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Schott FK61 glass

Started by andreC, 10/14/2008 04:51PM
Posted 10/14/2008 04:51PM Opening Post
Anyone know what glass the Schott FK61 is equivalent to; fpl51 or fpl53?
Posted 10/14/2008 06:41PM #1
Andre Coleman said:

Anyone know what glass the Schott FK61 is equivalent to; fpl51 or fpl53?

I think the Abbe number for FK-61 is slightly lower than FPL-51. I don't know who makes it but it is my understanding that this is the glass used in the various 102mm F/7 ED refractors such as the Astrotech AT-102ED.

As Roland is quick to point out, it is the combination of the mating element with the ED glass that determines the overall color correction. In the case of the 102mm F/7 ED scopes, I have one, it gets a lot of use. I find that the color correction is much improved over an similar aperture/focal ratio Achromat but that one can still see a tad bit of haze around a bright star when in focus.

Posted 10/14/2008 07:11PM #2
Schott does not make FK61. The Chinese make an ED glass called H-FK61 which is a first generation ED glass similar to Ohara FPL51. There is no availability of any Schott ED glass last time I checked with them. They just don't make it in any production quantity.

You will find just about all ED lenses made in China will use the Chinese made H-FK61 material, no matter what the US marketeers try to tell you.

Posted 10/14/2008 09:20PM #3
Thanks, Jon and Roland.
More marketing, I guess...