July 10, 2012:  Big Blue gets a new 36" f/6.3 mirror

Last year the 31" f/7 primary of "Big Blue", the telescope housed in the Warren-Rupp Observatory near Mansfield, OH, was tragically broken when the secondary mirror and holder came loose from the spider and struck the primary mirror.  The collision didn't do that much to the secondary, but it cleaved the primary in half.

I got a surprisingly calm call from a club official on the same night asking if it was possible to make a replacement.  After finding my 50' tape measure, and measuring the length of the test path that I could manage diagonally across the main shop space to see if I could test such a long focal length mirror, I saw that I had enough room and said yes.

With a ~450" radius of curvature, I needed nearly a path 38 feet long to set up the test!  As a fallback, a known good flat could have been used to fold the optical path, but I wanted to avoid that.

Two quotes, one insurance claim (fortunately the mirror was insured), and some creative thinking later, a plan was born to NOT replace the mirror with an exact replacement, but instead to greatly improve the telescope.

A larger 36" f/6.3 primary could fit in the oversized tube, and it would be thinner and thus cool faster than the old primary.  A new mirror cell would be needed, and would update the mirror support to modern standards.  A rotating secondary mirror holder that can't possibly come loose and a second focuser would allow the telescope to be used over a much larger area of the sky.

So, instead of a tragedy, this became a serious upgrade.

Some might observe that I normally make f/3.6 mirrors, rather than f/6.3, and this is true, though I always enjoy a change of pace like this "slow" mirror.  The glass arrived some months ago, and I have now completed work on the primary mirror.  Some photos are included below to show the progress.

Below, the back of the 36" mirror is ground against the back of another smaller mirror, a 22".  (For more on why the back should be ground, see this installment of In the Shop.)  The mirror just fits on my medium-sized machine, which has a 34" turntable.  So, the mirror overhangs about 1", but this is not a problem for such a thick mirror, and actually helps with cleanup.

Grinding back of mirror

Next, a hydrostone (water resistant plaster) grinding tool is cast against the mirror, and after it dries, tiles are epoxied to its face to complete the tool (image below).  A socket is attached to the back to accept he pin from the machine's arm.  This type of tool can be made quickly and affordably, and is ideal for radii that are not likely to be used very often.  (A ~225" focal length mirror is not a common thing in my shop.)

Grinding tool

The grinding tool is then used to grind the face of the mirror from fairly coarse grits down to very fine grits so that it is then ready for polishing.  Below is a photo of early grinding work to break the tool in and get it into good contact with the mirror.  After this, the arm position is varied to control the curvature of the mirror.  A spherometer is used to measure the curve, and adjustments are made during various stages of grinding to bring the focal length to the desired value of 224"-225".

Grinding the face

Finally, after grinding down to 5 micron abrasive, grinding is finished, and the low-angle reflection of the mirror can be admired and photographed.  It is always a beautiful thing to behold.

Grind finished

A scratch-free grind is a major milestone for a large project.  The mirror was then polished for many hours until any traces of grinding pits were removed.  After polish was complete, the figure of revolution looked good, so it was then "figured", or polished to the proper parabolic shape.

Figuring was completed on July 9, following careful testing.  The figure of revolution was checked a last time, dried cerium oxide was removed from the edge of the mirror, the bevel was touched up, and several small circles were scribed in the glass in the exact center of the mirror to make it far easier to collimate.  Most of my clients really like this feature, because it allows collimation stickers to be precisely centered, and the direct reading of laser collimators.

Here's a photo of a somewhat tired optician, happy that the project is completed slightly ahead of schedule, and trying to smile as he takes a photo of himself.  The little sign says: 36" f/6.3 - COMPLETE.

Me with the finished mirror

After final inspection of the surface, it is covered with acid-free paper and a piece of cardboard to protect it, and the primary mirror and new secondary mirror are packed in a crate for shipment to the coater.

The coated mirror will then be tested in its new mirror cell (inside the shop) to make sure the edge support is working properly.

If all goes according to plan, the mirror will be reinstalled in "Big Blue" and the other upgrades done before Hidden Hollow 2012, a yearly star party that features the telescope, which I may start calling "Bigger Blue".

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

Mike Lockwood
Lockwood Custom Optics

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