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Is there a F/5 150mm+ ED doublet?

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#1 Marcsabb

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Posted 24 January 2019 - 03:18 AM

That's the question. I know that a fast refractor based on the classic doublet scheme is prone to chromatic aberrations, severe spherochromatism, etc... as the lenses get bigger and no matter the type of glass used. I was recently talking to a friend who bought a Startravel 150 F/5 because he wanted something compact and fast, but more tolerant to vibrations and rough handling than reflectors, and he lamented the lack of options on this front. Oddly enough, people who want to spend money on large 'sweepers' can find achromats doublets in the 180-200mm range even at F/6, so at least it seems there is some demand for fast and large doublets, even at the cost of living with aberrations. 

 

 

 

 

 


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#2 sg6

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Posted 24 January 2019 - 03:54 AM

Half suspect there will be one somewhere, maybe a more direct look on the sites of Longpreng and Barsta etc would turn one up as the LP site has lots of refractors on it that do not appear in retailers.

 

However an f/5 150mm doublet will show CA immaterial of the glass being ED or not ED. It will be a question of how much CA is present.

 

ED glass is just another glass, and a doublet is just that = a doublet. ED is not magical, it is just a bit better glass that for some unknown reason we call "ED" and I cannot identify why it is "ED". Where does ED start and why ?

 

There would also be SA present owing to the steeper curvatures that would have to be present at the edges - that would "soften" the image formed. Blue passing through the edges focus different place as blues through the central area, same for Green and Reds etc.

 

As making a 150mm ED would cost - getting at big lump of good ED glass I would not be sure how big a market there would be as at the end anyone would still have an "achro", one with ED glass and still have CA and SA all at say 5x the cost.

 

To get good performance you would likely need f/7.5-f/8. Even that would not be overly good.

 

I would say that little gain is made, just shorter and it is the shorter that reintroduces the problems of CA and SA.



#3 junomike

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Posted 24 January 2019 - 04:03 AM

Closest you'll probably get is the Istar 150 F5.

Not really an ED per se but should be better than a standard Achro


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#4 Marcsabb

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Posted 24 January 2019 - 05:36 AM

Closest you'll probably get is the Istar 150 F5.

Not really an ED per se but should be better than a standard Achro

Maybe it's better than the doublet found in the Startravel, but it is still achromat and it costs more than the entire scope itself! 

 

Incidentally, I wonder how much would it cost to have an artisan create a custom cell for the Startravel using ED glass. Depending on the design, it could have an aberration comparable to a F/10 - F15 achromat, in a lightweight and compact package (7kg and around 70cm with the hood removed). It would still be a compromise but it could be the ultimate takedown scope for people who love large slabs of glass! lol.gif


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#5 The Ardent

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Posted 24 January 2019 - 05:49 AM

https://www.cloudyni...-refractor-r926


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#6 Jon Isaacs

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Posted 24 January 2019 - 06:06 AM

Maybe it's better than the doublet found in the Startravel, but it is still achromat and it costs more than the entire scope itself! 

 

Incidentally, I wonder how much would it cost to have an artisan create a custom cell for the Startravel using ED glass. Depending on the design, it could have an aberration comparable to a F/10 - F15 achromat, in a lightweight and compact package (7kg and around 70cm with the hood removed). It would still be a compromise but it could be the ultimate takedown scope for people who love large slabs of glass! lol.gif

 

The existence of the APM 6 inch F/8 ED doublet is quite recent and partly the result of the interaction between Markus Ludes and members of this forum. A 6 inch F/8 is pushing the limits of what is possible. An F/5.. 

 

As far as cost for a custom unit,  consider the mass produced APM is around $2500. I think you'd be lucky to have someone build a cell and produce an objective for $2500.. And I doubt you'd find anyone both willing and capable. 

 

Jon


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#7 CHASLX200

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Posted 24 January 2019 - 06:12 AM

FC would be a problem also as well as color and cost.



#8 Jon Isaacs

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Posted 24 January 2019 - 06:14 AM

ED glass is just another glass, and a doublet is just that = a doublet. ED is not magical, it is just a bit better glass that for some unknown reason we call "ED" and I cannot identify why it is "ED". Where does ED start and why ?

 

 

There are answers to your questions.  It's probably time for some reading. 

 

http://www.csun.edu/.../roland/ed.html

 

"What gives the ED material the ability to correct color? It is the chemical

composition of the material which is based on fluorides instead of oxides.
Fluoride based glasses have a different dispersion characteristic caused by
absorption bands that are further away in the ultraviolet (this also allows
them to pass UV light more readily)."

 

 

ED glass is not just a "bit better" than the standard crown-flint , the less expensive versions like FPL-51 reduce the longitudinal focus error by a factor of about 3.5.

 

Jon


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#9 Marcsabb

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Posted 24 January 2019 - 06:55 AM

The existence of the APM 6 inch F/8 ED doublet is quite recent and partly the result of the interaction between Markus Ludes and members of this forum. A 6 inch F/8 is pushing the limits of what is possible. An F/5.. 

 

As far as cost for a custom unit,  consider the mass produced APM is around $2500. I think you'd be lucky to have someone build a cell and produce an objective for $2500.. And I doubt you'd find anyone both willing and capable. 

 

Jon

The APM scope you mentioned is undoubtedly excellent value for money but, to accommodate the longer focal length, the tube is over 1 meter long and weights 10,5Kg. It will also require a stouter mount than a Starfire 150. I do agree that, considering also the glass used, F/8 is already pushing the limits to be considered an apochromat. An F/5 would have worse characteristic for sure... in terms of chromatic aberration alone, even admitting that a doublet designed around ED glass will reduce the CA to the levels obtainable by a F/10-F/15, that would be a ratio of 1.7-2.5, still below the Sidgwick standard (but more easily filterable). 

 

Compromises, compromises, compromises... smile.gif


Edited by Marcsabb, 24 January 2019 - 06:56 AM.

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#10 Jeff B

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Posted 24 January 2019 - 07:25 AM

No and there will never be one.  You can make one on paper but not in practice as the glass, very steep internal element radii and aspheric corrections required and lens element mounting and alignment requirements are beyond the consumer market, even with a triplet..

 

Sorry.


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#11 Marcsabb

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Posted 24 January 2019 - 07:40 AM

That's a nice beast! shocked.gif

 

Too bad Chromacors are not made anymore, they could have been an alternative solution for Startravel owners: https://www.cloudyni...cor-first-test/ (yes, there was one version specifically designed for this scope). 


Edited by Marcsabb, 24 January 2019 - 07:45 AM.


#12 Jon Isaacs

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Posted 24 January 2019 - 08:26 AM

The APM scope you mentioned is undoubtedly excellent value for money but, to accommodate the longer focal length, the tube is over 1 meter long and weights 10,5Kg. It will also require a stouter mount than a Starfire 150. I do agree that, considering also the glass used, F/8 is already pushing the limits to be considered an apochromat. An F/5 would have worse characteristic for sure... in terms of chromatic aberration alone, even admitting that a doublet designed around ED glass will reduce the CA to the levels obtainable by a F/10-F/15, that would be a ratio of 1.7-2.5, still below the Sidgwick standard (but more easily filterable). 

 

Compromises, compromises, compromises... smile.gif

 

A 6 inch F/5 ED would be considerably more expensive than the 6 inch F/8 ED and still be limited in capability.  A chromacorr corrects over a narrow field, not much use here. 

 

Lightweight,  perfect color correction,  large aperture. . That's a Newtonian with a coma corrector,  not a refractor .

 

A 6 inch F/5 refractor is a one trick pony.  A good achromat does the job. 

 

Jon


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#13 salico

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Posted 24 January 2019 - 09:32 AM

wasn't there an LZOS 150/6 APO out there once?https://astromart.co...t-fluorite-apo

I know, triplet...


Edited by salico, 24 January 2019 - 09:37 AM.


#14 jay.i

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Posted 24 January 2019 - 09:57 AM

wasn't there an LZOS 150/6 APO out there once?https://astromart.co...t-fluorite-apo

I know, triplet...

I just found that the other day, hadn't heard of it before. Looks like a beast, weighs 25kg... I have no doubt it provides epic views though.



#15 Jeff B

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Posted 24 January 2019 - 09:58 AM

I don't have the software, but I suspect a 6", F5,  FPL-53 based doublet with traditional mating glasses would have really bad spherochromatism, even if "best correction" is in green.  I've a feeling the sphero stuff would be soooo bad..ok how bad is it?...it would be so bad that it may very well puff up the spot for that color close to that of the un-focused color of an achromat at the same color.  Also, there would be no null in green due to the sphero as well.  That is worse than an achromat which can at least be designed and figured to have a null in green.  So it would be an expensive, poor performing fast doublet, relegated to at best, low power....which as Jon stated, "A good achromat does the job" and probably a notably better job at that.

 

So hug your homely achromat today and tell it that it's not such a slouch after all.

 

Jeff 


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#16 Riccardo_italy

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Posted 24 January 2019 - 11:38 AM

Indeed there is (almost, it's an f5.6), but you must buy a package of two

 

https://www.apm-tele...-inch-eyepieces

 

AFAIK, the ED lens needs the erecting prism to correct CA, and that's the reason why (probably) APM has not made a refractor based on that lens. However, they can make a rugged refractor with the 90° prism integrated....


Edited by Riccardo_italy, 24 January 2019 - 11:39 AM.

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#17 Marcsabb

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Posted 24 January 2019 - 12:07 PM

I don't have the software, but I suspect a 6", F5,  FPL-53 based doublet with traditional mating glasses would have really bad spherochromatism, even if "best correction" is in green.  I've a feeling the sphero stuff would be soooo bad..ok how bad is it?...it would be so bad that it may very well puff up the spot for that color close to that of the un-focused color of an achromat at the same color.  Also, there would be no null in green due to the sphero as well.  That is worse than an achromat which can at least be designed and figured to have a null in green.  So it would be an expensive, poor performing fast doublet, relegated to at best, low power....which as Jon stated, "A good achromat does the job" and probably a notably better job at that.

 

So hug your homely achromat today and tell it that it's not such a slouch after all.

 

Jeff 

Can't spherochromatism be corrected by increasing the spacing between the two lenses? I'm pretty sure Takahashi employed this trick with the Sky-90, a pretty fast F/5.5 fluorite doublet.

 

 

Indeed there is (almost, it's an f5.6), but you must buy a package of two

 

https://www.apm-tele...-inch-eyepieces

 

AFAIK, the ED lens needs the erecting prism to correct CA, and that's the reason why (probably) APM has not made a refractor based on that lens. However, they can make a rugged refractor with the 90° prism integrated....

 

A shame APM doesn't sell at least the lens cell! Maybe asking Markus nicely... lol.gif



#18 junomike

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Posted 24 January 2019 - 02:17 PM

Maybe it's better than the doublet found in the Startravel, but it is still achromat and it costs more than the entire scope itself! 

 

Incidentally, I wonder how much would it cost to have an artisan create a custom cell for the Startravel using ED glass. Depending on the design, it could have an aberration comparable to a F/10 - F15 achromat, in a lightweight and compact package (7kg and around 70cm with the hood removed). It would still be a compromise but it could be the ultimate takedown scope for people who love large slabs of glass! lol.gif

Any specialized glass (requiring uncommon glass and or custom work) is going to be excessive in price.

A reasonably costing  6" F8 is going to be 3 - 6X more than the 150 F5 Achro and getting the Focal Ration down to F5 would be even more still.

Once you go out of the normal realm (F/L, Ratio or Glass) things exponentially go north.

For this reason most people make compromises (150 F5 Achro, 130 F7 Apo, Etc)


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#19 Jeff B

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Posted 24 January 2019 - 03:05 PM

Can't spherochromatism be corrected by increasing the spacing between the two lenses? I'm pretty sure Takahashi employed this trick with the Sky-90, a pretty fast F/5.5 fluorite doublet.

 

Up to a point, yes.  However, as you increase the spacing you are further increasing the sensitivity of the centering and tilt of each element relative to each other in an already extremely sensitive design.   The Sky 90 is also 90mm in aperture.  If you scale that design up to 150 such that you get similar performance, you will more than likely end up with an F stop right back to our old friend F8.  Also the Sky 90 allows for under correction on purpose to, it seems, help bring red, blue and green to a more common focus point.  That's one of the "compromises" they talk about.  It seems to me, they are trying to get the smallest overall spot diagram from blue to red.   That may be more useful for imaging than visual, but that is debatable too.

 

Jeff


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#20 Tjn58

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Posted 24 January 2019 - 10:20 PM

As mentioned on an earlier post. although not an ED doublet, I have an Istar 150mm F5 R50 scope.  It provides wonderful views with no CA present at 200x (at least none these older eyes can detect).  I  use it with my Universal Astronomics altaz mount.


Edited by Tjn58, 24 January 2019 - 10:21 PM.

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#21 Marcsabb

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Posted 25 January 2019 - 01:40 AM

As mentioned on an earlier post. although not an ED doublet, I have an Istar 150mm F5 R50 scope.  It provides wonderful views with no CA present at 200x (at least none these older eyes can detect).  I  use it with my Universal Astronomics altaz mount.

You piqued my interest. I've read the description of the R50 "anastigmatic" lens on the Istar page but there is little info on how this lens achieve better correction. Spot diagrams also seems to display astigmatism confused1.gif  I've read somewhere that Istar lenses are made more corrected towards the blue at the expense of red; for visual use at medium magnifications, especially with a prism, it is usually a good enough compromise to be considered a semi-apo. Is that the case?


Edited by Marcsabb, 25 January 2019 - 02:32 AM.


#22 SeattleScott

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Posted 25 January 2019 - 04:53 AM

I believe these Istar scopes use Lanthanum to achieve considerable CA control at a lower cost than ED glass. Of course they are not cheap but cheaper than a 6” Apo. If considering this, one might also consider the Vixen NA140 (although not a doublet).

Scott

#23 Traveler

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Posted 25 January 2019 - 07:41 AM

Not a 6" but there was once a 7" APM 180mm F6 Semi Apo on the market:

https://www.cloudyni...-apo-refractor/

 

To my knowledge only avail. on the used market now...which is to bad...(are you listen to us dear Markus?).

 

I would buy such an instrument (6 or 7 inch version @f6) for sure. Hope that you (Markus) are be able to reply on our wishes in the Vendor and Group Announcements depatment here at CN about these fast F5-F6 and with apertures  > 6 inch ED refractors.


Edited by Traveler, 25 January 2019 - 08:12 AM.


#24 junomike

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Posted 25 January 2019 - 08:03 AM

As mentioned on an earlier post. although not an ED doublet, I have an Istar 150mm F5 R50 scope.  It provides wonderful views with no CA present at 200x (at least none these older eyes can detect).  I  use it with my Universal Astronomics altaz mount.

Did you build this yourself (using the Istar lens cell)?

Post a pic if possible.

Also, I'm intrigued by the lack of CA as my understanding is a 150mm F5 R50 would be equal to a 150mm F7.5 Achromat which should exhibit significant CA at 200X's.



#25 Marcsabb

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Posted 25 January 2019 - 08:37 AM

Did you build this yourself (using the Istar lens cell)?

Post a pic if possible.

Also, I'm intrigued by the lack of CA as my understanding is a 150mm F5 R50 would be equal to a 150mm F7.5 Achromat which should exhibit significant CA at 200X's.

My thoughts, exactly. The Syntha 150 F/8 is pretty bad (or at least the one I've tested). On the other side, the TS 152 F/5.9 I own has less distracting aberration than I would expect and it's using K9+F4 type of glasses. 


Edited by Marcsabb, 25 January 2019 - 08:38 AM.



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