ATM Subaperture Color Correctors

[Roger Ceragioli <rogerc@as.arizona.edu>]

Hi, Richard, Ken, and everyone else.

Over the last two years Dick Buchroeder, Rick Blakley and I tried to 
design and make a subaperture color corrector for my 8" f/15 achromat. 
 Dick and Rick, who are both excellent designers came up with a bunch of 
interesting designs using three separated elements.  But in the end, due 
to my folly, we decided to try out Roland Christen's old "tri-space" 
design.  Dick paid for the glass, Rick improved the basic design, and I 
tried to make it.

We settled on this design because we could oil-space the elements and 
got excellent color and coma correction.  Alas, the design uses Corning 
B58-53 an abnormal dispersion glass (it also uses Schott KzFSN2 and SK11).

Well, the project flopped because the Corning glass was hopeless. 
 Terrible homogeneity.  I'm going to bring it up to the Bellingham 
conference next weekend to show people what can go wrong with optical 
glass.  The Schott glasses, on the other hand, are excellent.

On paper this type of design, where the corrector falls about 2/3 of the 
way down the tube, looks bad off-axis, but in practice would probably do 
just fine.  But aside from the B58-53 corrector nobody could find 
another contact triplet.  So we abandoned the project.  

The Chromacorr, I'm told contains 5 elements.  It works as advertised. 
 We tried one on my refractor and it reduced the outstanding color 
almost to nothing.  The main problem with it and with other subaperture 
color correctors is the lateral color they leave.  Stars off-axis in the 
field tend to look like short spectra.  That's a problem with almost all 
refractive systems containing widely separated lenses.  And if you start 
with a single-element objectve, the problem is likely to get much worse. 
 Even the Schupmann tends to show off-axis stars as spectra, unless you 
hold your head just so behind the eyepiece.

Cheers,
Roger