question on new telescope primary mirror

[sibelius (Valerio)]

Hi everyone
I 'm thinking to make a new mirror for a new telescope and I am thinking to make it out of marble.
I have one question.
Let's suppose that I can get a parabolic surface for a fixed focal length that I previous decide and let's suppose that at this point I need to proceed to a fine grinding.

Now my question is, how can I proceed to fine grit to smooth the surface without loosing the surface shape?
I have already an idea but I would like to ear some other bell.

Thanks
V.

[Rod]

Hi Valerio

For a telescope mirror you need a homogenous material. That’s difficult with natural materials. Is there any reason you want to use marble? Plate glass or Pyrex-like are usually not hard to get.

Rod

[OzEclipse (Joe Cali)]

Hi Valerio,

I've worked with various carbonates in Earth Sciences research for 40 years.

Marble is a metamorphosed ocean sediment composed of microfossils. It's very soft and not homogenious on a microscopic let alone atomic scale. You can polish marble to a shine but I'd be surprised if it can be made smooth enough to accurately reflect a wavefront. If you make marble thin to reduce weight, it's brittle and can easily crack. I suspect the surface will similarly be full of small pits. I can't say how thermally stable it is but as others have advised, pyrex or plate glass is the easiest material to use as an amateur optician.

What size blank are you looking for?

Joe

[sibelius (Valerio)]

Hi everyone, thanks for your answer.

The reason I decide to go with marble are:
1- I saw a telescope with a Carrara marble primary mirror and I have to admit that the image are not bad at all.

2- I have already a piece of marble that comes from the Pilbara; it is probably the most expensive marble available and the consistency reminds me more like an obsidian stone; if I hit two pieces together the sound is quite high compare with a Carrara marble; in my bathroom the reflection of the top of this marble is quite amazing.

3 - In the beginning I was searching for a glass blank but I had some trouble find one of the size 16inches and then I was thinking of the problem of aluminizing the final product

I was looking also at the refractive index of a general glass and marble and I would say that in a "good range" of wave lenght are comparable.

So what I have is basically this piece of marble of 495x403x30mm that I am planning to machine with my cnc.
I machine many piece of marble doing relief and incision and never had problems so I am thinking to have a go.

I am still thinking as I will proceed to smooth the surface to the final result.

thanks for the moment and please feel free to add any of your thoughts.
V.

[OzEclipse (Joe Cali)]

3 - In the beginning I was searching for a glass blank but I had some trouble find one of the size 16inches and then I was thinking of the problem of aluminizing the final product.


Why would you think that aluminizing marble will be easier than glass?

Before you start, make sure you find an optical coater who is willing to aluminise a marble substrate for you. Make sure you get overcoating because you may never be able to re-coat the mirror. Old coatings are removed with highly corrosive liquids some of which are acidic and that will dissolve the optical surface.


I was looking also at the refractive index of a general glass and marble and I would say that in a "good range" of wave lenght are comparable.

Light does not pass through a mirror so the refractive index of the material is irrelevant.



So what I have is basically this piece of marble of 495x403x30mm that I am planning to machine with my cnc.
I machine many piece of marble doing relief and incision and never had problems so I am thinking to have a go.

I am still thinking as I will proceed to smooth the surface to the final result.


You can probably chomp out the surface to make the two curves. CNC machining is only any good for very coarse machining, it isn't nearly an accurate enough machining method for something like this.

Depending upon the quality / grade of CNC machine you have, you need a surface accuracy 1000-10000 times more accurate than a CNC can provide.

You will still need another piece of marble to use as a tool to grind and polish the surface in the conventional way.


Good luck
Joe

[sibelius (Valerio)]

Hi Joe thanks for your message.
I am not planning to do any further treatments on the marble except fine grinding.
I m thinking to make a tool with opposite curve of the marble and stick on it a sand paper for the wet and dry sanding then mount this tool on a motorized lazy susan that spin the marble.
But is all theoretical, I am so slow and in the main time I might decide to buy a 16inch mirror, who knows? Thanks anyway

[OzEclipse (Joe Cali)]

Originally Posted by sibelius

Hi Joe thanks for your message.
I am not planning to do any further treatments on the marble except fine grinding.
I m thinking to make a tool with opposite curve of the marble and stick on it a sand paper for the wet and dry sanding then mount this tool on a motorized lazy susan that spin the marble.
But is all theoretical, I am so slow and in the main time I might decide to buy a 16inch mirror, who knows? Thanks anyway

Just use abrasive powder. Sand paper definitely won't work. And how will you work your way down to finer abrasive? Reading your comments makes me think you have not made a mirror before. This book by Jean Texereau describes grinding, polishing and testing in some detail. It would be well worth your while to read about mirror making before proceeding.

https://tinyurl.com/ma7z6bb2

[sibelius (Valerio)]

Thanks for the information, I will read it.
You are right I never made a mirror before.
My idea was, after making the surface, to cast a tool same size of the marble, fix this tool on a vertical metal bar centered on the marble center line, spin the marble on a motorized lazy Susan and let the tool work by gravity on the marble.
The idea is not to let the tool moving across (that's why the same diameter and working on the same center) trying preserving the initial surface.
Anyway, i will try to gain more knoledge and I will eventually post if I can achieve something.

Thank you very much for the moment.
V.

[OzEclipse (Joe Cali)]

You have it the wrong way round. The piece on top becomes concave (the mirror) the piece below becomes convex(the tool).

The edges do all the cutting, so the tool and mirror must move longitudinally and transversely across the surface. Otherwise you won't get a spherical surface.

You need the movement to generate the sphere. The length of the strokes needs to be varied in both axes. Mechanical mirror grinding and polishing machines have the ability to vary the stroke pattern, length and width. If the length and width of stroke is not regularly varied, concentric rings are ground into the surface.

You are thinking like a machinist not an optician. As I mentioned before, the most precise machining is 1000x too coarse for optical surface figures. You can machine out the very rough figures for mirror and tool but then you need to go conventionally

[sibelius (Valerio)]

Hello Joe

"You can machine out the very rough figures for mirror and tool but then you need to go conventionally"

That's exactly what I was asking at the beginning, so the answer to my question is that I cannot do it all by machine.
Well well well, I need a bit more time to think and digest what you wrote.

thanks for the moment
V.

[OzEclipse (Joe Cali)]

You can put the tool on top if it's smaller than the mirror. If they're the same size, then the tool needs to be underneath.
This video shows the use of a sub diameter tool on top.

https://tinyurl.com/2p93sucv

Better machine designs will also move the tool using two cams at right angles to drive the tool left/right & forward/back The stroke lengths can be adjusted.

With this design, the overhang amount will need to be frequently adjusted.

[Rod]

Hi Valerio

As ozeclipse suggested earlier, Texereau’s book is an excellent resource. Some online ones that are also very good are:

http://www.bbastrodesigns.com/tm.html#myArticles

https://stellafane.org/tm/dob/index.html

A 16 inch mirror is a very challenging if it is your first mirror. Some have done it but most start with a 6 to 10 inch mirror and then move up. I’ve made mirrors by hand and by machine. Personally I think you need to make one or two by hand before using a machine. That way you have more of a feel for what for you are trying to get the machine to do. Most of the texts are geared towards working by hand too. A simple motorised turntable running at 1 to 5 rpm though is helpful for working by hand if you have one or can build one easily. But it’s not essential.

Rod

[GrahamL]

Lots of great advice coming your way V.


Agree with all the above in that this is likely a very bad choice of substrate.. that you would like to try your hand at making your own mirror is a great idea .. best wishes to you in regard.