[AZ-Observing] Re: The Orion 120mm ED Refractor
- From: "Jack Jones" <Telescoper@xxxxxxx>
- To: <az-observing@xxxxxxxxxxxxx>
- Date: Mon, 7 Aug 2006 21:32:05 -0700
I was waiting for someone to mention "semi-APO". I think it's now falling out
of favor among the ad-hype crowd and now they are just going with "APO".
Shameless I know, in the ad world, nothing has a definition and buzz words
rule. I wonder if Orion started out saying the 80ED was a semi-APO and then
went what the hell who's gonna stop us, call them all APOs. I need to find an
old catalog. But they are definitely doublets, with Japanese FPL-53 ED glass.
No way are they going to be triplets at that price. And so far they are giving
a very high quality image with almost color free viewing with just a slight
yellow fringe on the Moon edge.
As for the mount vs. the optics, I'm always saying it's 50/50 just to
emphasize the point.
Jack
----- Original Message -----
From: "Roger Ceragioli" <rogerc@xxxxxxxxxxxxxxxxxxxx>
To: <az-observing@xxxxxxxxxxxxx>
Sent: Monday, August 07, 2006 6:12 PM
Subject: [AZ-Observing] Re: The Orion 120mm ED Refractor
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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