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February 2017 Newsletter

As many of you know, and perhaps wondered why, Optical Perspectives Group (OPG) has exclusively licensed the sale and manufacture of the Point Source Microscope (PSM) and MicroFinish Topographer (MFT) to Davidson Optronics, a division of Trioptics-USA.

The answer is that this arrangement relieves OPG of the routine day to day activities associated with the PSM and MFT, and gives us a chance to develop new applications for both instruments, and to do such things as publish a newsletter highlighting some of the new developments regarding these instruments. We will do our best to describe new features, accessories and applications in this and newsletters to come.

In the new product category comes the MicroFinish Topographer (MFT), for measuring surface roughness, coupled to a laptop computer using Apré Instruments software. Because the Apré software uses USB3 to communicate with the MFT phase shifter, the MFT becomes truly portable. The entire unit fits into a sturdy polypropylene case about 38x30x24 cm plus a laptop. This permits the easy transportation of the MFT from lab to shop floor, or facility to facility. Because of the unique mechanical design of the MFT it is largely immune to vibration and can be used without a vibration table in most cases. Of course, once a MFT is purchased, the PSM is easily removed from the MFT and can be used on a stand-alone basis with its PSMAlign software for doing optical testing and alignment.

Every so often we get a question that at first seems like it is impossible to do with the PSM. In the case that now comes to mind it was measuring the index of refraction of a liquid at various temperatures. There are many ways of measuring the index of liquids but none seemed very compatible with either the PSM or with varying the temperature. After kicking around many ideas we came up with one that used the PSM, was simple and was readily adapted to varying the temperature of the sample.

The idea was to use a concave spherical mirror as a dish to contain a sample of the liquid and to set the mirror on a thermo-electric heater/cooler. As shown in the diagram, the radius of curvature, R, of the mirror is first measured with the PSM. Then a little liquid is added to the mirror so the liquid effectively forms a plano convex lens. Then the optical thickness of the liquid is measured and recorded. The optical thickness tO is t/n where t is the physical thickness and n the index.

Then the radius of curvature of the convex side of the lens is measured looking through the plano side of the liquid so that the convex surface looks concave to the PSM. From our paper “Measuring the four paraxial lens parameters using an autostigmatic microscope”, Appl. Optics, 54, 9284-6 (2015), eq. (3) and Fig. 3, the distance from the plano surface of the liquid to the optical radius of curvature of the convex surface, or the focus of the PSM, is RO1 = R/n - tO. Since RO1, R and tO are now known by measurement, n = R/(RO1 + tO). How well can the index be determined? Very crudely, if the distance measurements are taken as conservatively good to 5 μm, then Δn is good to about 0.0002 for indices from about 1.4 to 2.0, a pretty typical range.

Using a slightly more complex version of this same idea the PSM can measure the radius of curvature of soft contact lenses while they are submerged in saline solution.

Finally, a note on something that should be obvious but sometimes is not realized until it is pointed out in practice. One of the best ways of using the PSM for alignment is to mount the PSM on the ram of a coordinate measuring machine (CMM) in place of the usual touch probe, and use the CMM as a large but precise x-y-z stage. While CMM’s are fairly expensive devices, if one is available and you have a need why not use it.

First, mounting the PSM is easy on most CMM’s. The touch probe is usually clamped into the vertical axis ram by a 3/8ths inch (9.525 mm) diameter rod. Loosen the clamping screw and carefully pull out the touch probe. Use the many mounting holes and some common lab hardware to attach the PSM to a precision ground 3/8ths inch rod and clamp the PSM in the ram.

Now run the PSM over to the CMM master ball and focus the PSM on the surface of the ball to get a Cat’s eye reflection off the ball surface and hit the Set Ref Pnt button to initialize the PSM origin, or crosshair on the screen. In the CMM software, set the radius of the touch probe to zero. This is because the focus of the PSM is a true point and has no finite radius.

Then move the PSM forward until it is focused on the center of the ball and the return spot is centered and in focus on the crosshairs. Click the CMM software button to record the CMM x-y-z location. Depending on the software this may have to be done 4 times without moving the PSM location because the software thinks at least 4 touch points are required to know the center of the ball. Now the CMM software knows where the PSM focus is in the CMM coordinate system.

Now use the PSM to pick up at least 3 datums on the optical bench into which you want to align optics. The best way to do this if provision has been made ahead of time is to pick up the centers of at least 2 tooling balls installed on the optical bench. Three are even better in case, for example, the optical bench has some wedge in its baseplate so the axis you want to align the optics to is not parallel the CMM baseplate. Now tell the CMM software where these datums are so the software knows the location of the optical bench in the CMM coordinate system.

Finally, going back to the mechanical and optical drawings of the system, find where the centers of curvature of the various elements are located in space. To install the first surface, move the PSM to the coordinates of that center of curvature and move the optical element around until the reflected spot from that surface lies on the PSM crosshairs and is in focus. The element will then be positioned so the center of curvature is knows to less than 1 μm laterally and about ± 3 μm along the axis of the PSM.

If the position of the PSM has to be changed to get to the center of curvature of another surface, change the PSM orientation and go back to the master ball and re-zero the CMM software. To double check that this has been done correctly you can re-check one of the tooling balls on the optical bench. It should have the same location as before the PSM orientation was changed. Below are several example photos of the PSM being used on a CMM.

 


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Case Studies & Testimonials

Why is proper alignment so important?

Here is a case of a very happy customer due to better optics.

A few days ago an astronomer friend of mine commented that he had gotten the optics of his telescope improved and the improvement reduced the time it took to get data by a factor of 3. For an astronomer this is a dramatic improvement since observing time on large telescopes can cost thousands of dollars an hour.

My friend did not say how the optics had been improved, but the important point is that better optics, whether due to figure errors, mounting or alignment mean more productive optics. I generally think of better optics as a better product leaving the manufacturing facility without thinking about how much the better optics mean to the productivity of the customer.