More "Schott FK61" Shennanigans

Okay, I've been through the online Schott data sheets and haven't been able to locate "Schott FK61" anywhere.

The closest I've found is Schott N-FK-51A (which used to be called simply FK51). By the numbers this looks less like FPL-53 and more like FPL-51. A bit better than FPL-51 but much closer to it than to FPL-53. H-FK61 by CDGM in China has the same specification as FPL-51. CDGM is the only "FK-61" reference I've been able to find.

Can anyone confirm whether there's any such ED glass as "Schott FK61"?

I am aware of Rolando's comments here:

http://www.astromart.com/forums/viewpost.asp?forum_post_id=627874&poll_id=&news_id=&page=

"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."

But this seems to directly conflict with Mr. Maris' claims here:

http://tech.groups.yahoo.com/group/Stellarvue/message/96600

"Schott FK51 is very close to OK4, Schott FK61 is close to FPL-51 and
Schott makes Calcium Fluorite (CaF2). Recently Ohara was considering
making CaF2 but they decided against it. I really do not have time
to start visiting inconsistently moderated chat rooms right now but
if it is being stated that Schott does not make ED glass, that is
just wrong. Sounds like someone is very misinformed."

Note that there's also a reseller called Astrotelescopes in Applegate, california, that sells a 102mm f/7 which it claims uses "Schott FK-61".

I would hate to be "very misinformed" whether due to my own inadequate research or due to a reseller making false claims. I also have an agenda, and it is not too sell telescopes but rather to discover the truth about available glass types. And yes, I can handle the truth.

Thanks!

- Jim

Ed Neuzil said:

All this angst over glass types cracks me up.

Yeah, me too, to a degree. But it seems important to resellers who use glass type in their formal or informal marketing, which means it likely resonates with potential customers.

It would be nice to know the truth, whatever it is. So Ed, is there such a beast as "Schott FK61"? Inquiring minds want to know!

Regards,

Jim

These are the glasses Scott makes at this time (latest list): http://www.us.schott.com/advanced_optics/english/download/positive_list_0812_for_usa.pdf

Here is the complete Abbe diagram showing all the glasses Scott makes: http://www.us.schott.com/advanced_optics/english/download/abbe_vdnd_us.pdf
There are none in the area above Vd=85. FPL53 and CaF2 have Vd=95 and OK4 has a Vd=91, so they are both higher rated ED glasses.

Schott makes the first generation FK51 glass, same as Chinese FK61 and Ohara FPL51. These glasses can be used to make ED doublets and triplets and are quite inexpensive compared to Ohara FPL53 and especially CaF2 (fluorite).

Rolando

James Barnett quoted Vic Maris as saying:


"Schott FK51 is very close to OK4, Schott FK61 is close to FPL-51 and

Schott makes Calcium Fluorite (CaF2). Thanks!

- Jim

Jim, it looks to me like Vic merely mis-typed a "5" as a "6" in the part you bolded above. The entire sentence makes more sense that way since everyone seems to agree that FK51, OK4 and FPL-51 have similar dispersion characteristics. I suspect that Vic has the chart that Roland posted handy and knows that Schott makes FK51, not FK61. Maybe your whole premise is "much ado about a typo?"

I don't have a stake in this never ending debate by the way. I have owned refractors made by TEC, Tak, SV and TV. All were/are pretty sweet telescopes.

Jim