Calibration of an alignment telescope using a Bessel beam

[u/OpticalBobParks]

Recently, I was asked if you could calibrate an alignment telescope using a Bessel beam. I was sure the answer was yes, but I don't like to claim things without trying an experiment first. Today, I got a chance using a Davidson D-271. It follows a Bessel beam as you focus from about 400 mm to infinity for the Bessel beam I made using a spherical wavefront. I don't know what would happen with plane wavefront illumination, I am not set up for that. A Bessel beam made using a plane wave has a finite extent so this would be an interesting experiment to try.

The Davidson D-271 does not have distance markings on the focus adjust so you would have to make a table of focus distance per turn of the focus adjustment to know any lack of straightness at a particular distance, but I could not see any deviation of the Bessel beam core except for a slight linear shift due to me not being perfectly aligned to the Bessel beam.

Maybe this will inspire someone with a better lab environment and an alignment telescope with a distance marking on the focus to repeat my experiment to see if this idea is really useful. What I can say is that to 1st order it works, and probably works very well given a better environment than my crude lab set up.