Second hand spherical mirror > to parabolic?

[Honeybadger152]

So I recently purchased a second hand telescope since I've an interest getting into Astronomy. I knew that the scope wasn't great but at £45 and less than a mile away, I saw some value in it.

Being new I didn't want to invest much money at all.

The scope is a celestron ps1000.

As said before I'm aware it's certainly not something a seasoned amature would buy.

 

My question is...

Would it be possible to re-grind the mirror to make it parabolic?

Regards

Chris

[Dr_Ju_ju]

Why do you think that you want to re-grind the mirror ??  is it damaged in any way ?? 

Personally, I'd just use it as ism and get as much enjoyment out of it, and if/when you want to take the hobby further, then look at buying something that meets your requirements, for what interests you the most....  

[Cosmic Geoff]

1 hour ago, Honeybadger152 said:

Would it be possible to re-grind the mirror to make it parabolic?

That is certainly possible.  I once re-figured a 4" low-quality telescope mirror myself, a long time ago.   Unless the focal length is very short, it probably requires re-polishing rather than re-grinding.  Thw question is, is it worth the bother?  There is a suggestion that this is a Bird-Jones design whick means that the focal length of the mirror is a lot less than 1000mm.

Even if you can successfully re-figure the mirror, the telescope tube might require some adaption (made longer or shorter) to get the reworked telescope to focus.   My advice would be, that unless you are determined to attempt this as an experiment or optical project, you should use the telescope as it is, and look out for another second-hand telescope that better suits your needs.

It is possible to buy new mirrors, but usually in larger and more desirable sizes, and there is a site selling cheap parts from smaller telescopes, whose name escapes me at the moment, who might have a suitable mirror or complete tube assembly. 

[Honeybadger152]

I should have explained a bit more about my thoughts towards my query.

My intentions are to use the scope as is for now just to learn the ropes and to get a feel for whether it will become a hobby of mine.

 

Personally, throwing a grand or 6 at a setup doesn't excite me and this is no dig at anyone who chooses to do that.

I'm a bit of a tweaker kind of guy. I get a lot of satisfaction from squeezing as much performance out of things as possible. Modding/ adapting.

I like the idea of perhaps making my own telescope but I'm kind of learning that it's a bit pointless as the costs would likely exceed that of a purchased one.

Thanks for the input though

regards

Chris

 

[Merlin66]

What about trying the Kelley-Alder Mirror Flex System??

https://skyandtelescope.org/wp-conte...MirrorFlex.pdf

[Honeybadger152]

10 hours ago, Merlin66 said:

What about trying the Kelley-Alder Mirror Flex System??

https://skyandtelescope.org/wp-conte...MirrorFlex.pdf

This is very intriguing. I can't help but wonder though why this wouldn't be common practice with spherical designs.

Surely the offset cost for the extra materials would be cheaper than the more expensive parabolising process?

Do you know anyone that's done it with before and after images?

[Merlin66]

No, I haven't seen anything other than the S&T article, but it does look very "doable"

 

[Chriske]

As a matter of fact, many years ago it used to be standard procedure during telescopemaking course at our local club.
There are many things to consider using flexed mirrors.
The procedure has to be performed ab-so-lu-te-ly perfect. There no room for error.
After a while we abandoned the procedure because some course members were to sloppy during the gluing procedure.
But there's a reason why you should not do this. When a flexed mirror need a new coating, all companies will refuse that mirror. Reason : they will not put your mirror with a neoprene disk glued on it's back in their coater.

[Chriske]

Never tried it myself(!) but maybe this is a easier way.
Going to try it one of these days.

[Astrobits]

4 hours ago, Chriske said:

As a matter of fact, many years ago it used to be standard procedure during telescopemaking course at our local club.
There are many things to consider using flexed mirrors.
The procedure has to be performed ab-so-lu-te-ly perfect. There no room for error.
After a while we abandoned the procedure because some course members were to sloppy during the gluing procedure.
But there's a reason why you should not do this. When a flexed mirror need a new coating, all companies will refuse that mirror. Reason : they will not put your mirror with a neoprene disk glued on it's back in their coater.

There's is a valid reason coating companies will not put such material in the coaters. All organic materials are likely to out-gas preventing the vacuum becoming sufficient for a good coating. Also the out-gassed material will end up in the pump oil which again prevents a good vacuum being achieved and the oil will then have to be replaced at considerably more cost than the profit from the job.. It simply isn't worth the risk for the coating company.

Nigel

[Chriske]

I know Nigel, but I do have a coater myself(lucky me..😃). Had never any problems coating flexed mirrors at all.

[Honeybadger152]

On 22/09/2021 at 07:27, Chriske said:

As a matter of fact, many years ago it used to be standard procedure during telescopemaking course at our local club.
There are many things to consider using flexed mirrors.
The procedure has to be performed ab-so-lu-te-ly perfect. There no room for error.
After a while we abandoned the procedure because some course members were to sloppy during the gluing procedure.
But there's a reason why you should not do this. When a flexed mirror need a new coating, all companies will refuse that mirror. Reason : they will not put your mirror with a neoprene disk glued on it's back in their coater.

I don't suppose you have any before and after images Chriske? Demonstrating the improvement in optical accuracy?

I have been searching the net far a wide just for some sort of practical demonstration but can't find much at all.

[davidc135]

Interesting plan.

Sounds like you have a 5'' f/4 sphere for a primary. As it is, it will have 1.75 waves of spherical aberration as against a basic standard of 1/4 wave 

To be evenly parabolised by the harness the primary will need to have an even thickness around the edge. And the overall thickness will play a part in the force needed.

It may be that the eyepiees that came with the scope, if they have H, HM or R written on them, would be unsuitable for a fast F/4 scope although OK for the slower F/8.

Although basic Plossls or Kellners don't cost much.

Another component to check is the size of the diagonal.

David

[Honeybadger152]

2 hours ago, davidc135 said:

Interesting plan.

Sounds like you have a 5'' f/4 sphere for a primary. As it is, it will have 1.75 waves of spherical aberration as against a basic standard of 1/4 wave 

To be evenly parabolised by the harness the primary will need to have an even thickness around the edge. And the overall thickness will play a part in the force needed.

It may be that the eyepiees that came with the scope, if they have H, HM or R written on them, would be unsuitable for a fast F/4 scope although OK for the slower F/8.

Although basic Plossls or Kellners don't cost much.

Another component to check is the size of the diagonal.

David

Hi David

The scopes rating

127mm mirror

f/1000mm

f/8

Eye pieces have  H on them

20mm

12mm

Then a super small one that i've gotten no use out of

3. something mm

3x barlow only 1 lens

 

So far i've had a good look at jupiter and its moons. That was from indoors with awfull light pollution.

 

[Chriske]

5 hours ago, Honeybadger152 said:

I don't suppose you have any before and after images Chriske? Demonstrating the improvement in optical accuracy?

I have been searching the net far a wide just for some sort of practical demonstration but can't find much at all.

I do have images somewhere, need to search my HD's.
But it works just perfect. As a matter of fact using this technique you easily can pull the mirror to any CC you require, from ellipse to hyperbola, as long the mirror is not to thick.
That's the beauty of the system.

[Gonariu]

On 19/9/2021 at 20:19, Honeybadger152 said:

Così di recente ho acquistato un telescopio di seconda mano poiché sono interessato ad entrare nell'astronomia. Sapevo che il cannocchiale non era eccezionale, ma a £ 45 ea meno di un miglio di distanza, ho visto un certo valore in esso.

Essendo nuovo non volevo assolutamente investire molti soldi.

Il mirino è un Celestron ps1000.

Come detto prima, sono consapevole che non è certamente qualcosa che un maturo esperto comprerebbe.

 

La mia domanda è...

Sarebbe possibile rimolare lo specchio per renderlo parabolico?

Saluti

Chris

I have seen the telescope on the internet, for sure it is one whose original focal length of the mirror is stretched with a Barlow. I think you should keep it that way and practice with it a little and then move on to a better tool. Certainly that the mirror can be transformed from spherical to parabolic but from what I have read from reviews in Italian it is quite expensive. I'll give you one, that of the Newton 114/500 of an amateur astronomer who yielded little with the original spherical mirror, parabolising it had become a superb instrument, but is it worth it considering the expense? The link is this: http://www.davidesigillo.eu/test_114.html.  I'm sorry not to be able to put something in English, I hope that Google translator makes a comprehensible translation of the text (from English to Italian Google translator does excellent translations, from Italian to English I doubt it, on the other hand English is the language international par excellence, few speak Italian in comparison so it is clear that the efforts to improve the program are above all from your language to the others). I don't know what to think about these "barlowed" instruments, today they speak very badly in the Italian forums but with one of them, 114/1000, for the opposition of Mars in 1986 and 1988 I saw some beautiful drawings in the magazine of astronomy "Orione" that once published in Italy.

Edited September 23, 2021 by Gonariu

[davidc135]

8 hours ago, Honeybadger152 said:

Hi David

The scopes rating

127mm mirror

f/1000mm

f/8

Eye pieces have  H on them

20mm

12mm

Then a super small one that i've gotten no use out of

3. something mm

3x barlow only 1 lens

 

So far i've had a good look at jupiter and its moons. That was from indoors with awfull light pollution.

 

Just to be clear. As Gonariu mentions above the F/4 primary works with a Bird Jones lens that acts like a x2 Barlow extending the focal length to 1000mm and at the same time hopefully correcting the spherical aberration.

David

Edited September 23, 2021 by davidc135

[Honeybadger152]

3 hours ago, davidc135 said:

Just to be clear. As Gonariu mentions above the F/4 primary works with a Bird Jones lens that acts like a x2 Barlow extending the focal length to 1000mm and at the same time hopefully correcting the spherical aberration.

David

So what would this mean should I want to parabolize the mirror? Would it become more like f/3.5 or does that ratio change based on its shape?

I understand that i would need to reposition the secondary + focuser and remove the correction lens

[davidc135]

2 hours ago, Honeybadger152 said:

So what would this mean should I want to parabolize the mirror? Would it become more like f/3.5 or does that ratio change based on its shape?

I understand that i would need to reposition the secondary + focuser and remove the correction lens

I doubt that any tiny change in the focal length would be noticeable.  David

[Honeybadger152]

14 hours ago, Chriske said:

I do have images somewhere, need to search my HD's.
But it works just perfect. As a matter of fact using this technique you easily can pull the mirror to any CC you require, from ellipse to hyperbola, as long the mirror is not to thick.
That's the beauty of the system.

So Chriske, it would appear this mirror is an f/4 rather than an f/8.

From what i have read it is super difficult to flex mirrors successfully at that ratio?

I ask all of you, just that Chriske has stated he has experience in flexing.

[Chriske]

If it is a f/4 mirror you should not flex it.
A while ago I posted a thread called 'Printsonian', well that was a flexed 10" f6 primary mirror.

[Chriske]

After a quick search found a few pictures from long ago...

 

Here you can see there's no mirrorsupport at the perimeter of that 10". The mirror is held in place by the central bolt that is glued at the back of the mirror.
Just below the mirror you'll see the neoprene ring (the pusher part of the flex unit).

[Honeybadger152]

2 minutes ago, Chriske said:

After a quick search found a few pictures from long ago...

 

Here you can see there's no mirrorsupport at the perimeter of that 10". The mirror is held in place by the central bolt that is glued at the back of the mirror.
Just below the mirror you'll see the neoprene ring (the pusher part of the flex unit).

Have you got a spare one of those that you don't use anymore haha

That's a pretty sick looking scope