Re[2]: ATM 6" f/8

[Dwight Elvey <elvey@hal.com>]

Larry Manuel <lmanuel@sprint.ca> wrote:
> 
> 
> 
> Asaf S.-T. wrote:
> 
> > I'm very intersted in knowing what I'll be able to see with a 6" f/8
> > telescope, with a:
> >
> > -1.52" diagonal,
> > -1.75" diagonal,
> > -1.3" diagonal?
> 

Hi Asaf
 Again, with out knowing what distance the center of the tube
is to the top of the focuser, these are all just hypothetical
guesses. You need to state that distance, please, PLEASE!

> I built my 6 inch f/8 with a 1.3 inch diagonal, and it worked like a
> dream.  When I installed a 7/8 inch diagonal, I believe I could see
> lower contrast planetary detail, especially when the seeing wasn't
> excellent.  So, I think a small diagonal helps make the scope less
> sensitive to seeing turbulence.

 At higher magnification, used for planetary, a smaller secondary
can be used. Your looking at a smaller spot and 100% illumination
of parts you aren't looking at will only add to glare from surfaces
inside the telescope. This is mainly an issue for proper baffling
of unused light paths. Given the guess of 5.5 inches for the
center to focuser, the 100% illuminated spot would be about 0.2
inches at focus. This is about 0.25 degrees of sky. With an
eyepiece of 50 degrees field of view, you would get 100% across
the eyepiece view at 200X or an eyepiece focal length of 6mm.
If you used the same secondary and an eyepiece of 32mm and a
field of view of 50 degrees, the effective field would be 1.3
degrees and would be vignetted except the 0.25 degrees in the
middle. So, in summary, you'd be able to see the rings on Saturn
with the 7/8's secondary but not much of the Andromeda galaxy.
The 1.75" would most likely give you 100% for most any eyepiece
you'd like to use but with 30% obstruction the diffraction effects
are about at the maximum one would want for a telescope.
It would still make a good secondary for 35mm prime focus
pictures. There is no perfect combination, it depends on the
application.
 As others have said, the size of the secondary for most viewing
isn't all that important unless you want to optimize it for
some special application. A secondary of 11/16 ( and the 5.5"
guess ) will have no 100% illuminated field and will 100%
dark at 1.8 degrees of sky. For eyepieces with long focal lengths
and wide fields of view, you'll need a bigger secondary for
shorter focal lengths and narrower fields of view, smaller
secondaries are better. Again, no right size for everything.
Dwight