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#26 Justin Fuller

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Posted 22 October 2019 - 04:48 PM

Yes, I've heard that. Do they make doublets?

I do hope that there will come a time when triplets and quadruplets go the way of the Dodo. I would like to see affordable doublets exhibiting no aberrations whatsoever, even per an imager's requirements. It's been a long time since the achromat was developed, nigh 300 years; yes, our 60mm f/11 spyglasses are that ancient. Likewise, primaries of Newtonians with a figure, a curve, exotic enough to eliminate all aberrations associated therewith, and to enable 8" f/2 instruments with secondaries the size of a quarter, or smaller even; as well.

Yes, pipe-dreams, I know.

Yes, I already knew. Can't have bloated stars, et al.


If anything refractors will likely incorporate more elements like we've seen happen with camera lenses. More elements allows more ability to correct for more aberrations, and as AR coating gets better, it makes many element designs possible. I wouldn't be surprised if a seven element refractor came out one day that focuses ROYGBIV within fractions of a nanometer of each other and is free of all other aberrations... then triplets will be considered antiquated.

#27 stevew

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Posted 22 October 2019 - 04:50 PM

And many that are, are much too expensive. 

 

We amateur astronomers are a difficult lot. We want perfection, but cringe at paying for it (I am no different myself). 

 

 

Clear skies!
Thomas, Denmark

But there are many lenses available today that approach perfection for great prices. 


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#28 gnowellsct

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Posted 22 October 2019 - 07:35 PM

CFF has, of course, a high degree of polish.  smile.gif

Yes, I've heard that.  Do they make doublets?

 

 

It was a pun, which happens also to be true.  As my post immediately above says, CFF makes only triplets to the best of my knowledge.  They do of course make mirrored scopes.  

 

Greg N



#29 Sky Muse

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Posted 23 October 2019 - 07:19 AM

If anything refractors will likely incorporate more elements like we've seen happen with camera lenses. More elements allows more ability to correct for more aberrations, and as AR coating gets better, it makes many element designs possible. I wouldn't be surprised if a seven element refractor came out one day that focuses ROYGBIV within fractions of a nanometer of each other and is free of all other aberrations... then triplets will be considered antiquated.

I wouldn't be surprised, either; and initially they may very well be branded "Tele Vue".  

 

Incidentally, when it had come time to get my definitive 4" refractor(nothing larger for me, thank you), in 2003, I had narrowed it down to two.  The TV-102 lost.



#30 25585

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Posted 23 October 2019 - 07:35 AM

And many that are, are much too expensive. 

 

We amateur astronomers are a difficult lot. We want perfection, but cringe at paying for it (I am no different myself). 

 

 

Clear skies!
Thomas, Denmark

Humankind....



#31 Astrohobby

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Posted 23 October 2019 - 11:00 AM

CFF makes triplets as far as I know.  At least my CFF is a triplet.  I don't remember seeing doublets on their web site.  But Agema is deep into doublets.   I don't know how many scopes they are turning out.  CFF is turning out enough that there are people here and there who have them.  

 

Greg N

Hi Greg, since you own a CFF too your are right. They don`t produce any dublets since Pal is convinced about the design and before all: oil spaced!

 

My 6"/f8 CFF was that good - given a good night! When aperature fever caught me I moved to a 7" and then after when it became available to a 8"/f8 with serial number one. Since I have it in a large dome (by a friend) on a dark side (which is unfortunately 130 Km away one trip) I have not made use of that quiete often in the receint times :-(!

 

Anyway my friend has upgraded from TOA 130 to a TOA 150 but has not so many expierence with the 6" right now due to health and weather issues. Low planets doesn`t help too!!

 

We`ll have a closer look on that in autumn - certainly a test of an 6" to an 8" isn`t fair.

 

Anywhy for my light polluted area here in Duesseldorf I recently bought a vintage TMB Apo 6"/f8 triplet from a nice astro fellow member here in Germany which swears on Fluoride (owning a 7" TEC).

 

Cheers  Oliver


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#32 gnowellsct

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Posted 23 October 2019 - 11:13 AM

Hi Greg, since you own a CFF too your are right. They don`t produce any dublets since Pal is convinced about the design and before all: oil spaced!

 

My 6"/f8 CFF was that good - given a good night! When aperature fever caught me I moved to a 7" and then after when it became available to a 8"/f8 with serial number one. Since I have it in a large dome (by a friend) on a dark side (which is unfortunately 130 Km away one trip) I have not made use of that quiete often in the receint times :-(!

 

Anyway my friend has upgraded from TOA 130 to a TOA 150 but has not so many expierence with the 6" right now due to health and weather issues. Low planets doesn`t help too!!

 

 

 

Cheers  Oliver

 

Sounds like you guys are putting a lot of apo makers' kids through college


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#33 gnowellsct

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Posted 23 October 2019 - 11:17 AM

If anything refractors will likely incorporate more elements like we've seen happen with camera lenses. More elements allows more ability to correct for more aberrations, and as AR coating gets better, it makes many element designs possible. I wouldn't be surprised if a seven element refractor came out one day that focuses ROYGBIV within fractions of a nanometer of each other and is free of all other aberrations... then triplets will be considered antiquated.

The thing is the multiple lenses in a camera are designed to control image and focus at many different aperture openings (f stops) and in every position, within a very short physical focal length, maintain a flat field for the final image.

 

Adding six or seven lenses to a refractor would quickly, in my view, encounter the law of diminishing returns, given that three and four element designs are already getting north of Strehl .95.  

 

It is true, however, that oculars routinely have six and seven elements.  But I'm not sure the need for more correction translates into the OTA. 

 

It may be that someone has tried putting together a scope with half a dozen lenses and found the results to be unsatisfactory.  After all there has been centuries of experimentation.  There are of course engineering issues including weight.

 

GN



#34 Alan French

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Posted 23 October 2019 - 11:48 AM

Camera lenses have far different requirements than telescope lenses. Requirements for telescopes are far more stringent, they are used at one particular focus, generally,l and I don't think anyone is hand figuring camera lenses to correct for spherical aberration at the visual peak. 

 

We might see some more Petzval designs, but I doubt we'll be seeing a trend toward more lens elements. Alignment, especially with air spaced elements, is critical. Getting it right takes time and skilled hands - which translates to higher cost.

 

Clear skies, Alan



#35 macdonjh

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Posted 23 October 2019 - 12:09 PM

CFF has, of course, a high degree of polish.  smile.gif

And Polish, too, now that Catalin has moved from Romania.  Perhaps that's what you meant when, in a later post, you said you were punning.

 

h, and to enable 8" f/2 instruments with secondaries the size of a quarter, or smaller even; as well.

 

 

Unfortunately, trigonometry prohibits that, at least if you want a fully illuminated image circle, too.

 

If anything refractors will likely incorporate more elements like we've seen happen with camera lenses. More elements allows more ability to correct for more aberrations, and as AR coating gets better, it makes many element designs possible. I wouldn't be surprised if a seven element refractor came out one day that focuses ROYGBIV within fractions of a nanometer of each other and is free of all other aberrations... then triplets will be considered antiquated.

A Nagler telescope!

 

Reading subsequent posts by gnowellsct and Alan French makes me remember some of the elements in modern eye pieces are used to extend eye relief- not required for the scope itself.  So perhaps four elements is a practical limit for the main optical system.  Maybe five?  Triplet up front for color correction and a doublet in back for field flattening.  Isn't there one of those out there (Petzval with a front triplet)?


Edited by macdonjh, 23 October 2019 - 12:10 PM.


#36 gnowellsct

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Posted 23 October 2019 - 09:01 PM

And Polish, too, now that Catalin has moved from Romania.  Perhaps that's what you meant when, in a later post, you said you were punning.

 

 

Bingo




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