[ATM] Semi-APO Perspective

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Elsewhere, there has recently been a discussion of "semi-APO".
I recently worked a design that I thought might, with some tweaking,
become one of these "mythological" creatures.  It had a buried flint
element that might be substituted by a special short flint or short
flint.  The net result was:

	http://users.rcn.com/rflrs/ssd02-13x.len

In comparing the F-C color error against a "normal" glass equivalent:

	http://users.rcn.com/rflrs/ssd02-14x.len

I was disappointed in that the color error for the "semi-APO" was
1192.9 microns vs. 1287.5 microns for the normal glass.  Then I
remembered something Valery Deryuzhin had mentioned; at the initial
level, the color error should be considered at the 70.7% zone, as
this generally gives the best indication.

The semi-APO has an F-C color error ov 523.4 microns versus 1026.6
microns for the normal glass -- just about half!

The f-ratio here is f/12.8, just about the right level that
the semi-APO is photopically diffraction limited.  For the 100 mm
aperture the normal glass lens has to be about f/18 to be diffraction
limited.  So much for the f/15 rule!


-- 

Rick S.

http://users.rcn.com/rflrs


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