FreeLists / az-observing / [AZ-Observing] Re: The Orion 120mm ED Refractor

[Roger Ceragioli <rogerc@xxxxxxxxxxxxxxxxxxxx>]

Hi, Brian:

 >>

 I read that as two objective elements, making it a doublet.  There's a

lot of confusion over the terms APO, ED, and Achromat, triplet etc.

     Since there are experts on the line here, am I correct in thinking
that there is no "in-between" range of color correction amongst
refractors?  That is, the ordinary doublet has a color error of
something like 1:2800 of the focal-length of the telescope, and
"apos" have error of around 1:10000 or smaller --- with nothing
in-between except by purposely mis-designing an "apo" to have worse
correction than you would get naturally (more-or-less).  I presume that
since the optical materials is a pretty mature technology that nothing's
changed in the last 10 years as far as materials go.

\Brian

>>

Dean Ketelsen wanted me to respond.

You can have any color error you like (almost)!  I have made a number of 
so-called "semi-apochromats", which is really a misnomer but invented 100 years 
ago ("halbapochromat" in German) when apos were first being built to describe 
what are actually "reduced color error achromats."  Over the past couple of 
years in addition to many actual apos and ordinary achromats, I have built an 
8" f/9 with the color error equivalent to a 5 or 6" f/15 standard achromat, and 
a 5.5" f/8 with somewhat less color error.

There are legitimate reasons (mainly cost and focal ratio) for allowing some 
color error back into the design, although purists would find this abhorrent.  
It's not purposeful "mis-designing," but rather facing reality.  Anyway, if you 
really understood what AP, TMB, and others are doing with their scopes then 
you'd see that they are not "pure" either.  The only pure color correction is 
given by reflectors, and even then the aluminum coatings don't reflect evenly 
across the whole visual spectrum.  So what's pure?

If you'd like to read something more in detail about refractors of all classes, 
you might like to look at my website:

http://alice.as.arizona.edu/~roger

where there is a wealth of information (and probably some pseudo-information 
too!).

Cheers,
Roger Ceragioli
Mirror Lab
UofA 

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