255 replies to this topic

[neaptide]

Hello all

 

I was wondering if there is a significant difference between fpl-51 and fpl-53 glass? I would primarily using my next scope for short exposure eaa. I am currently considering a Stellarvue 80mm Nighthawk Next Gen ED doublet with FPL-51 glass. Any opinions on the glass or the scope?

 

Thanks

John

[rolo]

FPL-53 is a better choice for sure...

[rkaufmann87]

Your question comes up almost weekly on these forums. You may want to search rather than create a new discuss. What you will find is there is a lot of information and you will need to provide much more information such as what  you will use your telescope for and try to be as specific as possible.

[Element79]

FPL-53 is nearly twice as effective as FPL-51 in controlling CA.  That doesn't mean that FPL-51 cannot be used to make a color free telescope but by using it the telescope will either have to be a triplet or the focal length will have to be made longer.

[Redbetter]

Hello all

 

I was wondering if there is a significant difference between fpl-51 and fpl-53 glass? I would primarily using my next scope for short exposure eaa. I am currently considering a Stellarvue 80mm Nighthawk Next Gen ED doublet with FPL-51 glass. Any opinions on the glass or the scope?

 

Thanks

John

I have recently acquired my first two ED scopes, an 80mm f/7.5 ED doublet with FPL-53 and a 110mm f/7 ED doublet with FPL-51 (or Chinese equivalent.)  While I like them both, the  FPL-51 shows some color on bright objects at high magnification.  Out of curiosity I even tested stopping down the 110 to see if increasing the focal ratio considerably would eliminate the color.  It reduced color at f/8.56 but the 90mm aperture still had considerably more color than the 80mm at f/7.5 ED.  From what little I have seen there is a noticeable difference.

[Cometeer]

If you go for FPL-51, I recommend at least a triplet. If FPL-53, some long focal length doublets works nearly as well as a triplet.

 

Some threads to get you started:

http://www.cloudynig...l-53-vs-fpl-51/

http://www.cloudynig...ass-what-gives/

http://www.cloudynig...-to-save-money/

[Agatha]

John (neaptide),

 

Doing a search for info here is some times a little frustrating.  Here is one thread that I found:    http://www.cloudynig...ass-what-gives/

 

FPL53 is a higher quality glass that is more expensive.  But, the design of the lens is very important. 

 

It is quite well known around here that the Astro-Tech 111 (no longer made) is a wonderful telescope.  It is a triplet using FPL51 designed by Thomas Back.  But there are doublets that use a FPL53 that are very nice.

 

You can try to find more info as there is a ton of it.  And hopefully more knowledgable members than I will stop in to offer good info.

 

Good luck   

 

edit:  Cometeer is much quicker and has provided good stuff.

Edited by Agatha, 18 January 2017 - 06:17 PM.

[neaptide]

Thanks for the links everyone. I am in the midst of doing the research now. As of now I am leaning toward holding out for a scope that has the fpl-53 glass. Any significant color distortion will drive me nuts. 

[rkaufmann87]

Thanks for the links everyone. I am in the midst of doing the research now. As of now I am leaning toward holding out for a scope that has the fpl-53 glass. Any significant color distortion will drive me nuts. 

Don't be so quick to judge, I have a triplet with the equivalent of FPL 51 glass and it is outstanding. You can see many sample photos by clicking in the Photo Gallery and searching AT130EDT to see some examples. There are also many examples of scopes that don't use FPL 53 glass that are outstanding from many manufacturers.

 

Before you focus on a glass type take some time to visit some star parties in your area and look through a various scopes, you may be very surprised at what you prefer.

[mattyk-usa]

I am 100% on board with Roger here.  Discussions of glass in the absence of a complete OTA design, especially with regard to color correction, will bear little fruit.  FPL-51 based OTAs can have excellent color performance, depending on the mating glass, design and manufacturing quality.  The same can be said of FPL-53.  

 

I think the one thing we can say definitively is if a scope designer/manufacturer starts with FPL-53 glass, they have an easier job of it making a well-corrected OTA for any given focal length. 

 

Both materials can be used to make a superb instrument.

[Jared]

One clarification to an earlier post...  Neither glass is higher "quality" per se.  They are both available at a range of quality levels.  It's just that FPL-53 has some characteristics that make it possible to get better color correction at a given aperture and focal length.  It is possible with either material and the appropriate mating element(s) to get color correction that is good enough to satisfy even the most discriminating observer, particularly when you are talking about 80mm scopes.  Personally, I wouldn't reject a given scope just because it has FPL-51 in it.  My favorite 80mm of all time has OK-4 glass in it which has an Abbe number somewhere between the two, and I have owned an FPL-51 triplet that was easily capable of meeting astrophotography requirements for color correction, let alone visual requirements.

 

If you want to add another layer of complexity to your decision... Triplet or doublet?  FPL-53 doublet vs. FPL-51 triplet?  Honestly, you can drive yourself crazy with this stuff.  The best approach is either to go to a star party and take a look at various scopes, or buy from a dealer with a generous return policy and work your way up the price scale till you find a scope that makes you happy.

[Jon Isaacs]

I am 100% on board with Roger here.  Discussions of glass in the absence of a complete OTA design, especially with regard to color correction, will bear little fruit.  FPL-51 based OTAs can have excellent color performance, depending on the mating glass, design and manufacturing quality.  The same can be said of FPL-53.  

 

I think the one thing we can say definitively is if a scope designer/manufacturer starts with FPL-53 glass, they have an easier job of it making a well-corrected OTA for any given focal length. 

 

Both materials can be used to make a superb instrument.

 

When one is talking about affordable doublets like the StellarVue Next Generation Nighthawk, my experience is that the color correction is very consistent with the differences between FPL-51 and FPL-53.  With the common mating elements, FPL-53 offers about 1.8x the color correction of FPL-51 and this translates into much less chromatic aberration seen at the eyepiece.   

 

FPL-51 based doublets I have owned:  WO 66SD, AT 72ED, Astro-Tech,  AT-102ED, Looked through:  Orion 110mm F/7 Premium

 

FPL-53 based doublets I have owned:  WO Megrez II FD, AT-80LE, Orion Eon 120.  Looked through ED-100.  

 

The 120mm Skywatcher is testament to what can be done with quality and/or glass choice.  At 120mm F/7.5, even though it's FPL-53 it ought to show chromatic aberration but I don't see it even on Venus.. 

 

The scope in question, it's not an ED-80 when it comes to color correction.  

 

Jon

[Jon Isaacs]

Personally, I wouldn't reject a given scope just because it has FPL-51 in it.  My favorite 80mm of all time has OK-4 glass in it which has an Abbe number somewhere between the two, and I have owned an FPL-51 triplet that was easily capable of meeting astrophotography requirements for color correction, let alone visual requirements.

 

 I am quite sure that your favorite 80mm was a triplet made in Russia.  

 

While the Abbe number for OK-4 is between that of FPL-51 and FPL-53, it is nearly that of FPL-53.. 

 

Here are the Abbe numbers (Vd) of some commonly used ED glasses:
FPL-51 (Ohara): 81.54
H-FK61 (CDGM): 81.61
FCD1 (Hoya): 81.61
OK-4 (LZOS): 92.1
FPL-53 (Ohara): 94.93
CaF2: 94.99 (Fluorite)

 

Jon Isaacs

[mattyk-usa]

 

I am 100% on board with Roger here.  Discussions of glass in the absence of a complete OTA design, especially with regard to color correction, will bear little fruit.  FPL-51 based OTAs can have excellent color performance, depending on the mating glass, design and manufacturing quality.  The same can be said of FPL-53.  

 

I think the one thing we can say definitively is if a scope designer/manufacturer starts with FPL-53 glass, they have an easier job of it making a well-corrected OTA for any given focal length. 

 

Both materials can be used to make a superb instrument.

 

When one is talking about affordable doublets like the StellarVue Next Generation Nighthawk, my experience is that the color correction is very consistent with the differences between FPL-51 and FPL-53.  With the common mating elements, FPL-53 offers about 1.8x the color correction of FPL-51 and this translates into much less chromatic aberration seen at the eyepiece.   

 

FPL-51 based doublets I have owned:  WO 66SD, AT 72ED, Astro-Tech,  AT-102ED, Looked through:  Orion 110mm F/7 Premium

 

FPL-53 based doublets I have owned:  WO Megrez II FD, AT-80LE, Orion Eon 120.  Looked through ED-100.  

 

The 120mm Skywatcher is testament to what can be done with quality and/or glass choice.  At 120mm F/7.5, even though it's FPL-53 it ought to show chromatic aberration but I don't see it even on Venus.. 

 

The scope in question, it's not an ED-80 when it comes to color correction.  

 

Jon

 

Jon

 

I"ll try to clarify the point I'm making (and it's one that I know you've made before yourself).  Simply asking whether one glass type is better than another is going down a rabbit hole of dubious benefit for the person deciding on a purchase, if that is the overriding criteria.  One needs to start with a finished product, much as you did above, and make a determination based on its performance.  

 

The discussion of of materials like FPL-51/53 is really academic (i.e. fruitful from an "understand the science" perspective but not terribly useful from a product comparison perspective), since no one I am aware of is making products that differ only in the material used in one objective element.   One must take into account all of the design elements, not just the glass in part of the objective assembly.  I think that message is important to convey to anyone starting down this path.  I put far more weight on designer/manufacturer than I do on materials (and I'll bet you do too ).

[neaptide]

The original post came about because I was looking at a preowned scope that advertised as a Stellarvue 80mm Raptor apo with FPL-53 glass. Turns out the scope is actually a Nighthawk Next Generation ED with FPL-51 glass. So I guess my question should have been " How is the color correction on a Nighthawk Next Gen."?

I know that if I see color distortion I'm not going to be happy with the purchase.

Anyone have any experience with this particular scope?

Edited by neaptide, 18 January 2017 - 07:52 PM.

[KevH]

The original post came about because I was looking at a preowned scope that advertised as a Stellarvue 80mm Raptor apo with FPL-53 glass. Turns out the scope is actually a Nighthawk Next Generation ED with FPL-51 glass. So I guess my question should have been " How is the color correction on a Nighthawk Next Gen."?
I know that if I see color distortion I'm not going to be happy with the purchase.
Anyone have any experience with this particular scope?

I owned a SV80ED which I believe came after the NHNG although the specs were identical. It was not an APO and readily showed fringing on bright objects. Anything over 100x on the lunar terminator showed thin violet fringes along all crater shadows. Jupiter's disc also showed a thin violet fringe. Not terrible but certainly visible. By comparison, my current Vixen ED81S shows no color on anything but is slower at f7.7 and uses fpl53...

Edited by KevH, 18 January 2017 - 08:28 PM.

[Scott Beith]

The original post came about because I was looking at a preowned scope that advertised as a Stellarvue 80mm Raptor apo with FPL-53 glass. Turns out the scope is actually a Nighthawk Next Generation ED with FPL-51 glass. So I guess my question should have been " How is the color correction on a Nighthawk Next Gen."?

I know that if I see color distortion I'm not going to be happy with the purchase.

Anyone have any experience with this particular scope?

http://www.cloudynig...on-deluxe-r1741

 

Here you go.  I hope this helps.

[Jon Isaacs]

I"ll try to clarify the point I'm making (and it's one that I know you've made before yourself).  Simply asking whether one glass type is better than another is going down a rabbit hole of dubious benefit for the person deciding on a purchase, if that is the overriding criteria.  One needs to start with a finished product, much as you did above, and make a determination based on its performance. 

The discussion of of materials like FPL-51/53 is really academic (i.e. fruitful from an "understand the science" perspective but not terribly useful from a product comparison perspective), since no one I am aware of is making products that differ only in the material used in one objective element.   One must take into account all of the design elements, not just the glass in part of the objective assembly.  I think that message is important to convey to anyone starting down this path.  I put far more weight on designer/manufacturer than I do on materials (and I'll bet you do too ).

 

 

I think the discussion of whether the difference between FPL-51 versus FPL-53 is worth discussing is academic.  Academically, one must take into account all of those design elements to decide.  

 

Practically though, when one is looking at an 80mm F/7 ED/apo doublet manufactured in China by either United Optics or Long Perng, the glass type seems to be indicative of the color correction.  It may be possible to use an out of the ordinary mating element with FPL-51 to get improved color correction but that does not seem to have happened.  

 

And reading Scott's review of the scope in question is consistent with my experiences with a variety of scopes manufactured by KU and LP using the two ED glasses. 

 

Jon

[SandyHouTex]

Hello all

 

I was wondering if there is a significant difference between fpl-51 and fpl-53 glass? I would primarily using my next scope for short exposure eaa. I am currently considering a Stellarvue 80mm Nighthawk Next Gen ED doublet with FPL-51 glass. Any opinions on the glass or the scope?

 

Thanks

John

FPL-53 is the equivalent of Fluorite.  FPL-51 is not.  Here's the Vd versus Nd chart from Ohara:

 

http://www.oharacorp...-chart-2016.pdf

 

When making a doublet, you want the largest Vd between the FPL element and the mating element on a straight line on the graph to provide the best control of chromatic abberation.  As you can see FPL-53 is at Vd = 95, whereas FPL-51 is only Vd = 80.

[Redbetter]

Tying this back into the OP's clarification in post #8:  "Any significant color distortion will drive me nuts"  

 

It is more straightforward to design a better corrected FPL-53 objective set than with FPL-51 equivalent.  So if one is sensitive to chromatic aberration, the FPL-53 would be a better choice assuming similar quality (although not necessarily price.)

 

As I have said, I like the 110mm ED with the FPL-51 equivalent and it suits my needs at this time, but it is not color free and I didn't expect it to be.  I am looking forward to seeing what it can do on Jupiter in some side-by-side with larger scopes, where any flaws will become apparent. 

 

I wonder if the small level of CA will hurt contrast in a noticeable way or if aperture limitations will be controlling?  (Aperture mask on larger reflectors will provide some comparison at equivalent aperture.)   So far I've only been able to get a glimpse of Jupiter in awful seeing through a the cloud deck and still rather low in the sky.  Not much conclusion to draw from that.  I know from experience that a lot of CA in small indifferently figured short ratio achromats results in a lot of blurring of detail on Jupiter, without considering the aesthetics.

[ewave]

 

Personally, I wouldn't reject a given scope just because it has FPL-51 in it.  My favorite 80mm of all time has OK-4 glass in it which has an Abbe number somewhere between the two, and I have owned an FPL-51 triplet that was easily capable of meeting astrophotography requirements for color correction, let alone visual requirements.

 

 I am quite sure that your favorite 80mm was a triplet made in Russia.  

 

While the Abbe number for OK-4 is between that of FPL-51 and FPL-53, it is nearly that of FPL-53.. 

 

Here are the Abbe numbers (Vd) of some commonly used ED glasses:
FPL-51 (Ohara): 81.54
H-FK61 (CDGM): 81.61
FCD1 (Hoya): 81.61
OK-4 (LZOS): 92.1
FPL-53 (Ohara): 94.93
CaF2: 94.99 (Fluorite)

 

Jon Isaacs

 

What about Hoya FCD100 ?

 

so far I found these links and is that accurate  showing a Vd ~ 95 ?

 

http://www.hoya-opti...vd_20150508.pdf

 

also here:

http://www.hoya-opti.../pdf/FCD100.pdf

[peterjkim]

theoretically, same performance as FPL-53. with the right mating glass and same degree of precision in polishing, it will deliver the same performance as FPL-53 based doublets or triplets. now whether they make it to that degree over at the factories ES sources them from...

[stevie44]

When one is talking about affordable doublets like the StellarVue Next Generation Nighthawk, my experience is that the color correction is very consistent with the differences between FPL-51 and FPL-53.  With the common mating elements, FPL-53 offers about 1.8x the color correction of FPL-51 and this translates into much less chromatic aberration seen at the eyepiece.   

 

FPL-51 based doublets I have owned:  WO 66SD, AT 72ED, Astro-Tech,  AT-102ED, Looked through:  Orion 110mm F/7 Premium

 

FPL-53 based doublets I have owned:  WO Megrez II FD, AT-80LE, Orion Eon 120.  Looked through ED-100.  

 

The 120mm Skywatcher is testament to what can be done with quality and/or glass choice.  At 120mm F/7.5, even though it's FPL-53 it ought to show chromatic aberration but I don't see it even on Venus.. 

 

The scope in question, it's not an ED-80 when it comes to color correction.  

 

Jon

Jon,

This is an old thread I'm reading as I begin searching for an 80ED, which, with your advice, I realize is the best refractor for me to begin with.  A member has an SV80ED Raptor for sale, but I read somewhere today that the glass is FPL-51.  I think I would be sensitive to CA, and I see you've written that FPL-53 has a 1.8x advantage over FPL-51 for CA, and so I'm thinking I may be best to pass on the SV80ED.  Just off the top of your head, can you name a few 80ED scopes with FPL-53 which would be good for me to keep an eye on in the used market?  (you already mentioned the Orion ED80 to me).  

Thanks for your patience with this newcomer and best wishes,

Steve

[Bill Barlow]

You could check out the SW 80ED f/7.5 doublet as it uses FPL-53 glass.  Also, the SV 80ED f/7 Access doublet also uses FPL-53.  Both sell for about $700-$750 new.

 

Bill 

[Joe G]

Jon's WO Megrez II FD uses FPL-53 glass, although the lens cell's say fluorite doublet on them (hence the FD).  They were actually FPL 53.  There are also siblings to this scope.  William Optics sold an APOgrade upgrade to many of the tubes that used a similar cell.  I own an Orion Express tube which looks very similar to the Orion EON 120 tube.  Very robust build.  Purchased the APOgrade.  I haven't really noticed any false color.

 

These scopes do come up on the classifieds for good prices.  I'm sure others will chime in.

 

Try putting a "wanted" ad in the classifieds.