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

["Paul Lind" <pulind@xxxxxxxxx>]

 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



----- Original Message ----- 
From: "Brian Skiff" <bas@xxxxxxxxxx>
To: <az-observing@xxxxxxxxxxxxx>
Sent: Monday, August 07, 2006 12:13 PM
Subject: [AZ-Observing] Re: The Orion 120mm ED Refractor


>     As Jack, Dwight, and others have intimated, depending on the
> intended use, the type of glass and supposed color-correction may
> be only a minor consideration.  If you intend to do imaging of some
> sort, then the mechanical aspects will be of higher importance---
> a rigid mount and couplings capable of dealing with the extra hardware
> without flexure.  If there is flexure in the system, slightly soft
> images are going to be the least of your problems.  For casual
> observing with eyepieces only, probably a large number of the
> near-identical offerings at the cheap end from various sources will be
> adequate.  Once you start thinking solar observing, or planetary, etc.,
> then you'll have to think about better correction, longer f/ratio,
> solid mount and the mechanical stuff at the business end of the
> tube assembly, all of whioch mean more $$$ unless you make the
> parts yourself.
>
>     What is Charles Juels using for his near-Sun comet search from
> Fountain Hills?  The first comet, C/2002 Y1, was done with what the
> relevant IAU Circular describes as "0.12-m f/5.0 refractor".
> Was this a 5-inch Meade with a focal-reducer?  The second comet
> (C/2005 N1) seems to have been done with a telephoto lens, since
> the instrument is described as a "0.07-m f/2.8 refractor", which
> sounds to me like a 200mm (focal length) f/2.8 telephoto.  Evidently
> the aim was to increase sky-coverage at the expense of going fainter.
> If he's running the fast telephoto wide-open and unfiltered, then
> the images will suck, but obviously succcessful for the intended purpose!
>
> \Brian
> --
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> 

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