April 25, 2013:  A very good telescope      All text and images Copyright Michael E. Lockwood, all rights reserved.

It is always a pleasure to see a good instrument come into my shop.

One client was having issues with a Maksutov-Newtonian telescope, and wanted me to verify that the primary mirror was good (spherical), and check the the overall system.

So, I set up the telescope carefully in front of my flat, and found a fairly well-corrected system, with some "weirdness" in it.  Here's a photo of the test setup, using my large test stand as a table to support the telescope, which rests in a set of adjustable V-blocks that I made some time back to align telescopes tubes with a flat.  The interferometer is pointed into the focuser.

Test setup

I worked on collimation to improve the asymmetry seen in the fringes, but obtained variable, unrepeatable results.  Clearly something else was going on.  I suspected the mirror cell was warping the primary mirror.

As it turned out, the mirror was glued into its metal cell, and the push-pull bolts were separated a bit.  So, tightening one of the screws was actually flexing the metal of the cell, resulting in a variable warping of the primary mirror!

Below at left is an interferogram showing some astigmatism.  However, the overall correction is quite good, as the fringes are mostly straight across a diameter.  Remember this is a double-pass test, so the actual error is half of what is seen.

Below at right is the mirror cell, after the primary mirror was cut free of the glue that can still be seen in parts of the cell.  It was glued at more than 15 points, which is massive overkill, and pretty much guaranteed it would be stressed/warped under at least some temperature conditions.  Not only was the cell getting warped, but aluminum shrinks faster than glass as it cools, so it would have increasingly stressed the mirror as it got colder.

Double pass test interferogramMirror cell with primary cut free of glue

A new mirror cell is being constructed for this telescope to prevent future unintentional pinching of the primary mirror.

Please check back for future installments of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

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