16 replies to this topic

[retroformat]

I've been entrusted with a small short focal ratio newtonian to "repair" because it can't come into proper focus.  On first inspection collimation seemed okay - just looking into the tube, I could see everything properly centered: my eye, the secondary reflected in the primary, the primary reflected in the secondary, the secondary seen directly in the focusing tube...

Taking it out for a star test, I did see a serious problem.  I was observing Saturn and could not reach a proper focus.  I could reach a partial focus, in that the planet was faintly and clearly visible, but then also surrounded by a large halo of unfocussed light.  Taking the scope in and out of focus produced a weird set of patterns.  Although weird, all these out of focus images were basically correctly symmetrical (radial symmetry), suggesting again, the collimation was good.  Far out of focus, both inside and outside focus, the image was a traditional "donut" - i.e. an image of the primary mirror with the secondary shadow in the center.  But nearer to focus, just inside and outside of focus things looked really weird.  Also the inside focus image did not match the outside focus image.

Very close to focus, just inside of focus, the image reminded me of what a Maksutov produces: a bright ring, with a series of what looks like interference fringes/rings fading towards the center.  Just outside of best focus the view was the opposite: a bright center, with rays and rings "radiating" and fading away from the center.  I attach a sketch of what I'm seeing, along with some quite poor image captures that I got with my cell phone.

It seems like there must be something wrong with the primary mirror.  Assuming the secondary is a flat, it shouldn't produce any optical effects at all.  But what if this newtonian had a spherical primary (say, installed/manufactured by mistake)?  What kind of optical figure would produce this kind of out of focus performance?

I have a fancy book, "Telescopes, Eyepieces, Astrographs," which is an amazingly detailed book about all sorts of optical designs, and the kinds of aberrations they produce.  But unfortunately it doesn't seem to cover optical performance outside of the focal plane; so this optical behavior remains a mystery to me...

 

 

[Jon Isaacs]

Was the scope thermally equilibrated?  

 

How large is the mirror, how thick is the mirror?

 

Jon

[deSitter]

You are describing a mirror with severe spherical aberration, probably because it is in fact a sphere and not a paraboloid.

 

A spherical mirror can give good performance if it is slow. It depends on the aperture. A 4" f/10 and a 6" f/12 sphere will give good performance. But anything f/8 or faster no matter what size should be parabolized.

 

-drl

Edited by deSitter, 05 November 2024 - 12:22 AM.

[Asbytec]

I've been entrusted with a small short focal ratio Newtonian...

 

My guess is the scope is a 130mm or 150mm beginner scope designed mostly for low power views. Possibly a (incorrectly labeled) "Bird Jones" type with a small corrector in the focuser. The correcting lens may be missing. As DeSitter said, that is classic spherical under correction typical of a fast spherical mirror. Some refer to them as "hobby killers", but they are descent with low power wide field views. Not Saturn. 

[deSitter]

My guess is the scope is a 130mm or 150mm beginner scope designed mostly for low power views. Possibly a (incorrectly labeled) "Bird Jones" type with a small corrector in the focuser. The correcting lens may be missing. As DeSitter said, that is classic spherical under correction typical of a fast spherical mirror. Some refer to them as "hobby killers", but they are descent with low power wide field views. Not Saturn. 

A friend won a Celestron 5" f/5 telescope as a door prize. This was the worst commercial telescope I ever evaluated. It had at least 2 waves of SA (so much I could not directly evaluate it) and was not even suitable for low power views.

 

-drl

[Jon Isaacs]

The image is labeled "avellone 4.5 inch newtonian focus problem".  So it is a small, fast Newtonian.  It has a three vane spider.  

 

I would like to know more about the scope and a photo or two would be helpful.

 

Jon

[Asbytec]

The image is labeled "avellone 4.5 inch newtonian focus problem". 

 

Jon

Is it a foreign branded model we're familiar with. I cannot find any images of it. 

 

 

A friend won a Celestron 5" f/5 telescope as a door prize. This was the worst commercial telescope I ever evaluated. It had at least 2 waves of SA (so much I could not directly evaluate it) and was not even suitable for low power views.

 

-drl

A Boy Scout brought his Celestron 127mm Power Seeker to an outreach.

 

I noticed the Celestron emblem and thought it was a nice looking scope. Until later when I realized what it was and got to play with it. I didn't have the heart to say anything since his father bought it for him. It is what it is. We had dark skies, so the low power star field was pleasing.

 

The focuser was plastic, as were the two supplied eyepieces and the Barlow. I bought him a couple of nice low and medium magnification Plossls and gave him my red dot finder. It's the best I could do with it...for him. I never did star test it. I think it had a 4mm high power eyepiece.

Edited by Asbytec, 05 November 2024 - 07:26 AM.

[retroformat]

Such great answers, thank you all!  I hadn't intended for this to be a fun puzzler, but you took it that way, and deSitter and Asbytec win the prize.  It is indeed a very fast 4.5" f/3.5 Rich Field Telescope made by Celestron.  I can't quite believe it, but what I'm hearing is that Celestron would choose to make such a horrible primary mirror, only in order to bring the scope's price down?  (or make more profit??)   ...or wait: would this scope have had a corrector lens in the focusser, that is lost / that I need to go find?  It seems to have just the regular 1.25" focusser.  Right now, this telescope produces images so bad, I wouldn't even want to give it away.

 

[retroformat]

I will give it one more try with a low power EP.  I am curious about that.  Admittedly, I was bothered largely by the high power views.

[Starman1]

I will give it one more try with a low power EP.  I am curious about that.  Admittedly, I was bothered largely by the high power views.

Give it a try with a 2X Barlow, which will convert the light cone to f/7, or even a 3X Barlow to yield an f/10.5 light cone.

The Barlow won't reduce other aberrations, but it might affect the SA.  

[Asbytec]

I can't quite believe it, but what I'm hearing is that Celestron would choose to make such a horrible primary mirror, only in order to bring the scope's price down?  

 

Yes, it's intended to bring the price down. These are entry level scopes designed for rich fields and low power lunar observing. You can probably see Saturn's rings and Jupiter's main belts at lower magnification, too. And some of the brighter showpiece nebulae. It's not too dissimilar from my first 60mm achromat except for the focal ratio. 

[retroformat]

I had that little scope out last night, this time to try a lower power EP.  With a 30mm EP it wasn't too bad, actually.  We'll find a 40mm EP for it, and it will make a decent loaner scope for large objects under dark skies.  Thanks again for everyone's replies.

[dan_h]

Such great answers, thank you all!  I hadn't intended for this to be a fun puzzler, but you took it that way, and deSitter and Asbytec win the prize.  It is indeed a very fast 4.5" f/3.5 Rich Field Telescope made by Celestron.  I can't quite believe it, but what I'm hearing is that Celestron would choose to make such a horrible primary mirror, only in order to bring the scope's price down?  (or make more profit??)   ...or wait: would this scope have had a corrector lens in the focusser, that is lost / that I need to go find?  It seems to have just the regular 1.25" focusser.  Right now, this telescope produces images so bad, I wouldn't even want to give it away.

 

The label on the focuser states that this is a 1000mm FL scope so most likely a Jones-Bird configuration with a corrector lens in the focuser tube.  I suspect that someone has reworked something to make it function as an RFT.  

 

The label also provides a Celestron part number  32114.   Easy to find on line. 

 

dan

Edited by dan_h, 12 November 2024 - 12:44 PM.

[Starman1]

Yes, definitely.  The lens in the focuser is missing.

You might get better images using a Barlow, though the focal length would be longer.

[Mike Lockwood]

Just a quick guess here, but the star test results appear to indicate undercorrection, which would be consistent with a spherical primary mirror.

 

Maybe I missed something in the discussion above, just wanted to mention this.

[CrazyPanda]

There also appears to be some kind of TDE/TUE if one side of focus has a hairy outer edge.

[azure1961p]

My guess is the scope is a 130mm or 150mm beginner scope designed mostly for low power views. Possibly a (incorrectly labeled) "Bird Jones" type with a small corrector in the focuser. The correcting lens may be missing. As DeSitter said, that is classic spherical under correction typical of a fast spherical mirror. Some refer to them as "hobby killers", but they are descent with low power wide field views. Not Saturn. 

I recall a guy at a star party with a 130mm newt, like F5  and the double double was split but immersed in this haze I knew wasn't seeing related and I knew what it was.  I didn't have the heart to say anything. 

 

Pete