Support Your Local Mirror

The Hale Telescope's 14.5-ton mirror is honeycombed on its underside. The triangular pieces are hollow to reduce the weight of the Pyrex disk. The round areas are the locations of the mirror supports.

The mirror supports are truly remarkable devices. From The Perfect Machine: "The thirty-six supports were precision machines, an assembly of levers, counterweights, gears, and ball bearings like a fine wristwatch, but large enough that it took two men to carry each of them."

"The design of the support mechanisms was clever. The levers of the mechanisms were designed to push up, against the tops of the pockets in the back of the disk, to counteract the force of gravity pulling down on the disk. The supports were four inches behind the actual surface of the disk."

Each of the 36 mechanisms has 33 different places where something can rotate, slide, or pivot (That's 1188 degrees of freedom!) to change the tension on the surface to ideally keep the mirror in its perfect parabolic shape.

All of this happens by gravity alone, there is no active control (Maybe someday, but not now). As the telescope pivots to point to a different location in the sky the pieces within the supports relocate and change their tension on the mirror.

Over the years pieces within the mirror supports can bind up a bit, limiting how easily the parts can move and how effectively the mirror's shape is maintained. During the recent engineering run one of the mirror supports was removed and examined in great detail.


Here is a photo of the mirror cell with the instrument removed from the Cassegrain cage. The big hole at the top is the where the light comes through to the science instrument. The red fans are fans that can blow air on to the backside of the mirror to help the mirror reach the proper temperature. All of the rest of the openings on the mirror cell are locations of mirror supports.

A close-up photo of one of the mirror supports.

More on the mirror supports and something that may surprise you later.

Thanks for the Memories

Earlier this week I had the pleasure of meeting Ronald Florence, author of The Perfect Machine. It was Ronald's first return to Palomar in more than a decade. We spent a few hours poking around domes and going over what is new and old around here.

His book is a complete history of the building of the Hale Telescope and the Palomar Observatory. As a tour guide and trainer of tour guides at the observatory I have worked hard to transfer much of the knowledge contained within its pages into my head and into the heads of others. Tour groups work their way through the dome as our visitors hear a distillation of the facts and figures, blended with the stories from The Perfect Machine. When it comes to the Hale Telescope its science awes and its history grounds.

Without his wonderful book my job would be enormously more difficult and I am indebted to Mr. Florence.

Thank you.

Don't Stop Believing

The career of George Ellery Hale was one of the most remarkable in all of science. His unrelenting drive and determination brought forth the world's largest telescope four times. First at the Yerkes Observatory where the 40" refractor was completed in 1897.

Hale then moved on and in 1904 founded the Mt. Wilson Observatory. There the first modern research telescope, the 60 ", was completed in 1908. It was followed up by the 100-inch Hooker Telescope, completed in 1917. Edwin Hubble used the 100" to discover the true nature of the "spiral nebulae" (galaxies) and the expansion of the universe.

Again Hale moved on to a bigger project. In 1928 he secured a six million dollar grant to build what would become the Palomar Observatory.

The Palomar story is brilliantly chronicled in the book The Perfect Machine by author Ronald Florence. The book has inspired a TV documentary called The Journey to Palomar, which follows Hale's remarkable path.
TV viewers will have to wait until after the November elections to see The Journey to Palomar on PBS, however a special advance-screening will be held at the University Club of Pasadena on Saturday February 23, 2008.

The event and screening of the film is to help offset broadcast costs for this PBS Special. To sweeten the offer, some valuable extras are included for those who attend, such as dinner and a preview of one of the next giant telescopes on the horizon ($150 donation). The $250 donation level includes the same plus some pretty amazing stuff: a rare tour of the Hale Solar Lab, a behind-the-scenes tour of the Mt. Wilson 100-inch and viewing through the Mt. Wilson 60-inch.

A discount on those rates is being offered for astronomy club members. Those who sign up at the $250 level will get a $75 discount. At the $150 levels, astronomy club members will receive a $25 discount. Plus, all donations are tax deductible.

Full information on the sneak preview and the extras are detailed here, but to get your discount you should call the filmmakers at 310-313-6005.

I highly recommend both the book and the documentary.