FPL-51 vs FPL-53 ???

[Robert_T]

Hi Guys, I note some APO/ED refractors with FPL-51 and some with FPL-53 fluorite lens... what's the difference?

cheers,

Rob

[AGarvin]

Hi Rob,

Here's an extract from what appears to be a report on apochromats:

Ohara is at present a major supplier of ED glasses (although other firms also melt them). They produce three varieties, called "S-FPL 51," "S-FPL 52," and "S-FPL 53." S-FPL51 is the least abnormal and delicate, while S-FPL53 (the latest to be marketed) is softer and can be broken rather easily, though it is less fragile than fluorite. Its optical properties, however, come closest to fluorite of any true glass.

I'm certainly no expert on this stuff, but from what I've read when one is thinking "apo", one also has to take into consideration the mating element as that is what will make the difference. Here is an article by Roland Christen (owner/founder of Astro-Physics telescopes) on colour correction. An extract states:

YES, there is a difference between various ED and Fluorite scopes, but it is
not really the ED or Fluorite that governs the amount of correction, but the
mating element. Normally, even the worst ED design will have 4 times better
color correction than a normal achromat, but it could easily be 20 times
better simply by choosing a different mating element.

Hope this gets you started (and it's ok to qoute these).

Cheers,
Andrew.

[ozstockman (Mike)]

Hi Rob,

you can also check this thread on cloudynights forum. It has the same question in the title :-)

cheers,

Michael

[Robert_T]

Thanks Guys, very helpful both responses

[gts055 (Mark)]

FPL53 is more expensive, than FPL52 or FPL51. You can achieve an equal degree of color correction with all three when given they are matched to suitable mating elements. The difference is that telescope using FPL53, will likely have a shorter focal length. A shorter focal length results in a wider field of view, as desired by imagers. Within each glass type, various grades of quality are availalable. Premium grade of any glass will be guaranteed to a specification regarding striae, bubbles, and clarity etc. Quality comes at a price.
Regards Mark


mmm, my post is late, that Cloudy Nights reference two posts above explains it nicely.