From: Michael Lindner 
To: atm{at}shore.net
Reply-To: Michael Lindner 




----------  Forwarded Message  ----------

Subject: ATM: Re: Flats
Date: Tue, 11 Mar 2003 14:54:15 -0600
From: dchaffee 
To: "mikell{at}optonline.net" 

Hi Michael,
Can you forward this to the list, since I am presently unable to post to
atm{at}shore.net

Thanks, Dan
----------------------------------------


I'm having some trouble with the recent predictions of the effects of
diagonals with the numbers of waves of curvature that have been specified.

I made a 1.6" minor axis diagonal that had smooth .25 wave convexity
and no TDE in 650nm and used it in a 9.6"f/7.4 newtonian.
 With  this diagonal, the scope tested with obvious astigmatism in a star
 test at 309x. I also performed a star test by fixing the diagonal at a 45
 deg, angle in front of a stopped down 6" newtonian and saw the same amount
of oval deformation at 360x.
I will not wave rate the astigmatism based on the star test, but it looked
 worse in this regards than what any serious planetary observer would
 consider for an excellent scope. And I am very dubious that the astigmatism
 alone was subtracting only one or two hundredth's from the strehl ratio. The
 scope actually performed fairly well on planets, giving a wealth of small,
 low contrast artifacts on Jupiter and Mars. The rille in the Alpine Valley
 was seen once, but never Saturn's Enke division. The diagonal was mounted
 with three dabs of silicone, just as all my diagonals have been.

I switched to a diagonal that is flat to under.1 wave curvature and see no
 signs of astigmatism in the star test. The primary was not possibly the
 culprit. The difference is that extremely fine detail is more often within
 the grasp of this
system, including several sightings of the Enke Gap and the Alpine
rille--somewhat more often. To put this in the context of overall optical
quality, I would say with the
 poorer diagonal, the scope performed nearly identically to an otherwise
 near-perfect scope of around 8" or so in terms of contrast and resolution. I
 suggest that a .1 wave
diagonal is not unjustified accuracy for a newtonian flat.

Dan Chaffee

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Michael Lindner
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