Re: ATM - Best Secondary Size

[Dwight Elvey <elvey@hal.com>]

Richard Schwartz <richas@idt.net> wrote:
> Dwight, thanks for your very correct answer to George on diagonal
> size.   There is way too much orthodoxy, and way too little thought give
> to this.  One size does NOT fit all.
> 
> One nit to pic....  it is the distance from the diagonal center to the
> FOCAL PLANE that matters, not the end of the focuser.   If there is a
> camera body, the focal plane can be way outside the end of the focuser.
> 
> . . . Richard

Hi
 Yes, that is what I meant but not what I said.
I'd have been closer to right if I said to the
end of the focuser with an eyepiece at focus.
This has two slight problems. Without a secondary
to try, where is the eyepiece. It also seems that
all eyepieces have focal points slightly different
than the flange.
 When one constructs a telescope, one can
move a lot of things around, there is no easy way
to specify this parameter. One could say that the
you should use the end of the focuser tube when
it was at the desired position of focus ( after
fitting parts, one would move the primary to achieve
this location ). I'd select the focuser position
based on how you expected to use it. If you were
optimizing it for visual only, you might want it as
close to the main tube as possible. If you expected
to use a shorty Barlow, this would set the limit
( I still have to measure this distance for Nils
some night! ). Using it for a combination of things
like Richard does, you might set this with the focuser
extended almost fully to allow for cameras, photometers
and eyepieces.
 While Richard is exactly correct, the distance from the
focal point to the center is the important measurement, when
building a telescope, it depends on which measurements
you intend to hold constant and which you intend
to do final adjustment on. In the standard sono tube
Dobson, the final holes for the primary can be done
last. In an exactly machined telescope, everything
is right and one doesn't need to do a final fit
( this has never happened in anything I've built,
I tend to be a drill to fit type ). In a truss, one
might do a final adjustment of the truss tube lengths.
 I hope I haven't confused things even more.
Dwight