Where to find a *good* optically flat mirror?

[Neil WyattModerator @neilwyatt]

You can get them relatively cheaply from various sources as newtonian telescope secondaries.

Orion Optics is one option.

For small ones, Astromedia are great, scroll right down: http://www.astromediashop.co.uk/Components.html

They should have something that will do the job for between £2.58 and £5.58 although there's P&P on top.

Neil

[Bob WorsleyParticipant @bobworsley31976]

H&W mirror is about 6" by 5" with corners cropped, about 1/2" thick, mounted in an aluminium casting.

For other metrology bits interested in a Watts Circular Division Tester? For measuring angles with circular graduation scale and microscope to read it. Calibration certificate says it goes to the few odd arc seconds. In a large and heavy wood box!

[Clive HartlandParticipant @clivehartland94829]

Autocollimation is where an injected graticule is put through the telescope and reflected back to the telescope and any deviation of level or postion is shown by the error against the telescope graticule.

Requirements are an injected graticule system and then a surface reflecting mirror. the telescope is usually a theodolite and has a special modified eyepiece which has a pass prism where the injected graticule can be sent to the Mirror.

Examples of use are transitting the hull of a Submarine to line up the torpedoe tubes and the Periscope. For this a specially modified T2 Theodolite is used. This has a selected fit, 'Stiff' axis so that the THeodolite can be mouned horizontally, to be able to look upwards and then transitting the scope to horizontal.

Normall accuracy is approx. one half secs.

One sytem I know of as i worked on it was to align the dishes on a satellite. Thiis being suspended high up and the theodolites on rigid pillars, costly and tedious.

Another was to align the test equipment for making of the head up displays by BAe.

[Pete RimmerParticipant @peterimmer30576]

Posted by Bob Worsley on 25/02/2021 11:21:07:

H&W mirror is about 6" by 5" with corners cropped, about 1/2" thick, mounted in an aluminium casting.

For other metrology bits interested in a Watts Circular Division Tester? For measuring angles with circular graduation scale and microscope to read it. Calibration certificate says it goes to the few odd arc seconds. In a large and heavy wood box!

Yes I'll be interested in both of those Bob. I will send you a message, thank you.

[Neil WyattModerator @neilwyatt]

Posted by Clive Hartland on 25/02/2021 13:13:14:

Autocollimation is where an injected graticule is put through the telescope and reflected back to the telescope and any deviation of level or postion is shown by the error against the telescope graticule.

A different application, but you may remember sending me a little laser module many years ago.

It eventually ended up in a laser collimator which I use for my telescopes.

Basically you centre the dot on the main mirror by adjusting the secondary, then adjust the main mirror to reflect the beam 'back up the spout'. The part with the laser in has an o-ring around one end and three tiny screws work the other to adjust it for concentricity.

Laser module, hand turned and anodised body, 3D printed battery and switch housing and laser printed decal. Nice mix of technologies!

Neil

[Pete RimmerParticipant @peterimmer30576]

Posted by Neil Wyatt on 25/02/2021 11:01:31:

You can get them relatively cheaply from various sources as newtonian telescope secondaries.

Orion Optics is one option.

For small ones, Astromedia are great, scroll right down: http://www.astromediashop.co.uk/Components.html

They should have something that will do the job for between £2.58 and £5.58 although there's P&P on top.

Neil

Thank you for the link Neil. I must confess to being pretty ignorant in the matter of mirrors and optics. It's difficult to come to an informed decision when you read some places that you need x quality and then hear that really only Y quality of mirror will do. I would never have thought of using telescope seconday mirrors with them being so thin in section.

I'm very appreciative and grateful for all the input, suggestions, offers and advice gained in the thread. Makes me glad that I started it because I've gained more than the offer of some suitable mirrors which was all I could have hoped for.

Edited By Pete Rimmer on 26/02/2021 19:59:26

[KWILParticipant @kwil]

edmundoptics.co.uk is a good source of things optical

[Neil WyattModerator @neilwyatt]

Posted by Pete Rimmer on 26/02/2021 19:58:21:

I would never have thought of using telescope seconday mirrors with them being so thin in section.

It rather depends, those ones are inexpensive because it is easy to make small 1/4 or 1/.8 wave flat mirror.

But for serious scopes (like my Newtonians) the secondaries are about a third as thick as their diameter.

Another source is old scanners, they often have long narrow front silvered mirrors of good quality.

Neil

[Pete RimmerParticipant @peterimmer30576]

I've bought this mirror from a member who was kind enough to offer it in response to this thread. It's in fantastic condition, the silvered face is 55mm and it's 20mm thick.

I intend to make a cast iron mount to hold the mirror, I have the materials and the facility but what is the best way to hold it inside the housing whilst allowing for adjustment in the vertical plane? The sled will be 2-piece with the base holding the mirror mount so I can allow for horizontal adjustments there. I have to think about fine-adjusting the mirror vertically.

I've done reading-up online and there seems to be a bewildering number of options and considerations. I'd like to keep it as simple as possible.

Pete.

[Michael GilliganParticipant @michaelgilligan61133]

Microscope condensers, for example, are usually adjusted by two screws working against a spring-loaded plunger.

Simple arrangement, and very effective.

MichaelG.

[Martin ConnellyParticipant @martinconnelly55370]

Tribrachs use for theodolites and total stations allow horizontal, vertical and angular adjustment. Should be easy to design one to suit your own needs.

Wikipedia entry on tribrachs

Martin C

[Pete RimmerParticipant @peterimmer30576]

Thank you for the replies.

It seems I have a lot to learn about mirrors and optics. I spent a good couple of hours yeaterday trying to figure out what was the problem with my autocollimator. When I rotated the tube from vertical to horizontal the lines were blurring and fading almost right out but appearing again when it hit the end stops. I figured there was something wrong with the instrument when in fact it was the mirror all along. It must be polarised or something. I swapped to a different mirror and the 'fault' went away

[Pete RimmerParticipant @peterimmer30576]

I want to thank everyone who has replied to my thread. I have secured a couple of Hilger & Watts mounted mirrors in great condition which I will be collecting this weekend.

[TendorParticipant @tendor]

Posted by Martin Connelly on 24/02/2021 09:17:30:

The disk in a scrap hard drive may be useable. If you have one to take apart the disk is aluminium so easily cut. Don't know how well it will work for your application but it may be a suitable stop gap until you get exactly what you want.

Martin C

I have gone a little way down the path of investigating this for the same purpose. My preliminary results were not very encouraging, but I report them here for the information of interested members.

I acquired an old HDD and extracted the three 90mm diameter aluminium disks. (Newer HDDs have glass platens and are only 65 mm diameter). From one of them I cut 5 small circles of 33 mm diameter – the largest possible (given the hole in the centre of the 90 mm disk).

Using an optical flat they were tested for flatness by rigging up a very rough 'monochromatic' light source (a compact fluorescent bulb with green cellophane over it – about 550 nm wavelength).

Attached are the fringe images of two of the ten surfaces available (both sides of the disk are recording medium). Only one was 'nearly flat' (parallel straight fringes). The other image is one of the "less flat" examples. Fringes are half a wavelength apart – about 275 nm in this case. One fringe deviation from straight is therefore about 275 nm. Hume's book on autocollimators mentions that reflectors should be flat "within a few millionth of an inch" (~ 75 nm)

My conclusion is that these are not really good enough for autocollimator reflector use. However, beware that this was a single test on only one platen. Others may be better.

[Mark RandParticipant @markrand96270]

Did you check the disk platter before cutting circles out of it? It may (or may not) have been more flat in that condition.

[Pete RimmerParticipant @peterimmer30576]

Posted by Tendor on 07/05/2021 13:43:56:Hume's book on autocollimators mentions that reflectors should be flat "within a few millionth of an inch" (~ 75 nm)

My mounted mirrors came with certification stating the measured flatness as 3 millionths and 7 millionths of an inch.

[Michael GilliganParticipant @michaelgilligan61133]

Not needed by the aficionados, I’m sure … but for the benefit of others : **LINK**

https://www.kemet.co.uk/blog/lapping/how-to-measure-flatness-technical-article

MichaelG.

[Author]

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