43 replies to this topic

[paulsky]

Hi,

Which of this lens are best for CA?
What telescopes have the FPL-51 and what others the FPL-53?

Paul

[GordonCopestake]

FPL53 is in theory better than FPL51 however the lens design is more important than the glass used. Many scopes use both types and i dont believe there is a definative list however the most common are the Synta ED80, ED100 and ED120 use FPL53, the WO 66 and 80 doublets (and clones) use FPL51.

[Mark Costello]

Are FPL-51 and OK-4 the same types of glass? Thanks.

[jrcrilly]

Are FPL-51 and OK-4 the same types of glass? Thanks.

They are catalog numbers of two different glass manufacturers. None of the offerings of either is identical to any of those of the other, but OK-4 is fairly similar to FPL-53 (not FPL-51).

[Mark Costello]

Thanks, sir. A few apo refractor makers are using OK-4 IIRC.

[jrcrilly]

Thanks, sir. A few apo refractor makers are using OK-4 IIRC.

So far as I know, LZOS (the manufacturer of OK-4) is the only company making telescopes optics from it. They sell objectives to several telescope builders.

-edit- On reflection, I think another Russian lens maker (Lomo) uses OK-4 as well.

[blandp11]

Small nit, OK-4 is closer to FPL-52 in qualities rather than FPL-53. The Abbe (Vd) number of OK-4 is 92.05 while the Abbe number of FPL-53 is 94.93 and FPL-52 is 90.28. While OK-4 almost splits the difference between -52 and -53, it is a bit closer to 52 if either must be invoked for comparison.

FPL-51 has an Abbe number of 81.54 for reference.

Philip

... but OK-4 is fairly similar to FPL-53 (not FPL-51).

[Teal'c]

Thanks, sir. A few apo refractor makers are using OK-4 IIRC.

So far as I know, LZOS (the manufacturer of OK-4) is the only company making telescopes optics from it. They sell objectives to several telescope builders.

-edit- On reflection, I think another Russian lens maker (Lomo) uses OK-4 as well.

Correct, LOMO is using it and it's supplied by LZOS.

[blandp11]

One can make no statement a priori about the colour correction of any lens that uses any particular ED glass. Lenses are NOT automatically ranked in this order:

Fluorite better than FPL-53 better than OK-4 better than FPL-52 better than FPL-51.

The above is not the case. The mating elements and numbers (1 or 2) and design of the lens matter for final designed correction. And even MORE important is how the lens is made and how well the final figuring and polishing is done. A good design with a poor execution makes for a horrible lens.

Some of the absolutely finest telescopes that one can now buy are made with FPL-51 glass. Some completely underwhelming telescopes are made with FPL-53. The opposite also applies.

It is the marketers that want us to focus on one factor (glass type) at the expensive of all others. This way they can give us that single attribute and race to the bottom on price. Guess what might be sacrificed to achive a price point? Actual image quality!

Philip

Which of this lens are best for CA? What telescopes have the FPL-51 and what others the FPL-53?
Paul

[Teal'c]

On what philip is saying, Scroll down to "is this a Fluorite triplet"

http://www.tmboptica...FAQ.pdf#search="TMB%20OK-4%20a%20ABBE%20%23"

[celestial_search]

FPL-53 is a more expensive glass (it is a more recently developed glass). I've read a lot of the comments made by Roland C. and other APO gurus and it often gets technical. The design of lenses and telescopes is ostensibly simple, but once you discuss air-spaced/oil-spaced, doublet/triplet, viewing/astrophotography, baffling/flocking, designs (Petzval, etc.), it really becomes technical. The glass used is just one factor among many. And we did not even mention fluorite!

Designs using FPL-51 are probably trying to keep costs and prices down.

I have a SV Lomo triplet (OK-4) and it is superbly crafted and the lens is excellent. I don't know if I could tell the difference visually between it and FPL-53. Roland, Vic, Al Nagler, TMB, and Mr. Takahashi perhaps!

[Teal'c]

I realize that Fluorite wasn't mentioned, but as Philip mentions, the overall design and execution is what matters not just a type of glass/crystal. That part of the link is what I was getting at.

[Rich N]

Some of you folks might want to get on the Yahoo "Refractors" email list. Roland Christen posts there now and then commenting on optics. He recently posted some interesting stuff about APOs and lens material.

Rich

[Thomas A Davis]

Comparing apples to apples, FPL-53 is better than FPL-51.
Yes the mating glasses and design matter, but if quality
mating glass and the proper design is used, you can't get
the same level of correction with FPL-51. If that were
not the case, why use the more expensive glass?

Thanks, Tom Davis

[Teal'c]

http://www.astromart...ews_id=&page=13

[spaceydee]

I don't know about these glass types per se, other than the little amounts I pick up reading the fora here. My own personal thought on the matter is how awesome is it that we have so many reasonably priced scopes that perform as well as they do. I've got the FC100 from TAK (I don't know for sure that glass it uses, it may be flourite crystal for all I know). I am so happy with my scope for what I use it for I don't see myself ever being interested in another 4" scope for visual use. Again, I just feel that we are all lucky to have so many good choices.

[blandp11]

The bad answer: "Because it'll sell more scopes."

The good answer: "Because I can design a scope to the specifications that I want".

Does anyone think the TEC-200ED or TEC-160ED are poor scopes because they are designed with FPL-51? Are the Synta 80ED, 120ED better because they contain FPL-53? Be very careful, if an APO is reduced to "if is contains FPL-53 it's good", then that is the only thing the APO will have.

Philip

... why use the more expensive glass?

[Jon Isaacs]

>>>Are the Synta 80ED, 120ED better because they contain FPL-53? Be very careful, if an APO is reduced to "if is contains FPL-53 it's good", then that is the only thing the APO will have.
----

I would have to say yes, the ED-80s are better with the FPL-53 than they would be had they been made with FPL-51.

My reason is simple, for any given design, ie aperture, focal ratio, color correction, when compared to FPL-51, FPL-53 requires gentler curves which are easier to fabricate.

This means that for a given manufacturing capability, it is possible to get simpler, more consistent results with FPL-53, FPL-53 is not pushing the manufacturing limits as much as it would were it using less expensive glasses.

It is my understanding that the problems that Meade had with their original ED scopes was related to the steep curves their inexpensive glass required and the difficulty in building a cell that held the glass to the tolerances required.

For Roland's designs where he is pushing the color correction far in the Infrared and UltraViolet, the FPL-53 allows him to achieve the best possible results. For simple affordable APOs, the FPL-53 allows them to be more consistent and easier to build.

So, I think it cost Synta/Orion more to use the FPL-53 and to do so they had to make it an otherwise no frills scope but by all accounts, the results seem to have been worth it.

So, this does not mean one cannot build an small fast doublet with other glasses but it does mean it is easier to do with the FPL-53 and that it will probably have color correction that extends further into the UV and IR.

Jon

[milt]

if an APO is reduced to "if is contains FPL-53 it's good", then that is the only thing the APO will have.

And it won't be a BIG Apo because FPL-53 melts are not available in wide enough slabs to make much over a 150 mm lens. Yuri posted the attached shot a couple years ago. Maybe Rich can confirm if Roland was able to squeeze his 155 out of FPL-53.

Attached Thumbnails

• 

[Thomas A Davis]

http://www.astromart...ews_id=&page=13

I don't know what you are replying to me with, as I don't
have an AstroMart account (it was pulled for no reason,
with no explanation some time ago). Once again, and I
will stick to this story, apples for apples, FPL-53 will
make for a better corrected scope than FPL-51 if a quality
design with the proper mating glass is used. Same for
fluorite. Can FPL-51 be used to make an excellent lens?
Of course, but it is easier to accomplish, cost aside with
the better glass. The curves can be gentler, and better
color correction can be had. Put another way, if all else,
including cost is the same, give me an FPL-53 lens over an
FPL-51, and a fluorite lens over the FPL-53. That is if
the design, and mating lenses are of equal quality.

It is not just a marketing issue, unless the buyer is not
interested in the better correction the better glasses
can provide, and for some, this may not be the case. They
may be well satisfied with a good FPL-51 design, but I'm
hoping that it is not being said that FPL-53 can't make
a better corrected lens, or that fluorite can't be used
to make one better than both FPL-53 or FPL-51. I wouldn't
want the point that while having FPL-53 or Fluorite in an
objective does not guarantee better correction, the idea
is lost that using proper designs and mating glass, that
they can't provide superior correction over lesser glasses.

If you want the best correction for photography or critical
visual use, FPL-53 can be of benefit in a well-executed
design. Problem is that some go the extra cost on using
FPL-53 or fluorite, then use either a poor design, or
cheap mating glass, negating the value of the better
FPl-53 or fluorite element. This was not what I was
referring to.

Sorry, I don't know what I'm responding to here, so I'm
trying to cover all bases.

Thanks, Tom Davis

[blandp11]

That was not the question. Better than a TEC 160ED or TEC 200ED? with Or better than a hypothetical 80ED made by, say, Takahashi with FPL-51? The answer is no, they are not and would not be better. The reason is the care that TEC and Tak put into their scopes. This is lost if we only look at the glass it uses.

AP also made the EDT line of scopes using FPL-52. Were they any worse corrected than the EDF's, even into the IR? No, they were not. They are however of a longer focal ratio.

I once asked Roland if the EDT version of the Traveler had FPL-52 rather than FPL-53. He answered that all but the very earliest versions of the Traveler used FPL-53, but some did use FPL-52. He mentioned that the colour correction was idistingushable but that it was harder to fabricate.

FPL-53 is valued for the reasons that Jon suggests, easier curves for a given level of correction, or better correction for some level of fabrication ease. And the ablity to make some fast focal ratio designs that would be impossible without it.

But I object to the idea that scopes can be ranked by the type of glass they use. This is simple not the case and it is folly to think so.

Philip

I would have to say yes, the ED-80s are better with the FPL-53 than they would be had they been made with FPL-51.

[blandp11]

The new run AP 160ED (160mm APO f/7.5) positively uses FPL-53, as stated by Roland C.

Philip

Maybe Rich can confirm if Roland was able to squeeze his 155 out of FPL-53.

[Thomas A Davis]

if an APO is reduced to "if is contains FPL-53 it's good", then that is the only thing the APO will have.

And it won't be a BIG Apo because FPL-53 melts are not available in wide enough slabs to make much over a 150 mm lens. Yuri posted the attached shot a couple years ago. Maybe Rich can confirm if Roland was able to squeeze his 155 out of FPL-53.

I've gotten essentially the same information as you on this,
that melts larger that about 160mm are tops for FPL-53.
Not an issue with fluorite (but cost comes into play) or
with the OK-4 used in the larger LZOS lenses. I do believe
that 155mm is possible from 160mm, as long as the finished
blank is that size. 5mm over the final clear aperture is
enough to provide for proper mounting with the retaining
ring. The new TMB Signature Series 155mm is to be FPL-53,
if I remember correctly, so it does not appear to be an
issue there. I believe you can remelt pressings and get
larger sizes but homogenity issues come into play.

Thanks, Tom Davis

[blandp11]

How about this to show what I'm addressing (or not!) as the case may be:


Q: Is FPL-53 a better glass than FPL-51?

A: Yes it is, in so far as many designs are easier to execute with it with shallower curves and that some other designs are possible with it that are impractical or impossible for FPL-51.


Q: So is a scope with FPL-53 better than one without it?

A: No way! Design and execution matter greatly!


Q: So why did you plunk down some coin for a TEC-180FL if FPL-51 is so great, huh?

A: Because I wanted an 180mm f/7 APO and it the APM are the only game in town. Both makers produce excellent scopes. My decision was not based on the equation of Fluorite is better than OK-4, but on the quicker cooldown of the TEC oil spaced versus air and TEC's absolutely fab attention to producing a great scope. This size and focal ratio may be at the point of impossibility or impractibility for FPL-51.

Philip

[Thomas A Davis]

Phillip,

If you are arguing with me, we have no argument. I agree
that TEC makes great scopes. They do also use fluorite in
some of their designs, so I guess Yuri does see a value
in it, though. Keep in mind as well, that FPL-53 is not
available for your 180, as that size is not made. You can
be sure that Yuri used the best possible mating glasses
to get the exceptional design you have. I don't think the
argument is at your level, but within the realm of what
is available in produced FPL-53 blanks. There are one or
two out there (not TEC) that may be trying to tout their
FPL-51 designs as being better or as good as FPl-53, and
it may not be the case in their situation. TEC makes some
of the best corrected instruments I've looked through.

I was not referring to you at all, and don't want you to
get the wrong impression about me. In fact, my initial
response was over the fact that I had a link, with no
explanation thrown at me that I could not get to, as a
response to my post.

Thanks, Tom Davis