Good morning, everyone.
Today I’m going talk about the Hale telescope again, but not about the parts of the telescope that I was originally going to talk about, because while researching the part I was going to talk about, I found this really interesting other part of the telescope to talk about, so I’m going to talk about that instead of what I was going to talk about. I just want to be clear about that.
The last time we talked about the Hale telescope, we talked about how the mirror was cast, ground, and polished. The next step for the mirror would be its journey to Mt. Palomar, and its installation into the telescope structure. What we’re going to look at this time is the supporting structure for the 200 inch mirror, a collection of mechanisms that are more important and more complex than you might imagine.
The mirror is subject to the stresses and distortions of the force of gravity, and in order for it to keep its shape as a precisely formed elliptic parabola, it needs to supported from underneath to maintain that shape. These forces change as the telescope moves to point at different areas of the sky, and the supports have to change along with it. If the support structure doesn’t operate correctly, the telescope becomes impossible to focus uniformly. The designers of the mirror were aware of this problem, and it is one of the main reasons why the mirror was cast with a honeycomb-like back.
Another Look at the Mirror
As you recall from last time, the mirror for the Hale is honeycombed in order to reduce its weight by 42% to a mere 40 tons. There are 78 hollows in the mirror’s underside, and in addition to saving weight, they increase the surface area of the mirror, allowing the mirror to rapidly adjust to changes in temperature. In addition to the hollows, there are 36 holes for the mirror supports, the locations of which can be seen in the photo #3 below. Inside each of the holes is mounted a very complex mechanism that is a mirror support.
Mirror Cell and Supports
In photo #4 you see the bottom of the Hale telescope showing the mirror cell and the 36 supports arranged in concentric circles around the center hole of the mirror. If you click on the photo you can greatly enlarge it, and if you look closely, you might notice that all 36 supports are different. Photo #5 is a map of the locations for all of the supports.
So what do all of these supports actually do, anyway (I hear you asking)? They push on the back of the mirror to counteract gravity.
The supports around the center hole exert an average force of about 700 pounds, and along the outside of the mirror the glass is thicker and the supports are spaced further apart, they exert around 1,100 pounds of force.
The forces that the supports need to apply to the mirror change as the telescope is moved to point at different parts of the sky. As the telescope moves, each of the 36 mechanisms has 33 different places where something can rotate, slide, or pivot to change the tension on the surface to ideally keep the mirror in its perfect parabolic shape. The amazing thing is, none of this is controlled. Once the supports are in proper adjustment, gravity does all of the work, and the supports adjust themselves as the telescope moves. Isn’t that cool?
In 1949 when the mirror was first set up and all of the supports were adjusted, the engineers found that the mirror was just a bit off from perfect. They added four spring scales (photo #6) to finish things up, and their locations are marked on the map in photo #5 as SU or SD. SU is where a spring pulls up and SD is where a spring pulls down. At mirror support P10 (lower right) the spring scale pulls up with a force of 5 ounces. At P1 (lower left) it pulls down with a force of 21 ounces. At P4 (upper left) it pulls up with a force of 16 ounces and at P7 (upper right) it pulls down with a force of 30 ounces. Remove just one spring scale and the telescope is out of adjustment. Talk about fine tuning, jeebus.
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.”
Newer telescopes have systems called active optics that use computers to control actuators underneath their mirrors to correct for mirror sag. Those systems may be more accurate, and probably cheaper, but they definitely lack the coolness factor of the Hale’s supports.
In January 2010, all 36 support mechanisms were removed, overhauled, replaced, and adjusted. Here are some photos from this work taking place.
The now defunct blog Palomar Skies is still a great place to find out things about the Hale telescope. All photos are courtesy of Palomar Skies.
The Perfect Machine: Building the Palomar Telescope by Ronald Brashear