[sac-forum] Re: Diagonal Size

Jack,

The "70% rule" is widely thought of as being the recommended
limit for light fall-off toward the edge of the FOV. Bartels
and others think that this parameter is appropriate for 
visual use because the eye can't detect the subtle drop in
transmitted light from the center to the edge. Others 
(Dave Kriege) recommend a big 100% sweet spot at least ~ 0.4" in 
radius. This guarantees a drop off of less than 30% for a 2"
focuser.

I think it is important to consider the field-stop radius of
your largest EP too. The largest around is 0.83" for the Nagler 31,
all others are less. I look at 70% at 0.80" as a benchmark when
I am thinking about Newtonian design.

A too-small diagonal or too-far diagonal-to-focal plane distance
could lead to vignetting in extreme instances. These are the figures
in red.

Check out:

http://home.earthlink.net/~flyj/designie5.html

I have used this a lot.

Matt

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Sent: Tuesday, January 07, 2003 12:11 PM
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Subject: [sac-forum] Diagonal Size


What's with Mel Bartels diagonal size calcuation program? 

http://www.efn.org/~mbartels/tm/diagonal.htm

Is 100% illumination sufficient or do you need to get out of the red to
26mm? What is the effect of increasing off-axis distance at < 70%???

Jack

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