From: "Richard F.L.R. Snashall" 
To: atmlist 
Reply-To: "Richard F.L.R. Snashall" 




Bob May wrote:
 > Measure the diameter of the inside of the tube and use that as the final
 > value.  It will be slightly larger than the largest lens that will
fit in a
 > 2" EP (the holding ring is actually the better diameter and that will be
 > problably another .1" at best) and will be a decently good fit for
the max.
 > you will be able to see.

This has a retaining ring; that was the 42.7 mm diameter I measured. Thanks
for the confirmation!

 > FWIW, short FL scopes will also have the problem of the length of the 2"
 > focuser tube so don't forget that in your vignetting stuf.

I assume you are saying here that, if the draw tube is to narrow, it cuts
off the outer part of the cone?

In any event, I have gone back to the initial design of my current
[infinite duration ;-] project, and, using 42.8 mm field diameter (just
over 2 degrees), reoptimized.  I have added two .len files to my text site:

        http://users.rcn.com/rflrs/qflat76x.len
        http://users.rcn.com/rflrs/qflat76x_near.len

The first is the photopic infinite object distance best focus, while the
latter is the same instrument at the near focus of 10m, moving the back
lens with the focal plane.  If one assumes 0.8 Strehl as the criterion,
this still meets it at this near focus.  I tried this just by moving the
focal plane, it takes considerably more than the 135 mm of draw in this
design; the edges of the field really go bad, too, with the Strehl dropping
below 0.5 at even 20 m near focus.

                Rick S.

 > Bob May
 > http://nav.to/bobmay
 > bobmay{at}nethere.com
 > NEW! http://bobmay.astronomy.net
 >
 >
 >
 >

--- BBBS/NT v4.00 MP