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Jones-Bird vs. Schmidt-Cassegrain

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#26 rik ter horst

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Posted 27 August 2015 - 01:09 PM

Here's a question:

Has anyone ever looked through a Jones-Bird that provided decent views, view comparable to a standard parabolic Newtonian of the same focal length?

Jon


Yes, I have :-) In the 80's Opticon (the Netherlands) made about ten 10" F/6 Jones Bird telescopes. Primary was a spherical F/4 mirror and the corrector was made of BK7 and F2 glass. Obstruction was 25% (linear). This design was very well corrected for spherical aberration and provided excellent views even with some chromatism visible. This chromatic aberration is comparable with a good 100 mm F/15 achromat. I might have some numbers regarding radii etc. I'll have a look. At least, I still have some planetary observations made with this instrument...
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#27 rik ter horst

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Posted 27 August 2015 - 01:28 PM

Here's an observation of the GRS from 1989 under so-so circumstances. 

Jupiter 250 mm Jones Bird.jpg

 

Must have another one somewhere..... 

 


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#28 rik ter horst

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Posted 27 August 2015 - 01:40 PM

Here's Mars from around that same period. (Tri-X and TP2415, remember??)

 

Mars 1989.jpg

 

 

And the instrument (by the way, equiped with an optical window!)

 

250 mm Jones Bird.jpg

 

Sooooo long ago ;-)

 


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#29 Jon Isaacs

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Posted 27 August 2015 - 02:50 PM


 

Yes, I have :-) In the 80's Opticon (the Netherlands) made about ten 10" F/6 Jones Bird telescopes. Primary was a spherical F/4 mirror and the corrector was made of BK7 and F2 glass. Obstruction was 25% (linear). This design was very well corrected for spherical aberration and provided excellent views even with some chromatism visible. This chromatic aberration is comparable with a good 100 mm F/15 achromat. I might have some numbers regarding radii etc. I'll have a look. At least, I still have some planetary observations made with this instrument...

 

 

 

 

 

 

 

:waytogo:

 

Rik:

 

I should rephrase my question:

 

"Has anyone ever looked through a Jones-Bird that was not made by rik ter horst that provided decent views, view comparable to a standard parabolic Newtonian of the same focal length?

 

Do remember how much those scopes cost?

 

Jon


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#30 rmollise

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Posted 27 August 2015 - 03:13 PM

Would a typical achromatic or apo barlow do better as a corrector?
 
But then again, I probably wouldn't bother sputting a $50 barlow into a $25 secondhand Bird-Jones scope.


No. A short f/l spherical mirror needs a corrector.

Edited by rmollise, 27 August 2015 - 03:15 PM.

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#31 rmollise

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Posted 27 August 2015 - 03:16 PM

I should rephrase my question:
 
"Has anyone ever looked through a Jones-Bird that was not made by rik ter horst that provided decent views, view comparable to a standard parabolic Newtonian of the same focal length?
 
Do remember how much those scopes cost?
 
Jon


I've seen a couple of Japanese scopes of this type from the 80s - 90s that provided OK views. Not maybe quite as good as a Newt, but OK. Long gone now.

#32 rik ter horst

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Posted 27 August 2015 - 03:37 PM

:lol: Jon, sorry!!


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#33 Sarkikos

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Posted 27 August 2015 - 07:35 PM

Seems you are the black swan among the flock of Jones-Birds!

 

:grin:

Mike


Edited by Sarkikos, 27 August 2015 - 07:38 PM.

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#34 Tourneciel

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Posted 28 August 2015 - 10:05 AM

 

I should rephrase my question:
 
"Has anyone ever looked through a Jones-Bird that was not made by rik ter horst that provided decent views, view comparable to a standard parabolic Newtonian of the same focal length?
 
Do remember how much those scopes cost?
 
Jon


I've seen a couple of Japanese scopes of this type from the 80s - 90s that provided OK views. Not maybe quite as good as a Newt, but OK. Long gone now.

 

 

Hello Rod,

 

Do you mean scopes like the Bausch & Lomb 8080 (actually, Towa MMC-80)?

 

Best,

 

Alain.



#35 rmollise

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Posted 28 August 2015 - 05:22 PM

It's been long enough, Alain that I can't remember what it was, exactly. Not bad, though, and that was likely it.

#36 freestar8n

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Posted 28 August 2015 - 07:02 PM

An sct is a cassegrain with a Schmidt corrector in front. The mirrors can be any figure at all and they would still be scts. An acf is an sct. Normally the primary is spherical but it doesn't have to be, and all combinations of conics are explored in siglers papers.

But all the commercial scts I know of have spherical primaries.

Frank

#37 Tourneciel

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Posted 29 August 2015 - 12:50 AM

It's been long enough, Alain that I can't remember what it was, exactly. Not bad, though, and that was likely it.

 

We discussed this instrument here: http://www.cloudynig...spotting scope.

 

Best,

 

Alain.



#38 rolo

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Posted 29 August 2015 - 03:58 AM

I've had a few of Vixen "wannabe" versions known as the Tasco 8V. They're better suited for target practice with high caliber rifles than telescopes....


Edited by rolo, 29 August 2015 - 06:55 AM.


#39 Jon Isaacs

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Posted 29 August 2015 - 06:20 AM

It's been long enough, Alain that I can't remember what it was, exactly. Not bad, though, and that was likely it.

 

My guess is that it was one of the Celestron "Hybrids", possibly the G-8N.  Ed Ting's review of the G-8N gave it a "marginally recommended if you like this sort of thing."

 

Jon



#40 Sarkikos

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Posted 29 August 2015 - 10:08 AM

Why bother with spherical primaries in Newts?  Is it that difficult to get a decent parabolic primary?  Put in a Paracorr and you're done.

 

I think these Jones-Birds are junk for newbies or something for ATMers to tinker with.  Nothing wrong with that if you're into tinkering.

 

:grin:

Mike


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#41 rmollise

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Posted 29 August 2015 - 02:59 PM

The reason is ease of manufacture of bottom of the line scopes. Though it's easier to mass produce parabolic primaries today.

Spherical is not really the problem. The old f/8 spherical mirrored Tascos did well. What's the crazy is the desire of the makers to put out a SHORT TUBED spherical mirror scope that sells for a few dollars. Utterly hopeless.

Edited by rmollise, 29 August 2015 - 02:59 PM.

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#42 Karl Fabian

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Posted 29 August 2015 - 03:51 PM

The reason is ease of manufacture of bottom of the line scopes. Though it's easier to mass produce parabolic primaries today.

Spherical is not really the problem. The old f/8 spherical mirrored Tascos did well. What's the crazy is the desire of the makers to put out a SHORT TUBED spherical mirror scope that sells for a few dollars. Utterly hopeless.

Couldn't agree more Uncle Rod. Back in the early 1980s had one of those Japanese made Tasco "Lunagrosso" 4.5 inch Newtonians with an F8 spherical miror which was an excellent performer. ( 4-1/2" F8 spherical will meet the Raleigh limit).  Also had an Edmund 4-1/4" "Polamar"F10.6 spherical Newtonian with excellent diffraction limited performance. At longer F ratios a sphere may not deviate from a parabolic shape in any meaningful way. A sphere can act like a parabola because the sphere can be within the diffraction limit of being a parabola with longer focal length primary mirror. 


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#43 rolo

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Posted 29 August 2015 - 04:15 PM

Disassembling is a Tasco Jones Bird is a cinch too....

 

https://www.youtube....h?v=guYwowJBFNk



#44 rolo

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Posted 29 August 2015 - 04:18 PM

 

The reason is ease of manufacture of bottom of the line scopes. Though it's easier to mass produce parabolic primaries today.

Spherical is not really the problem. The old f/8 spherical mirrored Tascos did well. What's the crazy is the desire of the makers to put out a SHORT TUBED spherical mirror scope that sells for a few dollars. Utterly hopeless.

Couldn't agree more Uncle Rod. Back in the early 1980s had one of those Japanese made Tasco "Lunagrosso" 4.5 inch Newtonians with an F8 spherical miror which was an excellent performer. ( 4-1/2" F8 spherical will meet the Raleigh limit).  Also had an Edmund 4-1/4" "Polamar"F10.6 spherical Newtonian with excellent diffraction limited performance. At longer F ratios a sphere may not deviate from a parabolic shape in any meaningful way. A sphere can act like a parabola because the sphere can be within the diffraction limit of being a parabola with longer focal length primary mirror. 

 

The Palomar was indeed a nice performing little scope!


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#45 Sarkikos

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Posted 29 August 2015 - 04:21 PM

As I already mentioned in post #19, my 4.25" f/10.5 Edmund Newt circa 1972 was pretty sharp.  But the OTA was also long, almost as long as the tube for my 10" f/4.8 Dob.  Not much sense in going that route today since decent parabolic primaries are readily available.

 

Mike



#46 Mitrovarr

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Posted 30 August 2015 - 12:07 AM

I think, if you could get good results out of the design with reasonable build costs, the major manufacturers would be all over it. I mean, you get some of the benefits of a long FL in a short tube, you get to manufacturer a spherical mirror instead of a parabola (which is both cheaper and easier to do well), and you get to make a small corrector lens instead of a big corrector plate like in an SCT. My guess is either it's hard to get very good results out of the design (better than parabolic optics, anyway) or it's very expensive to do so. Or, it's some combination of the above; you have to spend a lot to get something decent and then it's got issues anyway so you wonder why you bothered. Especially when you can get really good, reasonably priced parabolic mirrors these days and then slap a paracorr on it and get rid of its only serious aberration.


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#47 gunfighter48

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Posted 30 August 2015 - 07:31 PM

Sorry guys, you are correct, I didn't read the article close. Just skimmed over it very quickly.  That's what I get for trying to be technical when I'm tired and not focused!!!!! My apologies!! :foreheadslap:



#48 John J

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Posted 02 September 2015 - 02:33 PM

Why bother with spherical primaries in Newts?  Is it that difficult to get a decent parabolic primary?  Put in a Paracorr and you're done.

 

I think these Jones-Birds are junk for newbies or something for ATMers to tinker with.  Nothing wrong with that if you're into tinkering.

 

:grin:

Mike

Well what did you just do by putting a Paracorr in after the secondary? You just did for a parabolic mirror that a Jones Bird does for a spherical mirror. Why don't all parabolic Newtonians come with a coma corrector built into the focuser? I guess the biggest problem with the Jones Bird is simply collimation. If the elements were well made and easily removable and repeatably replaced for collimation there would be a lot more of these around.

 

As an idea why not just manufacture F4 spherical newtonians and add a "Spheracorr" to it?

 

JJ


Edited by John J, 02 September 2015 - 02:40 PM.

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#49 gdd

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Posted 02 September 2015 - 03:28 PM

Didn't the Hubble Space Telescope a spherical aberration that the fixed with the famous corrector. So you could say the HST is a bird-jones of sorts.


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#50 Mitrovarr

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Posted 02 September 2015 - 08:55 PM

Well what did you just do by putting a Paracorr in after the secondary? You just did for a parabolic mirror that a Jones Bird does for a spherical mirror. Why don't all parabolic Newtonians come with a coma corrector built into the focuser? I guess the biggest problem with the Jones Bird is simply collimation. If the elements were well made and easily removable and repeatably replaced for collimation there would be a lot more of these around.

 

I think it must be more than collimation. Bird-Jones scopes have a pretty bad reputation - even middle-end ones from back in the day usually did, and you'd think at least some of them would have been collimated right. Mak-newts and Schmitd-newts, which do the same thing with a full corrector plate, are far better regarded, despite having all of the downsides associated with a corrector plate up front (dew magnet, heavy, slow cooling, more glass).

 

My offhand guess is that doing the correction with a small post-mirror optic stresses the lens far more than doing it with a big pre-mirror corrector plate, so it's extremely hard to make a good lens (probably, it's the chromatic aberration). That would explain why cheap ones are so bad, the middle-end ones are not great, but some people have positive memories of high-end ones. It would also explain why the design is dead for anything but the lowest level of scope - if it costs more to make the lens than to make a corrector plate or a parabolic mirror and a coma corrector, there's really no point.


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