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question about bird jones corrector removal

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#1 calypsob

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Posted 23 February 2018 - 07:12 PM

Can you pull the corrector out of a Bird Jones reflector and have a functioning newtonian?  Someone has a celestron 127eq that they offered to give me and I really have no interest.  However, if the corrector can be removed I would not mind turning in it into a wide field newtonian.  



#2 Joe1950

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Posted 23 February 2018 - 07:20 PM

I believe the primary mirror in a Jones Bird scope is spherical, so there will be spherical aberration to deal with. Depending on the f/ratio, it could be problematic. Also I would imagine the position of the focal plane will change.

 

Just some things to consider.


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#3 ShaulaB

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Posted 23 February 2018 - 07:22 PM

http://www.iceinspac...ead.php?t=27116

a quote from this websi

What you have is a Jones-Bird design reflector telescope. It uses a spherical mirror rather than the more expensive parabolic mirrors used in a regular Newtonian reflector.
The Spherical mirror in combination with a Barlow/Corrector lens inserted into the focuser's draw tube is what gives you the total focal length of the telescope.. You "do not" want to remove that barlow/corrector. If you do, you will have all kind of spherical aberration problems. You wouldn't even want to use that mirror in a longer OTA. It won't work without the correction of that barlow/corrector lens, or some kind of corrector plate on the end of the OTA. Then you would essentially have yourself something similar to a SNT if you did that.te


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#4 calypsob

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Posted 23 February 2018 - 07:30 PM

I believe the primary mirror in a Jones Bird scope is spherical, so there will be spherical aberration to deal with. Depending on the f/ratio, it could be problematic. Also I would imagine the position of the focal plane will change.

 

Just some things to consider.

Ah that is right, I keep calling it a newtonian, its definitely not a newtonian. 


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#5 calypsob

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Posted 23 February 2018 - 07:32 PM

http://www.iceinspac...ead.php?t=27116

a quote from this websi

What you have is a Jones-Bird design reflector telescope. It uses a spherical mirror rather than the more expensive parabolic mirrors used in a regular Newtonian reflector.
The Spherical mirror in combination with a Barlow/Corrector lens inserted into the focuser's draw tube is what gives you the total focal length of the telescope.. You "do not" want to remove that barlow/corrector. If you do, you will have all kind of spherical aberration problems. You wouldn't even want to use that mirror in a longer OTA. It won't work without the correction of that barlow/corrector lens, or some kind of corrector plate on the end of the OTA. Then you would essentially have yourself something similar to a SNT if you did that.te

Yea I think im going to have to pass it up. 


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#6 drneilmb

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Posted 23 February 2018 - 07:49 PM

If you really want to get all the way into the ATM journey, you can take the spherical 5" mirror out of it and treat it as a curve-generated blank for making a different 5" mirror. I mean, it's free, right?


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#7 Joe1950

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Posted 23 February 2018 - 07:54 PM

It's still in the configuration of a Newtonian with the diagonal mirror and the focal plane off to the side of the tube.

 

A spherical mirror with a high f/ratio, of say f/10 for a 127, aperture would be a good scope as the spherical aberration falls below the ¼λ tolerance often considered acceptable. 

 

However, an f/10 primary, without the corrector lens assembly would make the scope much longer. The corrector lens in the light path allows for a shorter focal length primary, thus a shorter telescope length.

 

The Jones Bird type of telescope is often looked upon unfavorably. However, if designed and implemented correctly, it could be a quality scope. It's just that mass production scopes are not made to the optical standards needed in order to keep the prices low.

 

These scopes will perform at lower magnifications but will not hold higher magnifications needed for viewing finer detail.

 

Saying that, If you were only going to use the scope as a wide field, low power scope, spherical aberration may not be apparent enough to ruin the view. 

 

For example, if you took out the relay corrector and used an eyepiece (you would have to know the focal length of the primary) giving you a magnification of 25X, you would get an exit pupil of about 5 mm, which would give bright images at low power.

 

Depending on the native focal length of the primary, at 25X you may not be hampered by the negative effects of the spherical aberration and have a good low power, wide field scope.

 

However, to get the primary to come to focus at a usable spot without the corrector lens, you would surely have to physically move the primary forward. Also the secondary may then be too small to give you any measure of a 100% illuminated field and you would have to replace it with a larger one.

 

In any event, being free of cost and with a little work, you may be able to use the scope productively as long as you don't expect to jump up the power.

 

May be worth a try and a good learning experience.

 

joe


Edited by Joe1950, 23 February 2018 - 07:57 PM.

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#8 Messierthanwhat

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Posted 23 February 2018 - 07:57 PM

It depends on your tolerance for aberrations and/or how much of a challenge you want to take on. People have made usable reflector telescopes from shaving mirrors. I don't know any of those people personally, but I assumed they have great tolerance for imperfection. So, if you remove the corrector and make whatever adjustments (if any) are necessary to achieve focus, you can probably get images that are recognizable, even if not excellent. On the other hand, if challenge is your thing, you can re-figure the spherical mirror into a parabola, of you can try the flex method described in this article.


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#9 AUricle

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Posted 23 February 2018 - 09:54 PM

Hi Wes,

It took me awhile, but I finally remembered and was able to pull up a very good forum thread about getting the most out of your 127eq Bird-Jones, and making a few small mods that reduce the pain of collimation to a 2 minute exercise.

The English is a little broken, but the explanation is quite good and very understandable.

You might want to give it a go especially since you got a freebie.

This was written in 2010 I believe. You might or might not have to register on the site to see the forums. It's worth it. The pictures he provided have 'expired', but you won't need 'em.

Here's a link to the site:

http://www.astronomy...ker-127-eq.html



#10 Dave Mitsky

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Posted 23 February 2018 - 10:01 PM

 

I believe the primary mirror in a Jones Bird scope is spherical, so there will be spherical aberration to deal with. Depending on the f/ratio, it could be problematic. Also I would imagine the position of the focal plane will change.

 

Just some things to consider.

Ah that is right, I keep calling it a newtonian, its definitely not a newtonian. 

 

Well, it can be considered to be a catadioptric Newtonian.

 

Today's ultra-fast sub-f/4 Dobs that require a coma corrector to function properly aren't really Newtonians either from that perspective.

 

Dave Mitsky


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#11 Benach

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Posted 24 February 2018 - 01:59 AM

Agrees with Joe1950 here. At my club we have a 300mm Jones-Bird from the Dutch brand Opticon. Man, it beats a C14 hands down when it comes to sharpness and contrast. I have not measured the exact sizes but with my 21mm Ethos but M42 was about twice as big in the JB as in the C14 under approximately similar conditions.

It was was often treated by some members as a mediocre telescope. Well, until my own telescope is finished, I will use the JB, without any hesitance.
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#12 calypsob

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Posted 24 February 2018 - 12:42 PM

If you really want to get all the way into the ATM journey, you can take the spherical 5" mirror out of it and treat it as a curve-generated blank for making a different 5" mirror. I mean, it's free, right?

That would be alot of fun, I have been wanting to do an atm lately but I think the 127 would be better off donated to the club for now.  I think I am going to focus on a widefield astrograph in the 5-6" range.  That is why I considered the 127. 



#13 calypsob

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Posted 24 February 2018 - 12:44 PM

 

 

I believe the primary mirror in a Jones Bird scope is spherical, so there will be spherical aberration to deal with. Depending on the f/ratio, it could be problematic. Also I would imagine the position of the focal plane will change.

 

Just some things to consider.

Ah that is right, I keep calling it a newtonian, its definitely not a newtonian. 

 

Well, it can be considered to be a catadioptric Newtonian.

 

Today's ultra-fast sub-f/4 Dobs that require a coma corrector to function properly aren't really Newtonians either from that perspective.

 

Dave Mitsky

 

Interesting, I stand corrected!  Cat newt it is. 


Edited by calypsob, 24 February 2018 - 12:44 PM.


#14 calypsob

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Posted 24 February 2018 - 12:46 PM

It's still in the configuration of a Newtonian with the diagonal mirror and the focal plane off to the side of the tube.

 

A spherical mirror with a high f/ratio, of say f/10 for a 127, aperture would be a good scope as the spherical aberration falls below the ¼λ tolerance often considered acceptable. 

 

However, an f/10 primary, without the corrector lens assembly would make the scope much longer. The corrector lens in the light path allows for a shorter focal length primary, thus a shorter telescope length.

 

The Jones Bird type of telescope is often looked upon unfavorably. However, if designed and implemented correctly, it could be a quality scope. It's just that mass production scopes are not made to the optical standards needed in order to keep the prices low.

 

These scopes will perform at lower magnifications but will not hold higher magnifications needed for viewing finer detail.

 

Saying that, If you were only going to use the scope as a wide field, low power scope, spherical aberration may not be apparent enough to ruin the view. 

 

For example, if you took out the relay corrector and used an eyepiece (you would have to know the focal length of the primary) giving you a magnification of 25X, you would get an exit pupil of about 5 mm, which would give bright images at low power.

 

Depending on the native focal length of the primary, at 25X you may not be hampered by the negative effects of the spherical aberration and have a good low power, wide field scope.

 

However, to get the primary to come to focus at a usable spot without the corrector lens, you would surely have to physically move the primary forward. Also the secondary may then be too small to give you any measure of a 100% illuminated field and you would have to replace it with a larger one.

 

In any event, being free of cost and with a little work, you may be able to use the scope productively as long as you don't expect to jump up the power.

 

May be worth a try and a good learning experience.

 

joe

Interesting, so are you saying that the corrector is acting as a reducer ?   I always assumed that a bird jones was a fast F ratio primary with a barlowed element in the focuser.  It would be fun to eventually pursue a high quality bird jones design. 



#15 calypsob

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Posted 24 February 2018 - 12:48 PM

It depends on your tolerance for aberrations and/or how much of a challenge you want to take on. People have made usable reflector telescopes from shaving mirrors. I don't know any of those people personally, but I assumed they have great tolerance for imperfection. So, if you remove the corrector and make whatever adjustments (if any) are necessary to achieve focus, you can probably get images that are recognizable, even if not excellent. On the other hand, if challenge is your thing, you can re-figure the spherical mirror into a parabola, of you can try the flex method described in this article.

I would love to try and grind on a mirror one day when I have a bigger area for tools.  I definitely have a low tolerance for imperfection so I might be better off starting with a blank :) 



#16 calypsob

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Posted 24 February 2018 - 12:49 PM

Agrees with Joe1950 here. At my club we have a 300mm Jones-Bird from the Dutch brand Opticon. Man, it beats a C14 hands down when it comes to sharpness and contrast. I have not measured the exact sizes but with my 21mm Ethos but M42 was about twice as big in the JB as in the C14 under approximately similar conditions.

It was was often treated by some members as a mediocre telescope. Well, until my own telescope is finished, I will use the JB, without any hesitance.

Really?  Does it use a secondary spider to hold the mirror?  Ive never seen a professional grade bird jones.  It seems like a corrector made form fpl53 or fluorite could make for an excellent planetary imaging scope.  



#17 Joe1950

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Posted 24 February 2018 - 01:25 PM

The Jones-Bird configuration is essentially a catadioptric telescope - that is the primary image is formed by a combination of a reflective element, the primary mirror, and a refractive element, the corrector lens assembly.

 

Usually the corrector is placed after the secondary mirror in the eyepiece focuser housing, but in some arrangements it is placed before the secondary mirror.

 

The prime purpose of the corrector is to eliminate the spherical aberration inherent in the spheroid shape of the primary. As a secondary purpose of the corrector, it, like a Barlow lens, will effectively extend the focal length of the optical system without substantially extending the physical size of the telescope.

 

That's why you will often see a Jones-Bird telescope advertised as an f/8, but in a much shorter tube than would fit a conventional Newtonian f/8.

 

The difficulty is in making the corrector element precise enough to cancel out the spherical aberration of the primary and not add any aberrations of it's own.

 

In principal, the Jones-Bird can be as good as any other cat scope as long as the optical elements are figured properly.

 

Where it fails however is in the production and testing capabilities of the manufacturer. Proper figuring of any optical element, the primary mirror, the secondary mirror or a correcting lens element and subsequent testing of the system is an expensive process.

 

So, in order to offer a scope that is inexpensive, but which performs to some degree, the manufacturers don't pay as much attention to to the quality of the elements as one would like.

 

Newtonians, Schmidt Cass scopes, Maksutovs that are made in a mass production facility cas also suffer from sub par optics. You may get a great scope, you will probably get an average scope, but you also might get a poor performer.

 

It's all the economics of production.

 

If you want superb optics, you have to have them made and tested by a skilled optician and optical company such as Carl Zambuto or Mike Lockwood, just to name two. However, the cost of the scope will be more than the cost of a production made scope. But you will be assured you're getting a top of the line telescope and may feel that is best in the long run. 


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#18 Dave Mitsky

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Posted 24 February 2018 - 01:59 PM

If you want superb optics, you have to have them made and tested by a skilled optician and optical company such as Carl Zambuto or Mike Lockwood, just to name two. However, the cost of the scope will be more than the cost of a production made scope.

 

That's putting it mildly.  wink.gif

 

Dave Mitsky


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#19 Jon Isaacs

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Posted 24 February 2018 - 05:25 PM

Agrees with Joe1950 here. At my club we have a 300mm Jones-Bird from the Dutch brand Opticon. Man, it beats a C14 hands down when it comes to sharpness and contrast. I have not measured the exact sizes but with my 21mm Ethos but M42 was about twice as big in the JB as in the C14 under approximately similar conditions.

It was was often treated by some members as a mediocre telescope. Well, until my own telescope is finished, I will use the JB, without any hesitance.

 

 

I would be interested in knowing more about the Opticon "Jones-Bird. " My guess is that its not your standard Jones-Bird design. 

 

I think there is more going on with your standard Jones-Bird than just the quality of the optics.  Even the scopes like the Celestron Hybrids which were built in Japan by Vixen were marginal performers. 

 

Back to Opticon:  Opticon is a name I associate with Rik ter Horst, Rik is a member of Cloudy Nights and an optical professional as well as an amateur telescope maker.   Some of you may remember the 30mm solid Glass SCT Rik built or the 8 inch F/25 SCT with the 20% CO..   

 

 https://www.cloudyni...t-cassegrain/  

 

Here's an ATM project Rik posted a few years back.. 

 

https://www.cloudyni...f25-astrograph/

 

On the professional side, I find he has collaborated with Harrie Rutten in designing some optics for an observatory..

 

Here's a paper Rik recently presented at SPIE:

Directly polished lightweight aluminum mirror

 

Like I said, my guess is that Benach's club's  "Jones-Bird" is not a standard Jones-Bird design but a rather a state of the art sophisticated design executed to the highest standards.. 

 

Edit:  A little more work answered my own question:

 

https://www.cloudyni...-2#entry6753943

 

 

My 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"

 

Rik replied:

 

"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..."

 

Edit number 2:

 

Rik just posted the first image of the moon taken with his latest project, a 16 inch modified DK.  

 

https://www.cloudyni...128-cassegrain/

 

Jon


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#20 Jon Isaacs

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Posted 24 February 2018 - 05:37 PM

Another tidbit.. 

 

I believe that Newton's first Newtonian has spherical optics.. 

 

Jon


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#21 Benach

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Posted 24 February 2018 - 07:22 PM

Some answers:

The NLs are extremely small. Many astronomers, professional and amateur alike, know eachother well and often for decades. I know Rik ter Horst, Harrie Rutten and the original owner of Opticon, Hans Dekker, personally. Harrie is a wizard with optical design, Rik is a optical production wizard and Hans is a measurement wizard. I am not sure but it could be that the Opticon SCTs were designed by Harrie Rutten. What I do know for sure is that Rik and Harrie collaborated when it came to designing the BlackGEM telescopes. This is an array of 26" f/5.5 telescopes with a flat field of 2.7 degrees per frame, in 110MPixels. Here you can read more about their latest collaboration:

https://astro.ru.nl/blackgem/

Seen one of the telescopes in real life, and the design is extremely sophisticated.

 

The Jones-Bird in our club is 12" f/6 and although I do not know the exact prescription, I can ask Hans for it, no guarantees that it will be given. But I remember that the example in Telescope Optics Evaluation and Design 2nd edition, page 196, about a Plossl eyepiece which cancels out all the aberrations of a Jones-Bird telescope happens to be done with a scaled down version of exactly this instrument.

 

I know that the secondary mirror is attached without a spider but with a planparallel thin optical flat plate. But this is not comparable to a Chinese clone. It is a highly sophisticated design of which only a very small number were ever made.

 

It is on one mount together with a 300mm Takahashi Epsilon, one in the three known in the world!

 

Here you can see them together:

http://www.sterrenwa..._2008-11-17.jpg

 

The Jones-Bird is the telescope on the right, the Takahashi is on the left.

 

More questions? Feel free to ask.


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#22 donlism

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Posted 26 February 2018 - 06:03 PM

Another tidbit.. 

 

I believe that Newton's first Newtonian has spherical optics.. 

 

Jon

Is Newton still in business?  The website's down and nobody answers the phone.



#23 Jon Isaacs

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Posted 26 February 2018 - 08:11 PM

 

Another tidbit.. 

 

I believe that Newton's first Newtonian has spherical optics.. 

 

Jon

Is Newton still in business?  The website's down and nobody answers the phone.

 

 

I am not sure if Newton is still in business but I have the feeling that you're giving me "the business."   :)

 

Jon




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