FoucaultFringes

In the Shop

This is a new feature for my web site, where I show some of the projects that I have been working on.  In some cases you'll see a finished project, but in others you'll see the unsanitized truth as I clean up someone else's mess.



July 25, 2010Rotating a 25" mirror on the test stand

What happens when a client takes delivery of a 25" F/4.2, 1.5"-thick mirror from a professional optician with considerable experience?  One would expect reasonably good performance from such an optic, and pretty fast cooling.

What my client got was far from this.  The first optician produced an optic with serious astigmatism.  The second optician refigured it and improved it a bit, but the astigmatism was still quite bad.  I would be the third optician to work on it.   (Unfortunately stories like this are not uncommon.  I am the fourth optician that another client has worked with.)

Here is an animation of what the 25" primary looked like when I received it.  The animation is made from images that I captured while doing Foucault testing.  I rotated the mirror about 45 for each image in this animation.

With this type of testing, if the mirror is a good figure of revolution, the test images should basically look identical as the mirror is rotated.

Clearly, in this case, they are far from identical!

Watch the triangular "notch" in the edge of the mirror - this is a piece of tape that I placed on the edge of the mirror to make the rotation more easy to track.  As it rotates, the shadows at the top and bottom center of the mirror, where dark and light portions transition, should be nearly identical, but clearly they change substantially.  This is indicative of obvious astigmatism present in the mirror's figure.  The astigmatism was easily confirmed with my figure-of-revolution test.

Also, if you notice the "blotchiness" of the shadows, that's simply surface roughness from poor figuring technique.  Sharp-eyed readers will also see a prominent scratch left over from someone's "work".

The second optician uses interferometry, but still managed to miss the astigmatism and probably did not see the roughness.  From this the reader can see that an interferogram is no guarantee of quality unless it is done properly.

So, this mirror went straight to polish, where it remained until the astigmatism was gone.  I watched the coating polish off asymetrically as polish proceeded, and after many hours all traces of the coating and original figure were obliterated, leaving a nice near-sphere with no measurable astigmatism.  (The scratch would not polish out in a reasonable time, so it was agreed that I would not try to remove it, but is was partially polished away during my work.)
Bad 4.5" elliptical flat
With the coating removed, I tested the glass for strain and found a superb anneal.  Therefore, the astigmatism was polished in, and I had just polished it out.

Now, you might think that after refiguring the primary mirror, this client's optical problems would be over...... but you would be wrong.

At right is the client's  4.5" secondary mirror that was used with the 25" primary.  Obviously the fringes are not straight - it was approximately 1/2-wave concave.  (This type of interferometry is done by placing the flat on a reference flat and allowing the two flats to equilibrate to the same temperature.)

So, on top of the bad primary mirror, this poor client had purchased a flat that was supposed to be good, but which was really contributing to even more optical aberrations.

I stripped the coating, reworked the secondary mirror until it was quite flat, and sent both optics off for coating.

The results?  The client is happy with his optics for the first time.  He called to tell me he saw round stars and airy disks for the first time.  After learning to cool his mirror evenly, there is no astigmatism visible due to the optics.  I've been giving him advice on improving his primary mirror cell.  His other comments can be seen at the beginning of my customer comments page here.



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

Mike Lockwood
Lockwood Custom Optics

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