[sac-forum] Re: Diagonal Size


Thanks, that's good info.

Bartels doesn't explain anything at all to you, but my searching found this
nifty program and most welcome explanation/illustration:

http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

And I found the site you mention also,
http://home.earthlink.net/~flyj/designie5.html, however
 you may really like the former free/shareware program (It's called Newt and
you have to download it).

Jack


> 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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