Thread: Using Optical Flat to check Flatness of Granite Surface Plate

Not a very precise comment on my part really - but if the spectrum bands are separated by too much distance or there are too many of them they could interfere with the other light frequencies so that clear well defined fringes are not as easy to get. Sodium is the common (cheap) solution.

Michael

Peter,
have a look at
interferometer | eBay
and see what goodies you might find there.....

Originally Posted by jhovel

Peter,
have a look at
interferometer | eBay
and see what goodies you might find there.....

Nothing there Joe. These are the mirrors used fir autocollimation

Nikon | Instruments Products | Industrial : Autocollimator Accessories

My original intention was to scrape a cube and attach a first surface mirror to that. By flipping the cube and autocollimating on it I could auto-generate a precise 90 degree surface. However I'm also considering instead making it an adjustable 3 point stand, with one if the points a very sensitive micrometer arrangement so I can take my angular readings at the mirror end instead of the autocollimator end as is normally done. I have a very good reason for wanting that, but need to rush to work so can't explain now.

pete

Originally Posted by electrosteam

Hi all,
I have just acquired (at the right price) a 3 inch optical flat about 1/2 inch thick.
I remember from university physics laboratory days using a sodium light (I think) to check the flatness of a surface by observing interference fringes.

Is this approach workable for a granite surface plate ?
If it is, what is the work method and suggestions on a source of a suitable monochromatic light source?

So far, all I have tried was a squirt of WD40, the optical flat floats effortlessly across the surface.

Getting into totally unknown territory is fun !

John.

Sorry for the delayed response, I was just heading off to bed and realised I hadn't checked this as promised so raced down and had a look.

My little Mitutoyo plate is filthy as I use that all the time and I didn't think I'd get much to work off that, so wasn't exactly surprised when I couldn't get fringes. Instead I whipped the cover of the bigger Chinese delight but also couldn't get any fringes. I grabbed a mic and that works just fine without fail, so there wasn't anything lighting wise etc to spoil the party.

While flatness may be an issue, I think a bigger problem may be that the colour and texture of a granite plate means the light isn't reflected very well, and that's something that's essential for the whole interference caper to work. So while floating the optical flat around isn't a problem, seeing anything meaningful with it is.

No great surprises there, but I thought I'd just confirm what we all suspected.