TheSkySearchers-Welcome!

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:


(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.

(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

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

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.

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

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.

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.

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.

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.

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.

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.

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
[mention]mikemarotta[/mention]

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.

https://www.telescope-optics.net/sub_ap ... htm#bottom

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.

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.

I came back to this discussion because of a comment. I have personnaly never perceived comas. They may well be there, but I never noticed them. My mental focus typically is on the center.

SkyHiker wrote: ↑Sun Dec 29, 2019 6:48 pm Aha, so the use of coma correctors is plain nonsense huh.

I have no experience with them, though I see in another discussion that you are invested in the purchases.

SkyHiker wrote: ↑Sat Apr 17, 2021 7:17 pm Good idea Gordon! For galaxies and other smaller targets a 10" Newt is great. Not sure if it's an astrograph or visual but a Newt astrograph with a good coma corrector can be an amazing tool for astrophotography. With my 12" Newt astrograph the Paracorr 2 produces super round stars all the way into the corners and I'm now quite happy with the choice (though I have to improve the mount mechanics a bit). The Paracorr 2 CC is not cheap (and I have a Baader MPCC Mk III like yours) but, reading up on performance, I decided to get the Paracorr 2. No regrets at all. I haven't plugged the Baader in yet to see the difference, I may do it some time.

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 understood this only as noise. An open tube would seem to invite nothing but glare all over. In point of fact, the open tube and truss work just fine because light travels in straight lines -- barring relativistics effects of gravity. The parabolic reflector works because the angle of incidence is equal to the angle of reflection at the tangent to the curve and the shape of the parabola directs incoming rays to the focus -- assuming that the rays are entering parallel to the axis.

stewe wrote: ↑Mon Dec 30, 2019 1:31 pm 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.

So, I dug a bit deeper.
So, I have a better understanding of the problem that annoys other people.

The faster the mirror the more likely you will have coma either photographically or visually.
It is just inherent to the design be it parabolic or spherical.

The faster the mirror the more likely you will have coma either photographically or visually.
It is just inherent to the design be it parabolic or spherical.