.html
>
> Please let me know if you see errors.  The formulas used are ones I
> derived through geometry, and all the sizes I have checked agree with
> telescopes I have designed in the past.

A question or two, or possibly just nitpicking - you wrote:

3)  Where do you put the focal plane?  I like to place it ~0.25" above the 
top of the fully racked-in focuser.  Depending on your eyepieces, you might 
adjust this slightly, but it works for me.  Allow a little more room for 
in-travel for a Paracorr and possibly a barlow lens.  So, the variable d2 
below is the distance from the center of the secondary mirror (center of the 
telescope tube) to just above the racked-in focuser.

Using the sight-tube centering of the secondary to ensure that the fully 
illuminated FOV is centered in the view, the secondary will be offset 
towards the mirror. If so, the optical axis is offset from the "geometric" 
center of the secondary, towards the focuser. If you want the optical axis 
centered in the tube, you need to offset the secondary away from the focuser 
as well - else it is the optical axis that is de-centered towards the 
focuser. So where do you put the secondary end of the measuring stick? 
(careful not to scratch the aluminization ;-)  (This makes very little 
difference in practice, though). And the other end - is the 0.25" in-focus 
included in the d2?

Nils Olof


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