ATM Re: 8" mirror in 9"tube

[Andrew.Martyn@dao.defence.gov.au]

>A) I'm looking at going with a 9-1/8"ID (3/16" wall thickness) tube.

>B) Low profile (1.5" high) focuser.
This may be a bit optimistic as I have an 1 1/4" NGF low profile whose
minimum height is 1.75".

>C) 8" f/6.3 primary with f.l. of 50" (obviously)
>D) 8" from focuser to front of tube (minimum)

Definitely a minimum! One trick is that if you are getting vignetting due
to
the end of the tube is to:
1. Make the main tube as short as possible, ie only 4" past focuser, and
2. Make a tube extension of say 10" that fits around the outside of the
tube
and is thus larger.
>Blurb on whether to use a 1.83" or 1.52" secondary with above and 27mm
>panoptic using supporting calculations from NEWT and Mel's secondary
program.
>Wants 75% at edge of field, but 1.83" causes vignetting of 75% ray at top
of
>scope.


When working out the diagonal size, you have to consider what eyepieces you
are going to use. Your lowest power/widest fov is the 27mm panoptic. You
are
right when you work out the actual fov is 1.45 degrees. Directly related to
this is the actual field stop size on the eyepiece, which in the case of
your
panoptic is 30.5mm or 1.2" (from the teleview page).

So you want 75% illumination at the edge of the FOV. You plug in the 2
diagonal sizes into programs, For the:

1.83" you get 75% fov of 1.60 degrees and 1.40" diameter. Mel Bartels
program
should tell you that at 0.70" radius (ie 1.40" diameter) the illumination
is
75%. However, I have noticed some small differences between the results,
but
not enough to worry about. OK, so this meets your requirements, but the
problem
is that the 75% ray is vignetting at the front, effectively wasting
diagonal
width.

1.52" you get 75% fov of 1.22 degrees and 1.07" diameter. Mel Bartels
program
should tell you that somewhere between 0.50" and 0.60" radius (ie approx
1.07"
diameter) the illumination is 75%. OK, this tells us that the illumination
at
the edge of your 1.45 degree eyepiece is less than 75%, but what is it? For
this, use Mels program and simply look at what the illumination is at the
edge
of the field stop (which is 1.2" diameter or 0.60" radius). The answer
using my
rough inputs is 71%. This is not much different to the 75% you were after,
and
you don't have to worry about vignetting at the front of the tube.

Therefore the answer is (in my opinion) use the 1.52" secondary!