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.


James Stone
 

Roland – very interesting, thank you for sharing this. A related question – with the proper mating elements, can FPL55 approximate performance of FPL53 and FCD100? I’ve heard conflicting information on this and it appears presently available FPL55 is starting to be incorporated into fairly large apos. Thanks in advance for any feedback on this.

 

 

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, January 4, 2022 at 5:54 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] 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
 

I can safely say that FPL53 also lends it self to the same performance using its matching glasses and design. You would not be able to see the difference visually or image-wise. The design curves are not the same of course, so one cannot simply replace the S-FPL53 with the S-FPL55 without changing the various radii.

Roland

-----Original Message-----
From: James Stone <jrs7r@...>
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 5:28 pm
Subject: Re: [ap-ug] FPL53 compared with other SD glasses

Roland – very interesting, thank you for sharing this. A related question – with the proper mating elements, can FPL55 approximate performance of FPL53 and FCD100? I’ve heard conflicting information on this and it appears presently available FPL55 is starting to be incorporated into fairly large apos. Thanks in advance for any feedback on this.
 
 
From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, January 4, 2022 at 5:54 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] 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.


James Stone
 

Thank you very much for the response. It is most appreciated!

Best,

James

 

From: "chris1011@..." <chris1011@...>
Reply-To: "chris1011@..." <chris1011@...>
Date: Tuesday, January 4, 2022 at 6:47 PM
To: James Stone <jrs7r@...>, "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: Re: [ap-ug] FPL53 compared with other SD glasses

 

I can safely say that FPL53 also lends it self to the same performance using its matching glasses and design. You would not be able to see the difference visually or image-wise. The design curves are not the same of course, so one cannot simply replace the S-FPL53 with the S-FPL55 without changing the various radii.

 

Roland

 

-----Original Message-----
From: James Stone <jrs7r@...>
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 5:28 pm
Subject: Re: [ap-ug] FPL53 compared with other SD glasses

Roland – very interesting, thank you for sharing this. A related question – with the proper mating elements, can FPL55 approximate performance of FPL53 and FCD100? I’ve heard conflicting information on this and it appears presently available FPL55 is starting to be incorporated into fairly large apos. Thanks in advance for any feedback on this.

 

 

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, January 4, 2022 at 5:54 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] 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
 

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.


midmoastro
 

Wait a minute. Are you secretly trying to tell us about the upcoming release of the new AP 105mmF7 Starfire triplet apo? GASP! 
Seriously though, thanks Roland.
Todd