Purchasing an Optical Flat

For some reason I feel that I need an Optical Flat, and like everyone else I'm trying to get the most bang for a buck. Ox Tool did testing on a surplus Russian optical flat Russian Optical Flat Testing - YouTube
From the results of his Youtube test, it looks like the Russian flat are very attractive in quality and price. This is a typical eBay flat Optical Flat Parallel Top 6mm (Flatness .7mm - .28”) | eBay
If you look at the title of the listing "Optical Flat Parallel Top 60mm" and other listings will be "Optical Flat Parallel bottom 60mm". The difference between the 2 descriptions is wording "top and bottom". I'm going to make an wild assumption that wording of "parallel top" means that 2 surfaces, top and bottom, are parallel within a given measurement. The wording "parallel bottom" the top is not considered any measurable function of the optical flat.
An example "parallel top" would mean that it could be used to check the flatness and parallel of a micrometer spindle faces. The "parallel bottom" would not accurately function for this type of test.

Optical flats are made to test flatness by being in contact with an object. Unless it is made specifically for testing micrometer face parallelism (like the Mitutoyo set made for that) it is unlikely to have the two faces parallel. An ok price though.

If you want to try things out without spending much at all, look at Surplus Shed. The linked item isn't perfect , being only polished on one face, but at $10, it is an easy entry into the subject. Keep an eye on them and they will probably turn up something more appropriate in time. Edmund Optics will supply from stock without any wait, but at 30-40 times the price.

For a monochromatic light, a low pressure sodium SOX light, starter and ballast are very cheap.

No connection to either except I bought a number of 1/20th wave first surface mirrors from Surplus Shed for autocollimator targets. Even with postage to the UK, the price was good!

Mark Rand said:

I bought a number of 1/20th wave first surface mirrors from Surplus Shed for autocollimator targets.

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Wonder what your thoughts are on any of the many Molybdenum laser mirrors made for CO2 lasers so cheap and common on eBay?

Usable with a visible-light autocollimator, too, perhaps?

But one example of many - given I'm actually in an "ABC" (Anywhere BUT China) mode for the time being:

1pc Dia. 20mm Mo Mirror Reflector for CO2 Laser cutter engraver | eBay

Most of the optical flats I've used (in laser systems, not for checking parts) are purposely made wedged. If the two surfaces are perfectly parallel, you get unwanted interference fringes between them. They have to go to special trouble to make them parallel, like the mic checking flats.

Conrad Hoffman said:

Most of the optical flats I've used (in laser systems, not for checking parts) are purposely made wedged. If the two surfaces are perfectly parallel, you get unwanted interference fringes between them. They have to go to special trouble to make them parallel, like the mic checking flats.

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That can be as useful as not, other applications than the one(s) the OP entered the room with.

I lucked into a calibration wedge for my (3) Davidson Autocollimators. And TWO of the special parallel mirrors. And one spindle-mount mirror. And a handful of differential adjusting screws.

Now all I need to do is find the time to sort new DIY mounts for the incandescent lamps also tracked-down. OR - my preference - LED conversion.

Questioned one of the eBay sellers about the difference between the description of "Top and Bottom" for optical flats.
His response :
"Bottom plate use only to check flatness.
Top plate also may use to check flatness, but if you have any bottom and top plates you can use them to compare the linear dimensions of two samples by the interference method.
The reference and verifiable measures of length are mount to the bottom plate and then the top plate is installed on top, by observing the interference pattern you can make concluded ablout the sizes of the samples are they same or not."

thermite said:

Wonder what your thoughts are on any of the many Molybdenum laser mirrors made for CO2 lasers so cheap and common on eBay?

Usable with a visible-light autocollimator, too, perhaps?

But one example of many - given I'm actually in an "ABC" (Anywhere BUT China) mode for the time being:

1pc Dia. 20mm Mo Mirror Reflector for CO2 Laser cutter engraver | eBay

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I looked at thoes but there kinda small - possibly not big enough for a full beam return, thats not to say they would not work, just require more care alignment wise.

adama said:

I looked at thoes but there kinda small - possibly not big enough for a full beam return, thats not to say they would not work, just require more care alignment wise.

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Trying to leverage off what Gernoff did to not have to heft my SE's as often as otherwise might be required, so, yah. Small is OK for it.

Having already glommed some micrometer-head adjustable mounts missing their mirrors. I should add a few of these cheap-seats to the play-toys. Rugged enough vs optical glass, shakey old fart might drop one. Or so I'd hope, given I have become he.