36 replies to this topic

[Drothgeb]

So I’ve DPAC tested quite a few scopes now, and have noticed that my 2 ES 127 FCD100s are better corrected than any of my FPL53 triplets. The FPLs have straight bars in green and red, but all show a slight bit of over correction in blue. Both of the 127s have straight bars in all three colors. 
 

Is it the glass/design? Of did I just get lucky?

[ris242]

They are very close........in abbe number......it could be one way or the other.  And add FPL55 as well.

 

FCD 1 is a different story.

[sehoon104]

I remember Stellarvue stating how important it is to have the telescope more accurately configured instead of just the glass type. They claimed the quality of the optics remained the same for FCD100 and FPL53 to 130mm (Since there's no FCD100 glass bigger than 130mm). FPL53 is also known to not maintain its performance in larger size, FPL55 does. Probably due to quality control lottery or design of the telescope optics

Edited by sehoon104, 15 April 2025 - 11:20 PM.

[scout]

I remember Stellarvue stating how important it is to have the telescope more accurately configured instead of just the glass type. They claimed the quality of the optics remained the same for FCD100 and FPL53 to 130mm (Since there's no FCD100 glass bigger than 130mm). 

That's not true. Astro-Tech's AT150EDL uses FCD-100 glass.

 

https://astronomics....with-hard-case?

[Psion]

FCD100 and FPL-53 are very similar types of glass, and the quality of the objective depends on the other glasses used and the optical design, including the maintenance of high wavefront accuracy.

 

FCD100 vs FPL-53
Refractive index: 1.437 vs 1.438
Abbe number: 94.94 vs 94.99 (practically identical)

 

It is definitely not true that FCD100 is only used up to 130 mm. Below are examples of telescopes with FCD100 glass featuring 150 mm and 180 mm objective lenses.

 

https://www.apm-tele...t-uf30mm-koffer

https://www.apm-tele...-f56-mit-42-zta

[sehoon104]

Yeah looks like I may have misread or the article is old. I'd assume they're both same performance though. I won't expect common performance gap as OP suggests. But the telescope design matters. I don't know if Explore Scientific is doing something better than the FPL-53 triplets tested, but I wouldn't be too sure.

Edited by sehoon104, 16 April 2025 - 02:17 AM.

[John Huntley]

.... FPL53 is also known to not maintain its performance in larger size, FPL55 does...

At what apertures does FPL53 start to loose it's performance ?

 

Takahashi use two S-FPL53 elements in the TOA 130 and 150. Their performance is about as good as it gets. 

[quilty]

I think you find the diffs not inside the material (type or name) but in it's production quality. As melting shaping and polishing.
And in the mate lens and its fitting.
Can't imagine anyone to find such small diffs at Abbe numbers directly in the star pattern

Edited by quilty, 16 April 2025 - 04:47 AM.

[Drothgeb]

From the sound of it, it’s not the glass. It must be the design/execution then.

 

I’ve read that the ES127 FCD100’s have a polychromatic strehl potential of 97 (TOAs are 98). But I never thought one would live up to the claim. After testing two now, maybe they do, or at least upper 90s. After testing a number of scopes now (DPAC and Star), I’m just a little surprised to find out that my least expensive ES scopes have the best optics. 
 

I do want to mention that all of the scopes actually tested very well. But except for the ES scopes, I could detect a slight bit of overcorrection in blue. Honestly, that’s what I was expecting to see, and was really surprised to see straight bars in blue with the ES scopes. Their optics are much better than given credit for. They just need a better focuser.

[Drothgeb]

I remember Stellarvue stating how important it is to have the telescope more accurately configured instead of just the glass type. They claimed the quality of the optics remained the same for FCD100 and FPL53 to 130mm (Since there's no FCD100 glass bigger than 130mm). FPL53 is also known to not maintain its performance in larger size, FPL55 does. Probably due to quality control lottery or design of the telescope optics

I have a SVX (strehl 99.6). Great scope, but it’s corrected on red instead of green/yellow, so it’s not perfect in blue. Excellent scope for NB imaging though. 

[Eddgie]

Made to the same aperture and focal ratio, and the same level of quality,  FPL-53 and FCD 100, used with the best mating glasses available for each, should have about the same color correction. 

 

Made to the same aperture, but one being with a slower focal ratio than the other, regardless of whether they  are FPL-53 or FCD100, the slower scope would have better color correction than the faster one

 

The Scientific Explorer 127 FCD100s are f/7.5. Most FPL-53 telescopes are in this size range are f/7 or faster.  Now even at 130mm, an f/7 FPL-53 telescope will have virtually color free performance with a polychromatic Strehl of .973, but an f/7.5 FPL-53 or FCD100 instrument would have closer to .99, so while both would be virtually color free, on paper, and in very sensitive tests, the slower scope would have the advantage. 

 

So, the advantage in this case, with the assumption being that the the ES is f/7.5, and the other scopes were f/7, the answer is simply that the focal ratio is slower. If the FPL-53 scope was made to the same speed, the color correction would be about the same. 

Edited by Eddgie, 16 April 2025 - 07:22 AM.

[sehoon104]

At what apertures does FPL53 start to loose it's performance ?

 

Takahashi use two S-FPL53 elements in the TOA 130 and 150. Their performance is about as good as it gets. 

SFPL53 is close to FPL55, so I'd assume it holds the performance for larger apertures. But yes, it's harder to make fast scopes with greater correction. With similar glasses, the design and the speed of the scope explanation makes sense for any difference.

[Lagrange]

At what apertures does FPL53 start to loose its performance ?

 

Takahashi use two S-FPL53 elements in the TOA 130 and 150. Their performance is about as good as it gets. 

 

I think the only issue with FPL53 is that affordable high quality blanks aren't available in larger sizes, so 6" is about the largest lens you can realistically make.

 

Larger than that and you need something else such as FPL55, FCD100 or even fluorite (which is available in blanks larger than 16").

[Brian Carter]

From the sound of it, it’s not the glass. It must be the design/execution then.

 

I’ve read that the ES127 FCD100’s have a polychromatic strehl potential of 97 (TOAs are 98). But I never thought one would live up to the claim. After testing two now, maybe they do, or at least upper 90s. After testing a number of scopes now (DPAC and Star), I’m just a little surprised to find out that my least expensive ES scopes have the best optics. 
 

I do want to mention that all of the scopes actually tested very well. But except for the ES scopes, I could detect a slight bit of overcorrection in blue. Honestly, that’s what I was expecting to see, and was really surprised to see straight bars in blue with the ES scopes. Their optics are much better than given credit for. They just need a better focuser.

 

For what it is worth, my ES127 FCD-100 has the finest optics I've ever looked through and I've owned a lot of scopes over the years.  Everything about the views are just perfection.  

 

I am not a huge fan of Explore Scientific.  This is my second FCD-100 scope, the first was a 4" I bought new at the beginning of Covid.  It arrive very badly out of collimation and I had a very hard time getting it fixed because the issue was an incorrectly assembled lens cell.  Anyhow, it is collimated now and I've had fun taking pictures with it.  Visually, meh.  It isn't an awful scope, it isn't great.  Very well color corrected though.

 

So I think I got lucky with the 127mm, but ES has a big problem with quality control.  I've been reading your tests and I'm not surprised your samples did so well, pretty much matches my experience.  But I also wouldn't have been surprised if you reported a dud.

 

The Hex focuser on that thing had to go though, what a garbage focuser.

[RichA]

At what apertures does FPL53 start to loose it's performance ?

 

Takahashi use two S-FPL53 elements in the TOA 130 and 150. Their performance is about as good as it gets. 

Also depends on the quality of the glass batch and how well the lenses are ground and polished and what the design is. 

[Princess Leah]

I think the only issue with FPL53 is that affordable high quality blanks aren't available in larger sizes, so 6" is about the largest lens you can realistically make.

 

Larger than that and you need something else such as FPL55, FCD100 or even fluorite (which is available in blanks larger than 16").

In my limited experience FPL 53 appears to cause more scatter than fluorite.

[RichA]

In my limited experience FPL 53 appears to cause more scatter than fluorite.

Due to the variations in density in glass (because it's not a crystal) vs. fluorite.

[John Huntley]

Is light scatter more prevalent at glass-air boundaries than it is within the body of the glass ?

 

I've always assumed that coatings are a very important part of controlling / reducing the former cause. 

[Wildetelescope]

Due to the variations in density in glass (because it's not a crystal) vs. fluorite.

As you said above, it depends on the quality of the glass.  Just because a lens uses FPL 53 etc.. does not mean it is a good piece of glass.  Glass composition is only the first step.  Crystals can have defects that affect optics as well.  they are less likely, but they can happen.  Baffling, flocking and so on also effect light scatter.   You generally find scopes using Fluorite lens to be of a higher quality and more expensive, while FPL 53/FC100 lens are found across the price spectrum.  In my experience a good quality ED glass scope will compare well with a fluorite scope.  Both are Great, and I consider myself bless to be able to look through either one when I get the chance, lol:-). 

 

Cheers!

 

JMD

[Princess Leah]

Is light scatter more prevalent at glass-air boundaries than it is within the body of the glass ?

 

I've always assumed that coatings are a very important part of controlling / reducing the former cause. 

Regarding the objective - I relate scatter as a product of inconsistencies in polish /variations in composition of glass/rough coatings.

 

I think of good coatings as increasing light throughput - reducing reflections.

[Max Coe]

At what apertures does FPL53 start to loose it's performance ?

 

 

It doesn't. The properties of FPL53 are quite uniform, regardless of the size of the blank. It's not a function of aperture. It's a function of economics. Ohara would prefer that designers use FPL55 rather than 53 because its a little bit easier to work (less breakage, smoother finish etc.) They have been pushing 55 since its introduction for this reason, and they have scaled back production of 53 in the larger sizes, perhaps to force the issue. The problem is that the label "FPL53" is so famous that every designer wants to wave it like a banner on their premium products. If they had called 55 by a name like FPL53b, the shift to the newer formula might have been smoother for them. Which is better: 53 or 55? Neither. They are nearly identical, but 53 is famous and 55 isn't. 

 

Differences in scatter may be due to variation in the bulk properties of a particular batch, but that would be unusual for Ohara. During my years in the biz, I never returned a blank or strip to Ohara. I think that it is likely that the amount of scatter that is apparent in a particular finished lens depends more on the details of the polishing sequence than on the quality of the starting material. 

[RichA]

Batches, or parts of batches have been rejected by top scope makers because they had too much in the way of local variations in density.

If you want to go nuts and really winnow down a batch to its very best areas, you can use this:

 

https://neutrons.ornl.gov/usans

[quilty]

It doesn't. The properties of FPL53 are quite uniform, regardless of the size of the blank. It's not a function of aperture. It's a function of economics. Ohara would prefer that designers use FPL55 rather than 53 because its a little bit easier to work (less breakage, smoother finish etc.) They have been pushing 55 since its introduction for this reason, and they have scaled back production of 53 in the larger sizes, perhaps to force the issue. The problem is that the label "FPL53" is so famous that every designer wants to wave it like a banner on their premium products. If they had called 55 by a name like FPL53b, the shift to the newer formula might have been smoother for them. Which is better: 53 or 55? Neither. They are nearly identical, but 53 is famous and 55 isn't. 
 
Differences in scatter may be due to variation in the bulk properties of a particular batch, but that would be unusual for Ohara. During my years in the biz, I never returned a blank or strip to Ohara. I think that it is likely that the amount of scatter that is apparent in a particular finished lens depends more on the details of the polishing sequence than on the quality of the starting material.

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?

[Shed9]

I posted this on CN a while back now, appreciate the more technical of us already know this stuff but it is still interesting and useful sometimes.

 

https://www.hoya-opt...sreference.html

 

The Hoya chart is quite comprehensive, but appreciate it doesn't have FPL55 on there however as noted above, they are similar and Ohara's own cross reference compares both FPL53 & 55 to Hoya's FCD100.  

 

https://www.ohara-in.../product/01002/

[Max Coe]

Batches, or parts of batches have been rejected by top scope makers because they had too much in the way of local variations in density.

This depends more than a little on how carefully one writes the specs when ordering the glass. Since I was usually ordering small quantities for prototyping, I wrote tight specs on density, transmissivity, stria, etc. This forced the manufacturer to perform some of the QC before shipment, or in some cases perform an additional annealing cycle or whatever. This increased the the price, of course, but if you want good oats, you must pay a fair price. If you are willing to accept oats that have already been through the horse, you at least have fertilizer. Some manufacturers are easier to work with than others in this regard. I relied a lot on Ohara primarily because I liked the personality of their US rep. She was always pleasant on the phone and never shipped me a piece that didn't meet my spec. (In return, my net 30 was always on time.) If a "top scope maker" is rejecting material, step 1 of mitigation is to compare what was received against the spec that was written. BTW, I have no experience with Russian or Chinese glass manufacturers, so the above comments may not apply to them.