Using the PSM as an Autocollimator
Crosshairs in the center of the field
The equivalent calibration for the PSM is to focus on a specular surface to get a well-focused return spot, a Cat’s eye reflection, and click the Set Ref Pnt button to center the crosshairs on the return spot electronically. This operation is the equivalent of bore sighting a rifle scope. When the crosshair is centered on the Cat’s eye reflection the focus of the PSM objective is centered in the crosshair.
Thus, when a return reflection is also centered in the crosshair, the return reflection is coincident with the outgoing light focus. In this way you can be sure you are at the center of curvature of a concave mirror to better than 1 μm when the PSM is used with a 10x objective, the standard sold with the PSM. Obviously, a higher resolution is achieved with a higher power objective.
If by calibration you mean when the PSM says the return spot is 54.2 μm from the crosshair, is it really 54.2 μm or is it 54.4 μm?
For most users, simply setting the Calibration Factor to 1.00 when using the standard 10x objective is good enough. For those who want to know the distance to the last μm the answer is to use a calibrated line width standard and measure it to see if the value you get with the PSM is the same as the standard. If it is not, the Calibration Factor can be tweaked to give a precise reading.
When using an objective other than the 10x Nikon objective supplied with the PSM you may want to change the Calibration Factor to get the correct reading in μms. For those not having a suitable line width standard, we have standards traceable to NIST for sale in our webstore.
The PSM also works as an Autocollimator
Recently we have sold several PSMs to a customer that wanted the PSM strictly as an autocollimator.
They wanted to build the PSM autocollimators into their hardware as permanent measurement devices and did not have space or stiffness to support a standard autocollimator that tends to be 500 mm or so long and weighs several kilograms.
As you may (or may not) know, the PSM works as an autocollimator by simply removing the objective and it has a resolution of better than 1 arc second.
For this particular customer, the PSM was a perfect solution since the PSM is about the size of your hand and weighs about half a kilogram. Another advantage for this customer was that their target was rather small, less than 10 mm in diameter. This fit nicely with the PSM collimated beam output diameter of 6 mm, much better than a standard autocollimator with a beam diameter of over 25 mm where most of the light falls off the target or is reflected as scattered light.
I am publishing this article, because I got the call from someone who wanted a PSM as an autocollimator for just this reason, to incorporate in a machine vision, automation scheme with limited space.
This got me thinking that we have always stressed using the PSM for alignment and centering and have treated its use as an autocollimator as a very secondary use.
The call from the customer who wanted to use the PSM for an autocollimator, made me realize that there is a customer base who really needs a compact, electronic autocollimator. The PSM may be just what they are looking for.
About the Author
Robert Parks received a BA and MA in physics from Ohio Wesleyan University and Williams College, respectively. His career started at Eastman Kodak Company as an optical engineer and then went on to Itek Corp. as an optical test engineer.
He learned about optical fabrication during a 4 year stay at Frank Cooke, Inc. This experience led to a position as manager of the optics shop at the College of Optical Sciences at the Univ. of Arizona and where he worked for 12 years and had a title of Assistant Research Professor. During that time he had the opportunity to write about the projects in the shop and the optical fabrication and testing techniques used there including papers about absolute testing and the installation and used of a 5 m swing precision optical generator.
Mr. Parks left the University in 1989 to start a consulting business specializing in optical fabrication and testing. Among the consulting projects was one working for the Allen Board of Investigation for the Hubble Telescope where he stayed in residence at HDOS for the duration of the investigation. In 1992 he formed Optical Perspectives Group, LLC as a partnership with Bill Kuhn, then a PhD student at Optical Sciences.
The consulting and experience with Optical Perspectives provided many more opportunities to publish work on optical test methods and applications. While still at Optical Sciences, Mr. Parks became involved in standards work and for twenty years was one of the US representatives to the ISO Technical Committee 172 on Optics and Optical Instruments. For two years he was the Chairman of the ISO Subcommittee 1 for Fundamental Optical standards. Recently Mr. Parks temporarily rejoined Optical Sciences part time helping support optical fabrication projects and teaching as part of the Opto-Mechanics program.
Bob is a member of the Optical Society of America, a Fellow and past Board member of SPIE and a member and past President of the American Society for Precision Engineering. He is author or co-author of well over 100 papers and articles about optical fabrication and testing, and co-inventor on 6 US patents. He remains active in development of new methods of optical testing and alignment.