Bird-Jones (Jones-Bird)

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mikemarotta
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Bird-Jones (Jones-Bird)

#1

Post by mikemarotta » Sun Dec 29, 2019 2:31 pm

Most Newtonian reflectors have parabolic mirrors. A Bird-Jones has a spherical mirror because they are easier to manufacture. The spherical aberation is corrected with a lens inside the focuser.

See also Wikiepeda on Newtonians
https://en.wikipedia.org/wiki/Newtonian_telescope
and scroll down to Jones-Bird.

This Wikipedia image shows that a parabola focuses all incoming light to one point:
Image

(The other way to think of this is that your automobile headlights are parabolic mirrors with the lamp at the focus. The beam is a column of parallel rays.)

A spherical reflector must of necessity create a fuzzy image where the "focus" should be.
1372
(From the University of Texas physics department. Prof. Richard FitzPatrick.)
http://farside.ph.utexas.edu/teaching/3 ... de136.html

Of all the inexpensive beginner telescopes on the market right now (2019), the Celestron 127 EQ is apparently the best-selling model with the Jones-Bird design. The consensus in the hobby is that this design should be avoided by the amateur astronomer. It is offered because of the reduced cost and (as a positive feature) because it allows a shorter length tube for easier travel. Those factors being as they may, other solutions to those problems are better.

"A good way to tell if a telescope is a Bird-Jones is to compare the focal length to the physical length of the telescope tube. The above-mentioned [Celestron 127 EQ] PowerSeeker has a focal length of 1,000 mm, but the physical tube size is 508 mm. This means that there must be something to double the effective focal length, which would be the correcting Barlow lens." -- http://astrowiki.jmhastronomy.com/index ... _Telescope
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#2

Post by Richard » Sun Dec 29, 2019 4:33 pm

Good write up , I have had many of these scopes and mostly bad so not recommended but a few have been good , perhaps it all to do with quality control as these are usually cheap.
One must remember that quality scopes like SCT and Mak's also have primary that is spherical but the corrector is better
There are a lot of Newtonian scope with Spherical primary with no corrector
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#3

Post by SkyHiker » Sun Dec 29, 2019 5:09 pm

To be fair, a parabolic mirror only focuses a parallel beam in one point if the beam is parallel to the optical axis. The coma gets larger the further you get away from the axis. The spherical aberration of a spherical mirror is constant. So for a fair comparison you should compute a weighted average of beams over all incident angles.

Also, spherical aberration can be corrected by a properly shaped lens. This is what the designers had in mind so we can't blame the original design. Of course it is tempting to cut corners to reduce the price if the aberration is not too bad, which is why it has a bad reputation.
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#4

Post by yobbo89 » Sun Dec 29, 2019 5:15 pm

I think f ratio plays a huge role as well for spherical mirrors.
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#5

Post by stewe » Sun Dec 29, 2019 6:00 pm

Thanks for the post, Mike. I agree with Henk. The main problem with the J-B telescopes lies not in the optical design, but rather the poor manufacturing that commercial J-B scopes suffer from nowadays. This is due to the fact that their market niche shrank to the cheapest segment since fast parabolic mirrors became cheap to make (and which need a coma corrector to give sharp views anywhere off-axis). 20 years ago, when fast parabolas were quite expensive, Celestron made J-B scopes of higher quality, the G-8N (8") and the C-150HD (6", I used to have the latter). Having said that, these scopes also lacked very high reputation. But again, it was not due to their bad optical design or bad optical quality, but Celestron's failure to pay attention to the details (focuser, mirror cell, baffling, spider, etc. etc.). By the way, in these scopes, the corrector was a larger group of lens between the primary and secondary mirrors. Anyhow, it seems that the commercial production of J-B scopes got cursed somehow, and it does not seem like it will get a blessing, unfortunately.
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#6

Post by mikemarotta » Sun Dec 29, 2019 6:24 pm

SkyHiker wrote:
Sun Dec 29, 2019 5:09 pm
To be fair, a parabolic mirror only focuses a parallel beam in one point if the beam is parallel to the optical axis. .. Also, spherical aberration can be corrected by a properly shaped lens.
Thanks for reply. We can assume parallel rays coming in from almost anything we view as astronomers. That said, you are correct in that at some level of detail, aberration must be accepted or corrected. I worked for a year for Carl Zeiss as a trainer and technical writer. Intersecting my other hobbies, I have commemorative coins and stamps from Germany (East and West) celebrating Zeiss and Abbe. Every design has trade-offs or else there be just one over all others and nothing in close second place.

For myself, as much as I enjoy observational astronomy, I have been considering other media for experiencing the universe.
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#7

Post by mikemarotta » Sun Dec 29, 2019 6:40 pm

stewe wrote:
Sun Dec 29, 2019 6:00 pm
... But again, it was not due to their bad optical design or bad optical quality, but ...
Thanks. See my response to SkyHiker above. My motivation in posting was the lack of any clear statement here on what a Jones-Bird or Bird-Jones is. Like so much else, I had to google it. So, I put together something for the Beginner's Forum so that other people could have some context.

On that note, Woodblock from Shropshire wrote of his SW, which you might have known immediately as a SkyWatcher. I only learned of the firm recently. Here in Texas, SW is Smith and Wesson. I kid you not. I did not have a telescope for many years, then got one five years ago. I was unfamiliar with the red dot finder. When I mentioned having trouble with it, one of the guys in my local astronomy club said, "It works just like the one on your gun."

We use a lot of jargon here on this board. And we have an international membership. As a technical writer, it is my habit to introduce new words (initializations, abbreviations) with an explanation. YMMV.
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#8

Post by SkyHiker » Sun Dec 29, 2019 6:48 pm

mikemarotta wrote:
Sun Dec 29, 2019 6:24 pm
SkyHiker wrote:
Sun Dec 29, 2019 5:09 pm
To be fair, a parabolic mirror only focuses a parallel beam in one point if the beam is parallel to the optical axis. .. Also, spherical aberration can be corrected by a properly shaped lens.
Thanks for reply. We can assume parallel rays coming in from almost anything we view as astronomers.
Aha, so the use of coma correctors is plain nonsense huh.
... Henk. :D Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK
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#9

Post by stewe » Mon Dec 30, 2019 1:31 pm

mikemarotta wrote:
Sun Dec 29, 2019 6:24 pm
We can assume parallel rays coming in from almost anything we view as astronomers.
Mike, with all due respect, I think you might misunderstand something. What do you mean by parallel rays? Parallel with the optical axis? Coma is produced by rays not running parallel with the optical axis, i.e., rays from sources that are not at the dead center of the field of view.
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#10

Post by mikemarotta » Mon Dec 30, 2019 5:43 pm

SkyHiker wrote:
Sun Dec 29, 2019 6:48 pm
mikemarotta wrote:
Sun Dec 29, 2019 6:24 pm
We can assume parallel rays coming in from almost anything we view as astronomers.
Aha, so the use of coma correctors is plain nonsense huh.
stewe wrote:
Mon Dec 30, 2019 1:31 pm
Mike, with all due respect, I think you might misunderstand something. What do you mean by parallel rays? Parallel with the optical axis? Coma is produced by rays not running parallel with the optical axis, i.e., rays from sources that are not at the dead center of the field of view.
Yes, I get that. See my remarks above: I worked for Carl Zeiss as a trainer and technical writer. For extremely fine photographs, yes, indeed, you want to correct away as many aberrations as possible. The techniques for achieving that are available. My statement was a first order approximation for the Beginner's Forum. If you want to write something more technical for Astrophotography, then feel free to contribute.
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#11

Post by JayTee » Tue Dec 31, 2019 12:21 am

Gentlemen,

I moved this informative post over to this forum as it had left the realm of knowledge possessed by the beginner. Also, it will be easier to find this information by future searchers if it lives in this forum.

Cheers,
JT
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#12

Post by notFritzArgelander » Tue Dec 31, 2019 12:50 am

mikemarotta wrote:
Mon Dec 30, 2019 5:43 pm
SkyHiker wrote:
Sun Dec 29, 2019 6:48 pm
mikemarotta wrote:
Sun Dec 29, 2019 6:24 pm
We can assume parallel rays coming in from almost anything we view as astronomers.
Aha, so the use of coma correctors is plain nonsense huh.
stewe wrote:
Mon Dec 30, 2019 1:31 pm
Mike, with all due respect, I think you might misunderstand something. What do you mean by parallel rays? Parallel with the optical axis? Coma is produced by rays not running parallel with the optical axis, i.e., rays from sources that are not at the dead center of the field of view.
Yes, I get that. See my remarks above: I worked for Carl Zeiss as a trainer and technical writer. For extremely fine photographs, yes, indeed, you want to correct away as many aberrations as possible. The techniques for achieving that are available. My statement was a first order approximation for the Beginner's Forum. If you want to write something more technical for Astrophotography, then feel free to contribute.
It's not just photographs. Visual use shows coma as well. The diagram only works for focus at the exact center of the field of view. Coma sets in equally for visual or photographic use away from the center of field of view for incoming parallel rays.

Celestron's approach to the Bird-Jones corrector is (as I understand it) to simply use a Barlow and increase the effective f ratio to where the difference between a spherical and parabaloidal mirror makes no difference. This is different from the original concept.

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#13

Post by Lady Fraktor » Tue Dec 31, 2019 6:03 am

One of the many issues with these telescopes using a spherical mirror/ barlow combination is that they require a larger secondary mirror than is provided.
At best these so called beginner telescopes are a bundle of compromises wrapped in a to short tube.
Give me one properly made and at around f/10 and I will happily view with it all night.
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#14

Post by notFritzArgelander » Tue Dec 31, 2019 6:46 am

Lady Fraktor wrote:
Tue Dec 31, 2019 6:03 am
One of the many issues with these telescopes using a spherical mirror/ barlow combination is that they require a larger secondary mirror than is provided.
At best these so called beginner telescopes are a bundle of compromises wrapped in a to short tube.
Give me one properly made and at around f/10 and I will happily view with it all night.
Yep. The fault is not the design it is in the cost cutting that goes into the building of these things.
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