Help - 24in Mirror Flat?

[tweeb]

Hi,

Our Astro Soc has been left some of the estate of an astronomer who made his own telescopes and mirrors. One thing we have access to looks like a 24in mirror flat - its about 5in thick - and we've been told it weighs around 150lb - but we are not sure exactly what it is and if it is worth anything to anyone. Hopefully photo attached. We also have lots other smaller mirror flats!

Any help appreciated.

Thanks

Terry - Eastbourne AS

[vlaiv]

I doubt it is 24" mirror flat.

Mirror flats are used for optical tests on optical benches. They are of higher accuracy than most of telescope mirrors.

You can check this if one surface is perfectly flat / reflecting (coated).

Other than that - it is just 24" mirror blank - it is raw glass that is used to make mirror out of - by grinding in specific curve into it and later polishing and coating.

Mind you, 24" blank can cost a pretty penny depending on glass type. It is not as easily sourced as smaller diameters (up to 10"). If you decide to turn it into a telescope primary (and what else to do with such a blank? ) - then seek a very skilled telescope maker with years of experience - such a large mirror is going to be quite an undertaking for any man.

[Peter Drew]

I would investigate this one carefully. The colour of the glass suggests Zerodur which even as a blank would be valuable. If it is indeed an optical flat (the thickness is what would be required) and of good figure then the value would be "astronomical".   ?

[Rusted]

A large flat can be tested face up in a water bath [or transparent oil] with a monochromatic light source.

The fringes which appear must be perfectly straight. [Give or take the curvature of the fluid surface itself.]

An alternative is to set up the flat safely and examine the reflected image of the sky with a mounted telescope.

Any deviation from optical flatness will show in the distorted image as astigmatism or change in focus from the direct view.

A skilled and productive optician might well have invested the time and expense in a large test flat.

Or obtained one for testing his optics.

Despite the modern taste for thin mirrors that Zerodur blank is probably really valuable to somebody.

Finding that somebody and then getting the blank to them, in one piece, is quite another matter.

Alternatively, modern cutting techniques would allow a couple of 24" blanks to be sliced out of it.
 

 

[Chriske]

2 hours ago, Peter Drew said:

I would investigate this one carefully. The colour of the glass suggests Zerodur which even as a blank would be valuable. If it is indeed an optical flat (the thickness is what would be required) and of good figure then the value would be "astronomical".   ?

yep, that's Zerodur colour.
But maybe, just maybe it is Cer-vit

[Astrobits]

I would think that any potential buyer of that as a flat will want a written test analysis of the surface figure from a reputable source. Otherwise it is, in my book, just a blank, albeit with a hefty price tag.

Nigel

[Astrobits]

Rusted's suggestion of using a water bath is workable but you must view it at a very large distance to avoid the viewing geometry from showing curved interference fringes. Ideally the distance from your eye to the edge of the mirror should be significantly less than a couple of wavelengths or you will see a bullseye when the mirror is, in fact, perfectly flat. The best way to do this test is to use a collimating lens to produce essentially parallel beams reflecting off the mirror.  A lens smaller than 24" dia. will show part of the mirror and can be moved around to check the whole surface.

Nigel

[tweeb]

All - Thanks. Looks like I used the wrong terminology - suspect I should have said mirror blank. As are all the other mirror blanks we have from the estate. The guy had all the grinding materials as well but we just don’t have the room to store them - or anyone with the skills to make them into mirrors! Anyone want or need a 24in mirror blank, or know the best place to advertise? Proceeds go to EAS.

Thanks

Terry

[cjdawson]

If you write to the, oh what's the name of that organisation that all the astro societies sign up too.  I'm sure that once the word gets out, you'll have people interested.    A 24" blank, will no doubt make a lovely observatory grade mirror.

[Peter Drew]

14 minutes ago, cjdawson said:

If you write to the, oh what's the name of that organisation that all the astro societies sign up too.  I'm sure that once the word gets out, you'll have people interested.    A 24" blank, will no doubt make a lovely observatory grade mirror.

FAS (Federation of Astronomical Societies)   ?

[Peter Drew]

Worth mentioning on "Cloudy Nights" forum, the Americans have a very active amateur mirror making fraternity.   ?

[cjdawson]

17 hours ago, Peter Drew said:

FAS (Federation of Astronomical Societies)   ?

That's the ONE!