FPL53 compared with other SD glasses


Roland Christen
 

Howdy Astronuts,

There seems to be a never ending discussion about the quality and suitability of these various glasses to make a highly corrected triplet lens. There is a common myth about FPL53 that somehow it has higher performance. That is actually not true. The high performance of each of these SD glasses depends almost entirely on the mating element which must be matched in Partial Dispersion to the ED glass. Ohara has a matching set of crown glasses that produces the highest performance level with S-FPL53, but if you use them with Hoya FCD100, then the performance is not ideal. Hoya has a matching set of crown glasses that produces identical performance as the Ohara S-FPL53 set.

As an example, I have posted the performance curve for two identical lenses, 105mmF7 triplet apos, one using FPL53 and the other using FCD100 with each using their proper mating glasses. The lenses are both identical close spaced (for fast cooldown) and optimized for diffraction limited performance from deep red 636nm to violet 436nm. Spacing is 0.55mm between the elements.
The performance curves can be found here:


You can see there is almost zero difference in performance between these two SD glasses (in fact the FCD100 is very slightly superior).

The issue with FPL53 is that the original glass was FPL53 which is discontinued. It was replaced by S-FPL53, which has a slightly lower partial dispersion, so is closer to FPL55 and other SD glasses such as Hoya FCD100. People fixate on the low dispersion part of the glass, but that is not what's important. The important thing that determines overall correction is the Partial Dispersion number, and the index of refraction. To get essentially zero color error the optical designer matches the partials so that the mates cancel out the color error of the SD over as large a spectrum as possible.

Roland Christen
Astro-Physics Inc.

--
Roland Christen
Astro-Physics


thefamily90 Phillips
 

👍


From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Roland Christen via groups.io <chris1011@...>
Sent: Tuesday, January 4, 2022 5:53:50 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>; main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: [ap-gto] FPL53 compared with other SD glasses
 
Howdy Astronuts,

There seems to be a never ending discussion about the quality and suitability of these various glasses to make a highly corrected triplet lens. There is a common myth about FPL53 that somehow it has higher performance. That is actually not true. The high performance of each of these SD glasses depends almost entirely on the mating element which must be matched in Partial Dispersion to the ED glass. Ohara has a matching set of crown glasses that produces the highest performance level with S-FPL53, but if you use them with Hoya FCD100, then the performance is not ideal. Hoya has a matching set of crown glasses that produces identical performance as the Ohara S-FPL53 set.

As an example, I have posted the performance curve for two identical lenses, 105mmF7 triplet apos, one using FPL53 and the other using FCD100 with each using their proper mating glasses. The lenses are both identical close spaced (for fast cooldown) and optimized for diffraction limited performance from deep red 636nm to violet 436nm. Spacing is 0.55mm between the elements.
The performance curves can be found here:


You can see there is almost zero difference in performance between these two SD glasses (in fact the FCD100 is very slightly superior).

The issue with FPL53 is that the original glass was FPL53 which is discontinued. It was replaced by S-FPL53, which has a slightly lower partial dispersion, so is closer to FPL55 and other SD glasses such as Hoya FCD100. People fixate on the low dispersion part of the glass, but that is not what's important. The important thing that determines overall correction is the Partial Dispersion number, and the index of refraction. To get essentially zero color error the optical designer matches the partials so that the mates cancel out the color error of the SD over as large a spectrum as possible.

Roland Christen
Astro-Physics Inc.

--
Roland Christen
Astro-Physics


fernandorivera3
 

Roland thanks for taking the time to educate us 👍

Fernando


Roland Christen
 

I have added a third design which shows a somewhat different optimization. In this one the mating glasses are slightly different to get better correction in the blue-violet. The red end of the spectrum is slightly lower, but the polychromatic spot size is smaller and more energy is focused into a smaller pixel area.

https://www.astrobin.com/9rkfnj/C/

Roland

-----Original Message-----
From: chris1011@...
To: main@ap-ug.groups.io <main@ap-ug.groups.io>; main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Tue, Jan 4, 2022 4:53 pm
Subject: FPL53 compared with other SD glasses

Howdy Astronuts,

There seems to be a never ending discussion about the quality and suitability of these various glasses to make a highly corrected triplet lens. There is a common myth about FPL53 that somehow it has higher performance. That is actually not true. The high performance of each of these SD glasses depends almost entirely on the mating element which must be matched in Partial Dispersion to the ED glass. Ohara has a matching set of crown glasses that produces the highest performance level with S-FPL53, but if you use them with Hoya FCD100, then the performance is not ideal. Hoya has a matching set of crown glasses that produces identical performance as the Ohara S-FPL53 set.

As an example, I have posted the performance curve for two identical lenses, 105mmF7 triplet apos, one using FPL53 and the other using FCD100 with each using their proper mating glasses. The lenses are both identical close spaced (for fast cooldown) and optimized for diffraction limited performance from deep red 636nm to violet 436nm. Spacing is 0.55mm between the elements.
The performance curves can be found here:


You can see there is almost zero difference in performance between these two SD glasses (in fact the FCD100 is very slightly superior).

The issue with FPL53 is that the original glass was FPL53 which is discontinued. It was replaced by S-FPL53, which has a slightly lower partial dispersion, so is closer to FPL55 and other SD glasses such as Hoya FCD100. People fixate on the low dispersion part of the glass, but that is not what's important. The important thing that determines overall correction is the Partial Dispersion number, and the index of refraction. To get essentially zero color error the optical designer matches the partials so that the mates cancel out the color error of the SD over as large a spectrum as possible.

Roland Christen
Astro-Physics Inc.

--
Roland Christen
Astro-Physics


Marcelo Figueroa
 

Thanks for posting this, people really go crazy on the subject without knowing much of what they are talking about. It's good, to see someone who does. 


Ram
 

On Tue, Jan 4, 2022 at 05:27 PM, Roland Christen wrote:
https://www.astrobin.com/9rkfnj/C/
 
Roland, are you considering bringing back the Traveler?
--Ram 


Ronald King
 

Thanks for posting this.  I agree with Marcelo.  People really go crazy over this and fixate on one particular glass without knowing the whole story.


Jeff B
 

Well, I can tell you all right now that never, ever, happens on Cloudy Nights........

Anybody want to buy a nice bridge?

Jeff

On Wed, Jan 5, 2022 at 10:40 AM Ronald King via groups.io <kiloromeo=verizon.net@groups.io> wrote:
Thanks for posting this.  I agree with Marcelo.  People really go crazy over this and fixate on one particular glass without knowing the whole story.


Steven Panish
 

Great information Roland!  Thanks.
Steve

On Wed, Jan 5, 2022 at 11:37 AM Jeff B <mnebula946@...> wrote:
Well, I can tell you all right now that never, ever, happens on Cloudy Nights........

Anybody want to buy a nice bridge?

Jeff

On Wed, Jan 5, 2022 at 10:40 AM Ronald King via groups.io <kiloromeo=verizon.net@groups.io> wrote:
Thanks for posting this.  I agree with Marcelo.  People really go crazy over this and fixate on one particular glass without knowing the whole story.