Apochromatic correction (1 Viewer)

Wehr · Jul 9, 2004

Wehr

Leif said:
The Leica APO Televid 77 uses 'flourite glass' according to the manual and 'flouride glass' according to the web site. Given that it gives a superb image it could use blue cheese for all I care.

I think this is not enough. But if you combine blue cheese and red jam you'll get an extraordinary UV and IR blocking.
The picture will become a little dark, but without any chromatic abberation.

Wehr · Jul 9, 2004

CDK said:
I was under the impression that in scopes and binoculars, APO is used as marketing talk. Back focal distance for blue, red and green. I am sure that someone once told me that no one has APO correction in scopes. Fluorite is not used (CAF2), but fluoride, which I think is different.

That's right. Unfortunately there is no definition as an industrial standard. So everybody can call his optics APO. Even Achromats should be colourfree, if you strikly follow the words sense - achromatic=nonchromatic.
It is not important, that an APOs CA-line hits cero for three colours. The actual goal is to have the CA-line both as flat as possible and as close as possible to the cero-line. Future APOs will be made from SD-glass and the next step are real Superachromats forcing CA-line to cero for four colours (soon available from Pentax and Zeiss). But even APOs and Superachromats are not really colourfree. Using prisms in the pass of light damages a good deal of colour-correction.

Pinewood · Jul 9, 2004

gthang said:
In chemistry, fluoride is the name of any chemical compound containing Fluorine.

PS. Periodic Table facts on FLUORINE: Atomic number 10; Type: Inert Gas (Noble Gas).

Fluorine is a halogen, group 17, not an inert gas.

Arthur Pinewood

Leif · Jul 9, 2004

Wehr said:
I think this is not enough. But if you combine blue cheese and red jam you'll get an extraordinary UV and IR blocking.
The picture will become a little dark, but without any chromatic abberation.

Wehr: Excellent! I'll see if the Warehouse Express site lists red jam under scope accessories.

AlanFrench · Jul 9, 2004

henry link said:
I'm pretty certain Takahashi still uses "real" Flourite. Their approach has been to improve the techniques for working with the material. I've owned 5 Takahashi scopes over the years (3 now) and have never experienced any of the problems attributed to CaF2. The oldest scope (FC-50) is about 15 years old with an uncoated Flourite element and still produces a nearly perfect star test. The oldest scope with a multi-coated Flourite element (FS-78) is 10 years old and shows no signs at all of coating deterioration. These are both 2 element APO's, which is possible because the focal ratio is F:8.

I believe you are correct. There is a very simple test to see if a lens element is true fluorite - shine a green laser through it. In real glass you can see the beam in the lens element. In fluorite the beam is invisible within the material.

Clear skies, Alan

henry link · Jul 10, 2004

Alan, Thanks for the tip. Now if I can just find a green laser. Henry

AlanFrench · Jul 10, 2004

Henry,

They have become popular with amateur astronomers as the beam is nicely visible at night, and can be used to point out constellations and the locations of objects in the night sky. Perhaps you have a club in your area and could locate someone with a green laser.

Clear skies, Alan

Apochromatic correction (1 Viewer)

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