26 replies to this topic

[Irido]

I was scrolling through AliExpress, and found many listings for some mirrors that looked quite peculiar to me. They're spherical mirrors, 203 mm in diameter, but with purported focal lengths of 800 or even 750 mm! Just searching "203 mm telescope mirror" yields many results. They're extremely cheap as well, available for about 80 or so dollars, shipping included. What are these intended to be used for? Such a fast spherical mirror would surely exhibit terrible spherical aberration, enough to preclude its usage in any sort of uncorrected Newtonian. I'd think they're a bit too slow to be used for Mak-Cass or Schmidt-Cass, not to mention that they lack a central hole. What is the intended use for these mirrors? Is there any manner by which an amateur could correct one for use in a DIY scope? I'd presume not, beyond just slowing them down a whole bunch; making something like a corrector plate for a Schmidt/Mak-Newt is surely beyond the reach of all but the most skilled amateurs. Any insight would be appreciated!

[Astrojensen]

I think the Chinese actually do sell spherical 8" f/4 Newtonians to unsuspecting customers. 

 

 

Clear skies!

Thomas, Denmark

[Irido]

I think the Chinese actually do sell spherical 8" f/4 Newtonians to unsuspecting customers. 

 

 

Clear skies!

Thomas, Denmark

Well, I suppose I shouldn't have had anything but the lowest possible expectations for unscrupulous AliExpress dealers. Do they sell accompanying correctors of any sort, or just leave those who bought such horrible instruments with the 8" equivalent of the department store special?

Edited by Irido, 11 November 2024 - 05:04 PM.

[Jon Isaacs]

What is the intended use for these mirrors?

 

 

The basic purpose is to separate unsuspecting customers from some of their hard earned cash.

 

Jon

[Pinbout]

Where's the link

[Astrojensen]

Do they sell accompanying correctors of any sort,

 

No.

 

 

or just leave those who bought such horrible instruments with the 8" equivalent of the department store special?

Yep. 

 

 

Clear skies!

Thomas, Denmark

[ShaulaB]

Have you considered grinding your own mirror? There may be a few folks around where you live who still know how to make good mirrors. If the goal is to save money, making your own mirror won't save you much, as it will eat up a lot of your time and space at home.

[Irido]

Have you considered grinding your own mirror? There may be a few folks around where you live who still know how to make good mirrors. If the goal is to save money, making your own mirror won't save you much, as it will eat up a lot of your time and space at home.

 

I haven't considered doing it myself; I tend to be dead awful at such precision work . I do wonder if these mirrors could be a suitable spherical starting place for those willing to remove the coating, refigure them to paraboloids, then recoat them. My guess is that they're unlikely to be of much use there either, as they're not cheaper than 8 inch blanks, and grinding a spherical surface tends to be the easier part of mirror making, from what I can tell.

[Pinbout]

 

and grinding a spherical surface tends to be the easier part of mirror making

maybe but then you have to polish it out. that's about 1 hour per inch.

[walt r]

If the glass is good and well annealed then they would be good as a Blank with a pre-polished curve.

 

I recently re-figured a couple of 4.25" f/4 mirror (1970's surplus). They were supposed to be parabolic but the figures were horrible.

 

Best part of refiguring at same f/ was not needing to polish out a ground surface.

 

Or one could re-grind to a different f/ like I just did to a surplus 6" f/10 mirror, again with a bad figure to f/5.

Been working on this one for a couple of months and now down to the finial touches.

[davidc135]

For the more adventurous, a corrector plate for a Schmidt Newt needs only to correct for 2.8 waves compared to around 16 in an 8''sct, so manageable.

 

A two element Houghton corrector with spherical surfaces will be flat fielded and free of coma, good for an astrograph but, of course, plenty more surfaces to polish out.

 

David

[chantepierre]

With friends we bought a lot of 6" f/5 mirrors sold as defective on Ali. The glass is good, the figure is just very bad. You can see that as cheap pre-polished blanks ready to refigure, just need to strip the coating on arrival.

[BGRE]

If nothing else they could be used as samples with likely pathological defects to hone/compare tests for measurement/detection of the defects.

[PalomarJack]

I would not even waste time trying them out. Make a tool/lap and re-figure them to parabolic. Most of the work is done for you.

[Astrokg]

There are many of us out there that cannot grind mirrors let alone get them recoated (nowhere in my country offers recoating). Therefore the aliexpress mirrors are very tempting. Some say they are parabolic, is that to be believed? There is even one with a good review saying it's confirmed as parabolic. We really need to sort out whether these mirrors are any good, do you simply have to pay a bit more? I have been very happy with lenses from aliexpress, have been looking at the mirrors and wondering what if? Has anybody has any real experience with these?

[Astrokg]

Here is link for 6 inch mirror with good reviews
https://a.aliexpress.com/_mskLtlZ

[triplemon]

If its parabolic, it does not automatically mean its good. And "good" can vary vastly, depending on the intended use and expectations. And as mentioned before a 6" f/5 is a very different animal as an 8" f/4

It might well be this is a mirror from the same source and quality as what rock bottom quality toy scopes use. Or it could be much better. Since this particular ad is a reseller that mostly seems to deal in microscope stuff, it could even be a mirror from a very different source every time anyone orders one ...

There is all kinds of sellers on aliexpress from major companies selling high end products factory outlet style to individuals selling home made stuff. The good thing is, most things sold on aliexpress also are subject to return policies and refunds just like many other reputable dealer offer, so taking your chances is less risky than some claim.

Edited by triplemon, 14 November 2024 - 05:41 PM.

[CrossoverManiac]

I think the Chinese actually do sell spherical 8" f/4 Newtonians to unsuspecting customers. 

 

 

Clear skies!

Thomas, Denmark

They might have been actually worth something if they weren't aluminumized and sold as mirror blanks that only needed fine grinding to finish.

[perfessor]

Here is link for 6 inch mirror with good reviews
https://a.aliexpress.com/_mskLtlZ

I note that it is an f/5 mirror, not f/4.  So if that's what you want ...

 

I once bought one of the 6" Bird-Jones scopes at a garage sale, worth the money at $15 I thought.  My plan was to take the spherical f/3 primary and refigure it to parabolic, then make a rich-field scope.  I may yet do that someday. Just for fun.

Edited by perfessor, 19 November 2024 - 03:21 PM.

[Astrokg]

Here is the f/4 203mm link
https://a.aliexpress.com/_mN5HlGX

It openly admits being spherical. So back to the original question, is this mirror of any use? So cheap you could knock up a test setup in no time I think. What is spherical aberration anyway? I have a refractor for planets but wanted a large aperture to help see dimmer things. They tend to be blurry blobs anyway so would the spherical aberration make much difference for them eg globulars ?

[davidc135]

Here is the f/4 203mm link
https://a.aliexpress.com/_mN5HlGX

It openly admits being spherical. So back to the original question, is this mirror of any use? So cheap you could knock up a test setup in no time I think. What is spherical aberration anyway? I have a refractor for planets but wanted a large aperture to help see dimmer things. They tend to be blurry blobs anyway so would the spherical aberration make much difference for them eg globulars ?

The spherical aberration of any spherical mirror is given by

 

SA = Ax22.55/f#³ for wavelength = 550nm and where A is aperture in inches.

 

So, for the 8'' f4 that's 2.8 waves; terrible. There won't be any particular point of focus, the outer zone focusing first and the mirror centre focusing last as the ep is moved further from the mirror. Each point of zonal focus is surrounded by a fuzz from all the other zones.

 

There will be a disc of least confusion, where the fuzz is smallest, that might approximate to a star if the magnification is low enough.

 

Interesting to see how views look at x30.

 

David

Edited by davidc135, 21 November 2024 - 10:18 AM.

[Astrokg]

That is great info David.
I have seen 200mm spherical with 1600mm focal length instead.
So f/8.
SA = 8x22.5/512
= 180/515
= 0.35

Would that be much better!?
I see the SA is inversely proportion to cube of f number, so it gets a lot better for longer focal lengths.
This makes me think if you want to go cheap and spherical make a long focal length telescope ...

[davidc135]

But at f8 it's an awkward beast for low power, whilst 0.35 waves will prove disappointing at higher powers. 1/4 wave is competent if the surface is smooth but with cheap optics it could be anything.

 

It's hard to be positive about 8'' spheres unless you intend to do the finishing optical work. By the time you've put everything else together to make a scope it seems better to pay the extra for a decent mirror.

 

David

[Astrokg]

I have found a long thread on this very topic...
https://search.app?l...l1,sh/x/gs/m2/4

[davidc135]

On the plus side, an 8'' f8 should work well at low powers with much reduced coma and a near flat field.

 

In the future, if you ever felt like tackling amateur optical work, a finished f8 sphere offers a good introduction for a beginner with a relatively small amount of (tricky) figuring and smoothing needed to transform it into an excellent performer.

 

However, there is a cost, mostly in the recoating.

 

David