How the PSM Caught a Potential Problem

Recently a customer was using the Point Source Microscope (PSM) to align a slow, singlet objective lens and find its focus. 

The customer had a plane retro mirror behind the objective and should have seen a nice round spot at best focus. Instead, he saw a vertical line image as in the following screen shot of the PSM computer display.

The customer knew the PSM was working correctly because he did get a small, round image when he focused at the center of a good grade steel ball. He also knew the lens was correctly aligned to the PSM because he had used the PSM in the autocollimator mode and had reflections from both sides of the lens centered on the PSM electronic crosshairs.

When he did decenter the PSM and looked at the reflected image on a white card he saw a line image. In addition, when he tried to refocus over a 25 mm range he could not find a circle of least confusion. All this pointed to a severely astigmatic return wavefront since the lens was from a trusted vendor. The test set up is shown in the picture below. The rear of the PSM is in the foreground and the objective is at the far end of the optical table with an undersized plane mirror behind it.

I suggested that it might be the small, plane retro return mirror was not flat because the mount was squeezing it. He assumed this was not the case but said he would check.

Not long after I got an email with this picture showing the great improvement in the image after remounting the return mirror.

Clearly, the problem is not completely solved, but the image is many times better than before and the source of the problem isolated. The PSM laser diode source is in the maximum intensity mode that makes the image larger than it should be due to saturated pixels in the camera, and there is still some astigmatism in the wavefront or the spot would be round. The red line just under the horizontal line image is 100 μm long for scale. The lens was about 2 m from the PSM so the roughly 250 μm long image has an angular width of about 5 seconds of arc. The height of the image is slightly more than one would expect due to diffraction.

This is just one example of how the PSM can spot a problem in a test set up before the problem becomes serious. The problem might be serious due to the time it takes to track down its source, or if the problem is not fixed, the issues it will create farther downstream if not corrected at the source.

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清 原 耕 輔   Kosuke Kiyohara
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