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

[Roger Ceragioli <rogerc@xxxxxxxxxxxxxxxxxxxx>]

Hi, Paul.
>  I've been following this thread and the term "triplet" keeps coming up. 
> But, according to Orion's online catalog description of the 120mm:  "FPL-53 
> extra-low dispersion (ED) glass in one of the two objective lens elements." 
> 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.  People 
> assume that an APO lens is a triplet.  There's the loose definition of 
> apochromatic, and there is Abbe's definition.  In the loose definition, APO 
> means better color correction than a BK7/F2 doublet.  Ernst Abbe , the 
> discoverer of the optics of fluorite, defined an apochromatic lens as one 
> which not only makes three widely separated wavelengths of light parfocal, 
> but also corrects spherical aberration and coma each at two widely separated 
> wavelengths   Even a doublet can achieve this in some cases, as shown in a 
> long and excellent tutorial on refractor design by Roger Ceragioli of 
> Tucson.  Chapter 4b is devoted to apochromatic doublet designs. One of these 
> is a 150mm f/10 ZKN7/FPL53 highly corrected APO.  The article can be found 
> at  http://alice.as.arizona.edu/~rogerc/ .
> So, if the Orion lens is a doublet, they can probably sell it for under 
> $2000, even with the high cost of FPL53.
>
> Paul Lind
>   
Well, sorry to disagree a bit, but I can't think of any doublet (or 
triplet for that matter) which can meet Abbe's standard literally.  My 
website says that, if I recollect correctly.  And by the way, the 
"parfocal" term comes from James Baker, not Abbe.  My fault for putting 
that stuff up and never yet revising my website.  Too busy at work and 
making lenses.

Anyway some months ago on the Astromart refractor forum and on the 
Antique Telescope Society website, I translated the text of Abbe where 
he actually defines what he meant by apochromat.  You might like to have 
a look.  I think the thread was "What Abbe said."  Nothing shockingly 
different.  I put it up just to get it in the record.

Truth is that you don't need anything like Abbe's level of correction to 
get excellent color-free telescope lenses.  Abbe was working on 
improving microscopes, which are a different kettle of fish, from 
telescopes with their tiny true field of views.

Don't know anything about the Orion scope, but if it's like my 
ZKN7/FPL53 doublet, then it has spherochromatism and chromatic coma to a 
small extent like all the apos on the market today.  It's impossible to 
completely get rid of that stuff, but it's not important in most 
telescope lenses.  Nobody's made a Gaussian apo to my knowledge!

Cheers,
Roger


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