Grinding own mirror gives you a spherical or a parabolic curve?

[Cath]

I can't help but come to the conclusion that everyone who grinds their own mirror is actually making a spherical curved mirror rather than a parabolic, am I right?

I can't see how a parabolic curve can be made with the technique everyone appears to use to grind their own mirrors, because a parabolic mirror's curve is not constant, it's a shape that focuses to a single point AND in phase.

The thing with a spherical mirror is that it does NOT focus the light to a single point, at least not without a correction lens or correction secondary mirror.

So, which is it you diy'ers are making I wonder, spherical or parabolic mirrors?

[Gina]

Parabolic I'm sure but over to the mirror makers - I haven't got into that yet

[JB80]

From what I understand is you start by grinding a sphere and then when you have reached your desired focal lengths you then add the parebola by changing the stroke at the last stage.

I think, I'm still reading the book before starting mine.

[JamesF]

AIUI, you grind a spherical surface first, then make it paraboiic during the figuring stage.

James

[JamesF]

There's some information here, though it isn't that detailed:

http://stellafane.org/tm/atm/figure/figure_intro.html

James

[Cath]

ahhhh I see, cool

Shame that page doesn't show the details.

[callump]

The natural mirror grinding process will create a sphere.

The first part of making a mirror is to grind and polish to a sphere of the required focal length.

Then the mirror is parabolised, by using special polishing strokes, that changes the figure from sphere to paraboloid.

But, at long focal length f/10 + there is not much difference between a sphere and a paraboloid, so for these you can just stop once you have a polished sphere.

All mirror grinding books will cover this in detail.

Callum

[Cath]

ooo I don't think I could leave a mirror with a spherical curve, I'd have to go the full hog and make it parabolic. I wouldn't like the thought of having to make do with spherical aberration after all the work I'd put into it.

Books! .. I think I gave up reading books the moment the internet became useful .. so thought I'd take a peek on youtube about the subject, a picture/video tells a 1000 words

The rest I'll leave to the reader.

[Gina]

I use the net a lot but still find books useful. It is often difficult to find all the info on line.

[Beulah]

I find access to all types of media handy - the skill is to be discerning to avoid the misinformation you find on the web.

It's advantageous to have that information to hand once the theory becomes practice - and that's where books come in handy. Videos to me are supplementary and have to be compatible with the method I follow.

[Bizibilder]

The difference between the initial sphere and the final paraboloid is so small that you actually polish the difference rather than grind it - if you see what I mean! In practice it is actually very easy to overdo the "figuring" as it is called - how do I know? I'm not telling....... (I was only 15 at the time - many many years ago now!)

[JB80]

I find access to all types of media handy - the skill is to be discerning to avoid the misinformation you find on the web.

It's advantageous to have that information to hand once the theory becomes practice - and that's where books come in handy. Videos to me are supplementary and have to be compatible with the method I follow.

That's how I look at it, a bit of everything is not a bad thing and without having the PC or laptop getting covered in grit and muck where I'm going to be working I'm going to want some reference close at hand.

I'm printing off the necessary information I'll need from either scanning the book or various web sources.

I find video entirely useful though as it gives you a different feel that you can't get from looking at diagrams.

[barkis]

The Paraboloid is a series of zones on the mirrors surface, which have different focal radii.

Of course the curve, which is effectively the deformed Sphere, has to be very smooth, and

its difference calculated according to the focal length of the proposed mirror.

Figuring a mirror, is a very rewarding and fascinating exercise, but requires extreme patience, and the will

to retrace steps if necessary.

Ron.

[billhinge]

I made 3 mirrors in the past (8.5", 4" and 10") but not recently and yes the surface is parabolic for most normal newtonian forms. In the good old days people relied on books which contained best part of a 100 years of telescope making knowledge. In the past I must have borrowed all the old famous books from the library, Texereau, Howard, Ingalls, Berry but you don't see them much these days.

In answer to the question, the mirror surface is figured to parabolic curve during polishing rather than grinding stage since only minute changes are required. The old rule of thumb used to be that at f12 you could leave the mirror spherical.

A good book source is here http://www.willbell.com/tm/index.htm

I would recommend Manual for Amateur Telescope Makers which is a more modern book but contains the necessary detail brought up to date, but I also have Amateur Telescope Making vol 1,2,3 which are interesting from a historic perspective (got mine from a second hand book shop for a few quid), theory is there plus lots of 'lost ancient knowledge' which could be re-defined with modern technology. Telescope Optics is an advanced discussion book but not really a mirror making manual. Star Testing Astronomical Telescopes is worth a read but more post build testing

If you want to look at how modern technology helps subscribe to the mailing list here http://www.atmlist.net/mailman/listinfo.cgi/atm

good luck