ED Glass (FPL51, FPL53 etc) - Should I care? - Forums

Topic starter

Sep 30, 2023

Hi all,

I currently image with a humble Redcat 51.

In the new year I'm hoping to compliment that with a longer focal length refractor, something with an aperture around 100-120mm with a focal length of 700-900mm.

I would consider myself a pretty average backyard Astrophotographer, I'm happy with my images but they're nothing special in the grand scheme of things. My question is how concerned should I be about the type of ED glass used in the scopes I'm considering?

Obviously I'm looking to improve and produce the best images I possibly can but how much weight should be given to the type of glass used?

Does it really make a huge difference for someone like me and how much bearing should it have on my decision making? All other things being equal am I likely to notice a difference between, FPL51 and 53 for example?

Scopes containing FPL53 elements and their equivalents seem to come at quite a premium as far as cost is concerned, when compared to scopes that simply state 'ED glass' without specifying the type.

Given my images and current level of proficiency, should I worry about glass type or will that only bear fruit for the super talented/experienced amongst us?

Thanks all
Danny

Engaging

Sean Mc

Sep 30, 2023

I’m a noob, so grain of salt here…

i don’t think that the glass is thr major factor. I’d pick some scopes in your price range and have a look at pics here. People have diffenet tolerances of abnormal stars in the corners. I see some pics that people rave about the optics and the stars are unacceptable to me. Then look at what people have done to modify a scope to make it reasonable. I really think it comes down to the design of the individual scope. For example, I’d take certain select svbony scopes over some much more expensive ones from more “premium” brands that claim “full frame”.

SemiPro

IOTD Staff

Sep 30, 2023 · Edited: Sep 30, 2023

https://www.stellarvue.com/optical-glass-types/

Here is this classic article if you have the time to read. The short version is that glass is something but not everything.

What refractors were you thinking of?

Erlend Langsrud

Sep 30, 2023

Yes, you should worry a little bit.

I think a medium/big FPL51 doublet of medium focal length will not quite give the "best images you possibly can"

A FPL51 triplet or FPL53 doublet is better.

Danny Lee

Topic starter

Oct 1, 2023

https://www.stellarvue.com/optical-glass-types/

Here is this classic article if you have the time to read. The short version is that glass is something but not everything.

What refractors were you thinking of?

Thanks Semi, interesting read.

There are a few from Askar I'm considering, the 107 PHQ and I'm keeping an eye on their new 103 and 120 APOs. Waiting for them to get into a few more hands first and for some images to be posted, there aren't many example images yet and not a lot of information about them. They're a lot more affordable than the PHQ series but there must be a reason for that.

TS Optics also have a few refractors in that class, their photoline and CF models. All are triplets and the PHQ is a quad.

Freddy Meiresonne

Oct 1, 2023

If you opt for FPL-51 then make it a triplet. If you opt for FPL-53 a doublet can work well expecially at F/7.

I have a FPL-53 doublet at F/7, i can't see much CA on photographs, if any…

Danny Lee

Topic starter

Oct 1, 2023

Freddy Meiresonne:
If you opt for FPL-51 then make it a triplet. If you opt for FPL-53 a doublet can work well expecially at F/7.

I have a FPL-53 doublet at F/7, i can't see much CA on photographs, if any...

Thanks Freddy, considering all of the scopes I'm looking at are triplets/quads I guess glass type doesn't need to of the highest priority.

andrea tasselli

Frequent contributor

Oct 1, 2023

You really should. I'd not touch anything less than a triplet with the right glass match between crown and flint but that's me, mostly. All other things being the same (and rarely they are) a Flourite/FPL53 airspaced triplet is to be preferred overall. Ed doublets are just achromats with reduce color dispersion.

Anderl

Oct 1, 2023

Care or you will always ask your yourself if you are missing out on quality.

Danny Lee

Topic starter

Oct 1, 2023

Thanks all, of course another issue is a lot of manufacturers and scopes don't specify glass type other than stating X number of ED elements.

Makes it tricky to compare one scope to another as far as glass goes.

Alan Brunelle

Oct 1, 2023 · Edited: Oct 1, 2023

The type of glass chosen by the optic engineer is based on trying to achieve a means to an end. That is the reduction of aberrations due to differences in color dependent focus position within the desired focal plane. If the ends are achieved well, then the means are irrelevant. Instead of looking at the glass used, look at the ability of the optic to correct chromatic aberration. Often spot diagrams at three different wavelengths are given by the manufacturer. If an optic has perfect color correct, why would you care what glass is in the optic? Within reason, you should not. However, if the engineers only can otherwise accomplish the task by using 20 optical elements, then you might be concerned about total light throughput regardless of f-ratio. Each transit of an optical element "consumes" some of the available light input. And it may only be a percent or two, but it does quickly add up in a compounding way. Exotic glasses tend to achieve the desired result with less glass elements used. However, you are only asking to compare two exotic glasses. The additional issue you should consider is how does that correction hold up in the native optic when you add reducers or other elements within the imaging train? As is typical of manufacturers, the listing of these exotic elements is used as a marketing ploy. They know that in this market, many many individuals eat this stuff up. Again all that matters is does the engineered design work. I'm not saying that they do not work. They do. I am saying they may not be necessary, or the difference between the two you brought up in your question may be irrelevant. Also what does your practice of this sport require of the optic?

Also note that the optical designs have to work harder to achieve good corrections the shorter the focal ratio. Longer focal ratios, less so. Buying a longer focal ratio optic and spending a lot of extra money on exotic elements may be a waste of that money. A longer focal ratio optic can be shortened with a focal reducer. The challenge may then be that this could bring the optic into poor chromatic correction. A focal reducer with an exotic element might then be preferable.

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enta

Oct 1, 2023 · Edited: Oct 1, 2023

Everything beyond 6" you need good glass or it won't work well.
FPL 53 or FCD-100 doesn't matter, all top tier glass.
FPL51 or FCD-1 is good glass but will have minor impact on the quality.

If you want to get the best results you need the best glass, if you just want to explore the sky it's not that important.
I wouldn't go without the "best" glass because I don't want to deal with the downsides because I cheaped out.

Björn Arnold

Oct 1, 2023

What matters the most is what the optical engineers and the manufacturing make out of it. Therefore, I'd always take a look at the diagrams and do some research on the experience of other people and especially images captured with the equipment of interest.

You can have the best glass but if the folks don't know how to make a good scope out of it, it's worthless.

CS, Björn

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Sean Mc

Oct 1, 2023

I fell for the “fpl53 triplet” trap. Can a 53 triplet be better? Absolutely. Is it always better? Hell no.

i bought an expensive 53 triplet. It had a bed cell, so i exchanged it for another… that had a bad cell :/

then i noticed that NOBODY online was posting images with aberration free stars. The manufacturer claimed full frame “illumination”. Stars start showing aberration before they reach apsc even though technically it “illuminates” full frame.

spot diagrams reflected this fact.

i now have a scope that has perfect stars, and the manufacturer doesn’t list the glass type.

Choosing a car because it’s more expensive and has carbon fiber doesn’t automatically get you a better car.

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Joe Linington

Oct 1, 2023 · Edited: Oct 1, 2023

I have an "unspecified glass" SharpStar (Askar) triplet (76EDPH) and an FPL-51 Doublet (SV503/102ed). In NB imaging, the SV503 produces sharper stars and lower FWHM. In wideband OSC imaging, the SV503 has some blue haloing. I use an Astronomik L3 UV/IR to reduce that and I am happy with the results. I am also planning on buying the Antlia LRGB filter set which cuts off a little higher than most to avoid the blue halo area. The 76EDPH shows no halos but again, slightly larger stars even though it's a shorter F/L. When I bought my 102mm scope, the cost difference between a FPL-51 doublet and an FPL-53 triplet was enough to buy a mono camera and filters vs OSC. I had to weigh what would give me a better boost in quality. I chose a mono camera and filters with the lesser scope and I think I made the right call. Maybe some day I will be able to afford a nice FPL-53 triplet to compare but I think that will be a long way off.

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SemiPro

IOTD Staff

Oct 1, 2023

Danny Lee:
https://www.stellarvue.com/optical-glass-types/

Here is this classic article if you have the time to read. The short version is that glass is something but not everything.

What refractors were you thinking of?

Thanks Semi, interesting read.

There are a few from Askar I'm considering, the 107 PHQ and I'm keeping an eye on their new 103 and 120 APOs. Waiting for them to get into a few more hands first and for some images to be posted, there aren't many example images yet and not a lot of information about them. They're a lot more affordable than the PHQ series but there must be a reason for that.

TS Optics also have a few refractors in that class, their photoline and CF models. All are triplets and the PHQ is a quad.

Yeah the refractor market is definitely saturated with new telescope these days. @andrea tasselli is right that if you can, try to look out for exactly how many of the elements are actually high quality glass. Some of the cheaper scopes will advertise the super high quality stuff, but then you find out that one of the three elements are actually the good glass.

Check out the spot diagrams if you can, but keep in mind that manufactures put their best foot forward with those and individual telescopes may vary.

Here is another point for you to consider; imaging time. Would you believe me if I told you that your Redcat51 actually outperforms the 107PHQ in terms of SNR collection?

In the red we see that the 107PHQ collects way more object signal than the Redcat51. This make sense, it having the bigger objective after all. In the blue we see that due to the large difference in focal length, the Redcat51 actually delivers more signal per pixel assuming you use the 2600MC for both. Taking your recent M31 for example, you would need roughly 8 hours of integration time on the 107PHQto reach the same SNR level as the 3 hours and 54 minutes you have on the Redcat51.

There are certainly those out there who find that trading speed for a higher resolution is acceptable. On the other hand, what I posted above is often what leads people away from refractors to reflectors when it comes to higher focal lengths. You need a considerable amount of aperture at higher focal lengths to get a decent focal ratio for imaging. That considerable amount of aperture is prohibitively expensive in the refractor space.

Of course, reflectors introduce the joy of collimation to the host of issues you have to deal with in addition to tilt and backfocus.

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Sean Mc

Oct 1, 2023

Except that if an object fits in the fov of the 107phq, it’ll collect more light on that object during a given time than the redcat will. The redcat might seem faster, but on a smaller target it loses out on resolution and collects more light from areas you’re going to crop.

it’s all relative.

SemiPro

IOTD Staff

Oct 1, 2023 · Edited: Oct 1, 2023

Sean Mc:
Except that if an object fits in the fov of the 107phq, it’ll collect more light on that object during a given time than the redcat will. The redcat might seem faster, but on a smaller target it loses out on resolution and collects more light from areas you’re going to crop.

it’s all relative.

The 107PHQ collects more light as a whole, but it is more spread out than the Redcat51 and thus less of it hits each individual pixel. On the Redcat, yes the object will be much smaller and have a softer resolution, but it's SNR will still be better than the 107PHQ given the same camera, no binning, and same integration time.

The 107PHQ will obliterate the RedCat51 in point sources (aka stars) but in extended sources (everything else), the Redcat51 still wins.

The crux here is that everything I am saying is assuming the same camera with the same pixel size. If we tried to get the Recat51 down to the same sampling size as the 107PHQ then yeah the show is over for the RedCat51.

This is where bigger telescopes get their advantage from. They can get away with cameras with larger pixels (or in the modern CMOS era, binning by 2x or 3x) and still having finer resolutions than smaller telescopes. That is their true advantage when it comes to imaging.

We can see that by making the resolutions the same, the Redcat51 loses its SNR advantage.

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Alan Brunelle

Oct 1, 2023 · Edited: Oct 2, 2023

Yeah the refractor market is definitely saturated with new telescope these days. @andrea tasselli is right that if you can, try to look out for exactly how many of the elements are actually high quality glass. Some of the cheaper scopes will advertise the super high quality stuff, but then you find out that one of the three elements are actually the good glass.

Please understand that this is incorrect and I am not sure Andrea was making that point. Exotic, ED, FP glasses etc. are not necessarily better. You should stop thinking of these different glasses as being high quality or better or good. They just have a different refractive index for particular wavelength ranges. And yes they do cost more. It is critical in these designs for the ED glass element to work with the non ED elements to get all the light over the targeted wavelength range to focus fully at the same image plane! That is why you see different optical designs using different glasses and air space, oil space, etc. In fact putting a great ED element together with poor "regular" glass will make a crap expensive telescope. I have not seen a multi-element ED-any telescope that has all ED elements. @Danny Lee, while a first go for information from forums is a fine start, please inform yourself just a little bit from established sources before you drop your money on this. And please read some of the very good sensible replies from just above on this thread from those who have had experiences with both ED and "regular" glass telescopes. Note above I asked what type of imaging you will be doing. As well stated in a following post just below that, if you will be exclusively doing narrow band imaging, then you absolutely do not need a mult-spectral ED focusing rig. You won't care if blue focusses at the same focal plane as red because you will be imaging those wavelengths separately and need to refocus. If RGB, the broadness of the filters might make sense for simple ED, but see a solution to this from the post above. And if OSC, you definitely will need good color correction. All other features and performances aside, I will repeat once again, as others here have stated, forget the ED when shopping. Look at the performance spot size and be sure the data they present is reliable. And decide if you will be exclusively narrow band or not.

As an aside, last week I rented an optic that contained 4 ED elements, of differing type! Yes 4. More elements (let alone ED elements) than many telescopes have or need. Are you going to argue that it must be better than anything else? What about the 13 other glass elements that make this thing work? (Yes a 17 element optic!) Do you just assume those elements are crap just because they use standard glass? Buy for the performance (substantiated performance) not the identity of what is in the tube. Your goal is the best image you can get. Cost usually is a factor, though in this community less so from what I have seen, then weigh the benefits vs. cost. If cost is not a factor, you will probably want a Astro-Physics refractor. Probably performance matched by no one selling telescopes today. But you will wait years to get it! Example, 110 Starfire: Only 3 elements. Only one ED element. 2 very carefully matched (see what I mean about design being critical) non-ED elements. I can guarantee those "regular" glass elements are not crap. And look up the web page for the Astro-Physics telescopes and see what information I am talking about.

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Matthew Proulx

Oct 2, 2023 · Edited: Oct 2, 2023

Alan Brunelle:
Yeah the refractor market is definitely saturated with new telescope these days. @andrea tasselli is right that if you can, try to look out for exactly how many of the elements are actually high quality glass. Some of the cheaper scopes will advertise the super high quality stuff, but then you find out that one of the three elements are actually the good glass.

Please understand that this is incorrect and I am not sure Andrea was making that point. Exotic, ED, FP glasses etc. are not necessarily better. You should stop thinking of these different glasses as being high quality or better or good. They just have a different refractive index for particular wavelength ranges. And yes they do cost more. It is critical in these designs for the ED glass element to work with the non ED elements to get all the light over the targeted wavelength range to focus fully at the same image plane! That is why you see different optical designs using different glasses and air space, oil space, etc. In fact putting a great ED element together with poor "regular" glass will make a crap expensive telescope. I have not seen a multi-element ED-any telescope that has all ED elements. @Danny Lee, while a first go for information from forums is a fine start, please inform yourself just a little bit from established sources before you drop your money on this. And please read some of the very good sensible replies from just above on this thread from those who have had experiences with both ED and "regular" glass telescopes. Note above I asked what type of imaging you will be doing. As well stated in a following post just below that, if you will be exclusively doing narrow band imaging, then you absolutely do not need a mult-spectral ED focusing rig. You won't care if blue focusses at the same focal plane as red because you will be imaging those wavelengths separately and need to refocus. If RGB, the broadness of the filters might make sense for simple ED, but see a solution to this from the post above. And if OSC, you definitely will need good color correction. All other features and performances aside, I will repeat once again, as others here have stated, forget the ED when shopping. Look at the performance spot size and be sure the data they present is reliable. And decide if you will be exclusively narrow band or not.

As an aside, last week I rented an optic that contained 4 ED elements, of differing type! Yes 4. More elements (let alone ED elements) than many telescopes have or need. Are you going to argue that it must be better than anything else? What about the 13 other glass elements that make this thing work? (Yes a 17 element optic!) Do you just assume those elements are crap just because they use standard glass? Buy for the performance (substantiated performance) not the identity of what is in the tube. Your goal is the best image you can get. Cost usually is a factor, though in this community less so from what I have seen, then weigh the benefits vs. cost. If cost is not a factor, you will probably want a Astro-Physics refractor. Probably performance matched by no one selling telescopes today. But you will wait years to get it! Example, 110 Starfire: Only 3 elements. Only one ED element. 2 very carefully matched (see what I mean about design being critical) non-ED elements. I can guarantee those "regular" glass elements are not crap. And look up the web page for the Astro-Physics telescopes and see what information I am talking about.

You're not factoring in that the 110GTX needs a flattener at the very least which is 2 lenses and the reducer/flattener is a triple lens ED. Most of these 4+ lens refractors are reducing and need more ED lenses because of CA otherwise.

I wouldn't waste any money on anything less than a Triplet regardless. You can get away with less ED on longer focal lengths but they really start to show their purpose when you get to shorter focal lengths. And yes I see CA in a lot of images on here that people say they don't see.

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Alan Brunelle

Oct 2, 2023

Matthew Proulx:
Alan Brunelle:
Yeah the refractor market is definitely saturated with new telescope these days. @andrea tasselli is right that if you can, try to look out for exactly how many of the elements are actually high quality glass. Some of the cheaper scopes will advertise the super high quality stuff, but then you find out that one of the three elements are actually the good glass.

Please understand that this is incorrect and I am not sure Andrea was making that point. Exotic, ED, FP glasses etc. are not necessarily better. You should stop thinking of these different glasses as being high quality or better or good. They just have a different refractive index for particular wavelength ranges. And yes they do cost more. It is critical in these designs for the ED glass element to work with the non ED elements to get all the light over the targeted wavelength range to focus fully at the same image plane! That is why you see different optical designs using different glasses and air space, oil space, etc. In fact putting a great ED element together with poor "regular" glass will make a crap expensive telescope. I have not seen a multi-element ED-any telescope that has all ED elements. @Danny Lee, while a first go for information from forums is a fine start, please inform yourself just a little bit from established sources before you drop your money on this. And please read some of the very good sensible replies from just above on this thread from those who have had experiences with both ED and "regular" glass telescopes. Note above I asked what type of imaging you will be doing. As well stated in a following post just below that, if you will be exclusively doing narrow band imaging, then you absolutely do not need a mult-spectral ED focusing rig. You won't care if blue focusses at the same focal plane as red because you will be imaging those wavelengths separately and need to refocus. If RGB, the broadness of the filters might make sense for simple ED, but see a solution to this from the post above. And if OSC, you definitely will need good color correction. All other features and performances aside, I will repeat once again, as others here have stated, forget the ED when shopping. Look at the performance spot size and be sure the data they present is reliable. And decide if you will be exclusively narrow band or not.

As an aside, last week I rented an optic that contained 4 ED elements, of differing type! Yes 4. More elements (let alone ED elements) than many telescopes have or need. Are you going to argue that it must be better than anything else? What about the 13 other glass elements that make this thing work? (Yes a 17 element optic!) Do you just assume those elements are crap just because they use standard glass? Buy for the performance (substantiated performance) not the identity of what is in the tube. Your goal is the best image you can get. Cost usually is a factor, though in this community less so from what I have seen, then weigh the benefits vs. cost. If cost is not a factor, you will probably want a Astro-Physics refractor. Probably performance matched by no one selling telescopes today. But you will wait years to get it! Example, 110 Starfire: Only 3 elements. Only one ED element. 2 very carefully matched (see what I mean about design being critical) non-ED elements. I can guarantee those "regular" glass elements are not crap. And look up the web page for the Astro-Physics telescopes and see what information I am talking about.

You're not factoring in that the 110GTX needs a flattener at the very least which is 2 lenses and the reducer/flattener is a triple lens ED. Most of these 4+ lens refractors are reducing and need more ED lenses because of CA otherwise.

I wouldn't waste any money on anything less than a Triplet regardless. You can get away with less ED on longer focal lengths but they really start to show their purpose when you get to shorter focal lengths. And yes I see CA in a lot of images on here that people say they don't see.

Maybe the 130 is a better example, with the flattener being 4 elements, none apparently ED. It makes sense that a reducer would make use of ED elements. Focal reduction definitely puts an extra refractive burden on the system, post objective corrections. My point is one should not buy an optic because of the type of ED glass and how much of it is advertised. Performance is the issue. And the use of non-ED elements does not equate to cheap or bad optics. Typically the diversity of refractive indices is is integral to the design.

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