77 replies to this topic

[SandyHouTex]

With the success of the Orion EON refractors, why hasn't a manufacturer come out with a larger 6 to 8 inch doublet using FPL-53 (or similar) glass for something less than the dollars required for the current premium triplet refractors from Tak, TEC, etc., or am I missing something.

I love my EON 120 but I wish something larger was available. Any rumors, etc..

[Thomas A Davis]

I believe the answer is that high quality FPL53 blanks are
not readily available in that size. They are mass-produced
for the Synta 120mm FPL53 scopes, but not in the larger
sizes. This creates two issues. One cost. Two, it creates
an issue with potential homogenity issues that would require
some blanks to be rejected for telescopes (for which the
manufacturer will not pay, as some high-end manufacturers
have stated). The cost between the FPL53 doublet, and the
triplet of the same size would not be as much as you might
think.

As to 8" FPL53 blanks, I know of no manufacturer making
FPL53 triplets. The closest are the LZOS lenses, but they
use OK4 glass.

Tom

[Mike Clemens]

There is something about 8" APOs that makes manufacturers say "I'm not going to do that again !!!!"

*LZOS excluded - probably because they can reject glass without eating retail value of the glass

[JayKSC]

With Orion now offering orders for its gargantuan 36-inch or larger truss dobsonians, maybe Orion will start to offer giant ED refractors too? (one can dream, if only to have some cool ads to enjoy) Considering the asking price of the giant dobs, I'd expect such huge refractors would command equally huge prices.

- Jay
South Florida

[skyward_eyes]

I was gonna say, I love dobs but I would at least like to see a 150ED from Orion. If they can make dobs up to 50" now they why is it so hard to get a 150ED?

[Doug76]

I wouldn't mind seeing an f/8 6" with ED glass, and FPL-51 would do just fine, if they can't make FPL-53. It would be cheaper too.

[zjc26138]

I wouldn't mind seeing an f/8 6" with ED glass, and FPL-51 would do just fine, if they can't make FPL-53. It would be cheaper too.

Ditto

[jrbarnett]

It's been done, basically. At least with an FPL-51 equivalent. The old Meade Ed doublets featured ED doublets in 102mm, 127mm, 152mm and 178mm.

There are several reasons why the ED doublet approach doesn't "scale". First is that with ED glass in a doublet, element centering is a major requirement. Heavier elements are harder to mechanically fix than lighter elements. The larger Meades were notorious for de-centering. later iterations of the Meade cell added both push-pull collimation and separate lateral centering screws, but did not make the scopes any less prone to needing centering adjustment. Note that this was at f/9. I believe the need for precise centering becomes even more acute as f-ration drops.

Second, as others have pointed out, larger ED blanks are very expensive. The price does not rise in a linear fashion. A blank for a 6" ED lens might be 2x the price of a 5" ED lens blank and 6x the price of a 4" ED lens blank. In part this is because it is harder to produce a huge hunk of glass without inclusions, bubbles and other flaws than it is to produce a flawless smaller hunk.

Third, many of the lenses coming out of China are mostly figured and polished using the latest generation of sophisticated, automated polishing equipment. ~120mm may be the size limit for such machines. Machine work is much faster and in the long run much less costly than skilled hand work. A large, hand worked Chinese ED lens might not be that much less costly than a large, hand worked lens of other country-of-origin.

Fourth, big ED doublets are relatively colorful unless you scale up the focal ratio. For example, to get the same level of color correction as a 100mm f/9 ED doublet, and 8" ED doublet using the same basic materials and design would need to be f/18. If you're going to live with color, you may as well opt for an f/15 8" D&G achromat for a fraction of the cost, and throw in a Baader semi-APO filter.

Fifth, now high quality Chinese-made triplets in larger apertures (~130mm) are hitting the market at relatively attractive prices. Oddly enough, the handful of larger Chinese triplets (152mm to 158mm) are not much cheaper (and sometimes more than) the beast large US-made triplets from A-P and TEC. I'm not sure whether we'll see large, affordable, Chinese-made triplets any time soon, but I do suspect that we'll see large triplets rather than large doublets, if we see anything.

Regards,

Jim

[Mike Holland]

...

Third, many of the lenses coming out of China are mostly figured and polished using the latest generation of sophisticated, automated polishing equipment. ~120mm may be the size limit for such machines.

Why would that be the case? Why can't they scale up a machine? Thanks.

Mike

[Mike Holland]

If these came out in prices that are in line with the scopes that the Chinese are making now they'd be very popular. For example, the 120mm EON (ED doublet) is $1800, whereas the 120mm Takahashi ED triplet is about $3700, so close to 2:1. If there were 150mm doublets for 1/2 the price of Takahashi or AP 6" apos people would buy them, even if they had a small amount of color.

Mike

[Doug76]

Used to be the Chinese, either side of the strait, didn't make bigger than 12" mirrors, now they make 16". Scaling up doesn't seem to be a problem for them.
As for the f/ratio, I figure a decrease in CA, if at a reasonable price, is the goal, since so many complain about how much there is in Achromats of 6" f/8 size (I'm not one of them, I consider the unobstructed view and better light transmission well worth some CA).

[jason_milani]

It is a big issue with ED glass. Making a mirror is much easier than making a lens (that's why refractors cost more than reflectors in similar apertures). Go over to the Astromart refractor forum and you'll find posts by R.C. explaining this very thing. He basically says that when you get over 5 inches in aperture things really get difficult and expensive when making an apochromatic lens.

[Jared]

I wouldn't mind seeing an f/8 6" with ED glass, and FPL-51 would do just fine, if they can't make FPL-53. It would be cheaper too.

Yuri (the owner and chief designer at TEC) has stated that the blank for the FPL51 element in his 160mm refractor costs him 3.8 times as much as the blank for the FPL53 element in his 140. Couple that with the difficulties in cell design for handling larger chunks of glass as well as the lack of guaranteed quality (without paying huge premiums) from Ohara and Schott in the larger blanks and I don't think we will be seeing inexpensive 6"+ refractors in the very near future.

[Doug76]

Someone will figure it out. They nearly always do when there is sufficient need. Have faith, baby!

[Mike Clemens]

> Someone will figure it out.
>

Aries started making 7" APO doublets... stopped.

Astro-Physics made 7" and 8" APOs... stopped.

TEC made 8" APOs.... stopped.

These are guys who can charge top end prices for their work, but they stopped because it was not worth it.

How are the guys who cannot charge top end prices going to figure it out?

Naturally, changes can happen, the gears of progress keep turning. Its just THOSE were the guys I'd expect to make the progress. The reality I guess is they can keep their plate full with products which dont cause undue graying of hair.

[Dick Parker]

I have made several Achromatic refractors. I (and a friend) wanted to make an 8 inch objective (that is two objectives) using FPL-53. That glass, was not available in the 8 inch size. We had quoted the cost for two pieces that would have to have been made for us. The cost, per piece, was $4,000. Now, we are hobbiests. Add to that the cost of the other two pieces of glass, the time and talent to make the lens, and the lens cell (not trivial) and you can see how the cost escalates quickly. And the available market diminishes equally quickly. An 8 inch refractor needs an observatory to be used to its full potential.

Dick Parker

[Jeff Morgan]

Someone will figure it out. They nearly always do when there is sufficient need. Have faith, baby!

Since the glass manufacturers cater to the photography market, the blanks will never be large enough (at a cost sane people would pay). What photographer is asking for an 8" diameter telephoto lens?

Given the limitations on blank size, larger achromats with sub-aperture correctors may be the way to go.

[SandyHouTex]

Thanks for all of the inputs guys.

I guess what I was thinking was something along the lines of what Meade did with their ED APOs a few years ago. As I recall their 6" and 7" APOs were significantly less than other premium APOs pricewise. In reading alot of reviews of those scopes I know the issues with the 7", but as I recall the 6" was pretty well done.

[Doug76]

Likely because they do charge high end prices, and the market is too slim for it to be worthwhile to them in the end.

[APM M.Ludes]

Hi Mike Clemens,

you state
" *LZOS excluded - probably because they can reject glass without eating retail value of the glass "

the thruth is, LZOS is like Ohara and Schott, they melt here own glas and they melt it in the quality they need it, so no reject needet. If Ohara or Schott would polish and sell here own apos, you can be shure, that they would take more care on the glas the melt for here own need :-)

Its a cost question. Schott and Ohara offer diffent quality , with here highest quality apo makers would be able to do great apos, but such glas cost a lot more then standart quality.

Markus

[APM M.Ludes]

we worked last monthes with our Optical designer , with LZOS in Russia and Chinese companys on the possibility to make such 6"F/8 ED Doublet Apo.
The risk of doing Optics in China and assembling in Russia is that china is using bad glas quality and bring russi into trouble. China informed us that they will not going to be responsable for bad quality if Ohara deliver not good FPL 53 . Also china is asking big quantitys.
By shipping polished and coated blanks to russi to produce matched cell and assemble correct and test , the final product is only a bit less expensive that a fully russian LZOS made doublet wherte we have a 100% gurantee of the quality.

B caclulating all cost for optics, mechanics , focusers, finder, shipments , taxes and dealer profit we come out at about US $ 6,000 retail for a high quality 6"F/8 Doublet Apo.

compare to other 6" apos its still much less expensive.

I am sitting here and thinking if $ 6,000 will be accept at the market or not. If yes, then we could start

so what do you customers say about this ballpark ?????

thanks

Markus Ludes
APM Telescopes
20 years APM anniversary , 1990-2010

[Clive Gibbons]

I am sitting here and thinking if $ 6,000 will be accept at the market or not. If yes, then we could start

so what do you customers say about this ballpark ?????

thanks

Markus Ludes
APM Telescopes
20 years APM anniversary , 1990-2010

I'm sure that's a fair price for a high quality 6" f/8 FPL53 doublet, but it would be out of my budget.

Also, consider that the TEC140 triplet isn't much smaller in aperture and would cost $800 less.
Tough competition.

[SandyHouTex]

I believe I would sign up for one of the 6" ED doublet APOs at the $6000 US if you make them Markus.

[Al Miller]

Maybe I don't know anything but, $6000 sounds a little steep when talking about a 6" doublet. Considering the 120mm (4.7") OTA's are under $2000 and aren't that much smaller. Now there is a 5" triplet for $2000 that has great reviews. Even the TMB 130 is under $4000. I dunno... why would a 6" doublet be so much more expensive?

** EDIT ** Nevermind this post. I went back and read the beginning of the thread and found lots of reasons why the cost would be so much higher.

[RogerRZ]

Price goes up exponentially on apo refractors. A 5" scope only has 25 square inches that needs to be perfect; a 6" has 36.