December 18, 2013:  A half-baked secondary mirror     All text and images Copyright Michael E. Lockwood, all rights reserved.

I picked up some optics on the way home from a trip - a 32" mirror for testing, and the 6.25" m.a. Pyrex flat that goes with it.  The flat was glued to a piece of aluminum.

First, let me say this - unless you know EXACTLY what you are doing, I never recommend gluing a high-quality telescope mirror to anything.  Smaller secondary mirrors are the only exception, say under 3" m.a., but there is still a chance of astigmatism as the glass and whatever it is glued to cool and change dimension at different rates.

Note:  The best thing to glue a piece of glass to is another piece of glass.  That piece can then be bolted carefully to something else.  The second piece of glass will expand underneath the bolts, allowing the change of dimension, and preventing the mirror from being significantly distorted.

Normally freeing the mirror from glue is no big deal - you simply slide a piece of wire, a razor blade, or a putty knife under the mirror, cutting the adhesive.  However - this assumes there is space between the mirror and what it is glued to.

In this case, there was no space!  Clearly the genius that did this did not anticipate ever having to remove the mirror.  So what to do?

After some consultation, it appeared that most adhesives would lose their strengths above ~400 degrees F.  So, into the oven the Pyrex flat went, slowly ramped up to 425 degrees F.  I left it there for a few hours, and found that at some point the flat had slid down its holder by ~1/2", as can be seen in the image below.  I figured the glue was done for, and cooled it off slowly.

(Disclaimer - I take no responsibility if you try this and something goes wrong.  You certainly don't want to do it with a mirror that is made from something other than Pyrex or another lower expansion substrate like quartz, etc.  Anything you try is done at your own risk.)

Half-baked secondary mirror

However, upon removal of the cooled-off items, attempts to separate the mirror from the metal were unsuccessful.  The glue was still holding, and there was still no gap between glass and metal.  So, at this point the flat was literally only half-baked!

Free at last!

After another thermal cycle the next day and some gentle persuasion with a putty knife, the mirror finally separated from the metal and it fell away, as can be seen above.  A razor blade is just visible wedged between glass and metal at left.

After cooling, I set the flat up for interferometric testing in my 24" collimated beam.  The results below show a flat that is decent, but it can likely benefit from some tweaking and can be improved further.  It certainly merits some more testing after the coating is removed.

Flat testFlat testing

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

Mike Lockwood
Lockwood Custom Optics

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