36 replies to this topic

[Max Coe]

You pointed out well the diffs between FPL55 and FPL53, specially the ease at machining like brittleness.

How would you do so considering fluorite/FPL53?

I can't, because I never designed with fluorite. Indeed, the only piece that I have ever owned was a blank of about 80mm diameter and 35mm thickness. It was in a box of stuff given to me by a colleague who retired. I passed it on to another colleague who was in the optical window business. He paired it with some of his own product and had a little 5 element refractor made to his own design. (When he told me he was doing this, I sent him a length of thin walled titanium alloy tube to use as the barrel.) The resulting telescope is mostly an office ornament, but he does use it occasionally. Few who have looked through it know - until they are told - that there isn't a single piece of glass in the whole scope. It's all crystals. 

[Princess Leah]

Max

 

If two telescopes are 80mm F6.

Both use FPL53.

 

One is a doublet, the other a well corrected triplet.

 

Am I right in imagining that the curvature of the elements in the triplet would be less pronounced and therefore spherical aberration should be better controlled in the triplet?

 

Thanks Leah.

[Psion]

Yes, if the triplet is made correctly, it will have less aberrations, and higher Strehl.

[Princess Leah]

Wasn't my question.

 

I was away asking if the curvature would be less pronounced.

[Psion]

You asked if the triplet will have less spherical aberration, but that has little to do with the size of the lens curvature but the overall optical design. So, you asked the wrong question.

[Princess Leah]

Not necessarily. It's a good question.

Also the more pronounced a curve, the more likelihood it will scatter light.

That is one reason pilot lights are built on as flat a plane as possible.

[maniack]

I would think the curvature of the elements is related to the focal length of the system. So the curvature would be similar if they are both 80mm f/6 systems. However the typical 80mm triplet is faster than the typical 80mm ED doublet.

[maniack]

Oh and I thought this was already covered somewhat in this thread.

[Max Coe]

Max

 

If two telescopes are 80mm F6.

Both use FPL53.

 

One is a doublet, the other a well corrected triplet.

 

Am I right in imagining that the curvature of the elements in the triplet would be less pronounced and therefore spherical aberration should be better controlled in the triplet?

 

Thanks Leah.

The triplet will be some combination of positive (P) and negative (N) elements. The most common  arrangements are NPN and PNP, although I have an early TMB that is PPN with two different ED glasses in the front. If there is only one N in the triplet, its curvatures may be about the same as the doublet's N, and the curvatures of two of the surfaces of the two Ps will probably be somewhat relaxed compared to the doublet. Similarly, if there is only one P in the triplet, the curvatures of two of the surfaces of the two Ns will probably be somewhat relaxed compared to the doublet.

 

As it happens, I have an 80mm f/6 triplet (NPN) and and f/6.2 ED doublet (PN). I have never disassembled the triplet, but the the doublet's C2 and C3 looked pretty extreme to me when I disassembled it for cleaning, um, 20 years ago. 

[Princess Leah]

Very interesting, thanks for explaining.

 

I also have a F6 triplet and a F6.25 doublet, perhaps they are the same configuration...

Edited by Princess Leah, 18 April 2025 - 01:04 PM.

[Max Coe]

My doublet is an elderly University Optics kit scope, circa late 1980's. The triplet is a much newer AT80EDT.

[Cheshire Cat]

To be honest I would be more interested in the polish and the company responsible for it.