[ATM] A challenging 28" project

[Richard]

Hi Dominic-Luc,

Friday, August 3, 2007, 11:31:41 AM, you wrote:

DLW> A low power one,
DLW> like 4X is not appropriate here. I could guess 100 X is too
DLW> much. Something like 20-40X is probably perfect.

It's not the power that matters, it's the numerical aperture, although
I admit they do tend to go hand in hand. The 'standard', if there can
be such a thing, on these spherical wave interferometers tends to be
NA 0.25 which often goes with a power of 10x. Works for me and is
easily capable of a beam of f/3 or better, ie mirror under test of up
to about f/1.5. To get a wider beam for Mikes' f/1.1 I would have
though you would want to go to a smaller NA, so a NA 0.1 5x may well
be the right direction rather than a NA 0.65 40x.

Mike is well aware of these interferometers and has plans to figure my next
Zerodur reference mirror for me. Some may recall I was advocating
their ease of use and construction under a thread about Offner Null
testing a couple of years ago.

The microscope objective needs to be reasonable quality. Zeiss and
Olympus can go fairly cheap on Ebay if you keep your eyes open. You
should know which standard the objective is so you know where to place
the laser (or laser collimator if you use one). There are dimensions
on the more common DIN standard at
http://www.edmundoptics.com/techSupport/DisplayArticle.cfm?articleid=244

The critical element is the small concave mirror which is the
spherical reference surface. Ideally, it would be optically perfect.
In the real world it needs to be better than 1/20th wave on the
wavefront. Surplus Shed sell (or used to sell) a Pyrex 2" f/2 uncoated
mirror for about $3. Mike has measured some of these, I think they
weren't bad, better than 1/4 wave. If you can figure one as a
reference element you can save a fortune. The rest is a small HeNe
laser (Ebay ~$50) and a beam splitter of some kind like a pellicle or
cube beam splitter (Surplus Shed ~$25). The former has less problems
in use than the latter, but is extremely delicate.

-- 

Best regards,
  Richard in the UK