Society for Popular Astronomy

It appears that no one can agree on what a SEMI-Apo telescop
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Author:  skyhawk [ Sat Mar 09, 2019 4:57 pm ]
Post subject:  It appears that no one can agree on what a SEMI-Apo telescop

It appears that no one can agree on what a SEMI-Apo telescope is.

If you read any advert doublets (apo ones, ???) such as the Skywatcher ED and Takahashi FS-60 as just two of many hundreds of examples are "APOchromats", and yet there are arguments that a real APO is a triplet.

From other people

One comment is "A semi-apo tends to be a doublet (achro) with "fancy" glass. FPL-51 or FPL-53 generally.

2 wavelengths are defined for the focal length usually Red and Blue but that allows the Green to drift off from the ideal. It is the amount of this "drift" that they claim makes it semi-apo. In an ED scope this difference is small, and hopefully small enough that the eye does not detect it, so it looks CA free."

Another is

"There is no formal definition of a "semi-apochromatic.""

Another comment

"The Red and Blue are at 0 whereas the Green has a difference of about 0.005 as shown

Now if that were for a normal achro then a semi-apo would have a difference of say 0.002 or 0.003, but less then an achro, however still more then an apo which should be Red=Blue=Green, so 0 difference."

"Manufacturers claiming semi-apo are fine, they at least state what you are buying, but there a a few that stated apo for what is a doublet and so cannot be apo.. "

So are manufacturers misleading us or have we become accustomed to believing a doublet with rare glass can be an APO.

Therefore do I have one proper APO and one .......... Semi APO or two APOS

apo.jpg [ 118.66 KiB | Viewed 383 times ]

Author:  David Frydman [ Sat Mar 09, 2019 9:20 pm ]
Post subject:  Re: It appears that no one can agree on what a SEMI-Apo tele

Horace Dall's camera obscura objective was, I think, a 108mm f/30 doublet corrected for four colours.
The view on the white 2 metre observing table was colour free even with the mobile 135x table magnifier.

Lichtenknecker optics of Belgium used to make 4 element objectives corrected for four colours, some quite large.
A friend had a Lichtenknecker objective, but maybe a simpler design.

I had a Ross 1500mm f/15 triplet objective that showed no false colour at all, so far as I could see.
It had a semi circular small cut out right at the edge in each element.
A metal rod went through these holes to ensure that the elements could not rotate.
The view through it was strange as it looked very unrefractor like with absolutely no false colour.

My Braoadhurst Clarkson doublet had rotated a lot from best position when I took it apart for cleaning. It had the correct pencil marks on the edge that aligned for best position.

For an achromat a 3 inch f/9, 4 inch f/12 and 5 inch f/15 is considered acceptable.
Conrady had stricter constraints. 3 inch f/15, 4 inch f/20 and 5 inch f/25.
So it depends on definition.

For me it is the performance that counts, not any possibly questionable name given.
Advertising is often not very honest.


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