Heating the dome just doesn't make sense from an energy standpoint when you realize that almost every night the dome is opened and all the hot air would escape.
Astronomically, it is actually worse than that. While having a warm dome would be nice for visitors enjoying a tour and a work crew performing maintenance it would force all of the telescope's parts to adjust daily back and forth from warm in the daytime to that night's much colder temperature. This can take a significant amount of time when the temperature variation is large. During a period of adjustment it would be difficult to use our large telescope and have any useful results.
The temperature equalization of the telescope is one problem. The other is convection. Hot air rises. Heating the inside of the dome and then opening the shutters would essentially force warm air to rise right in front of the telescope. The result would be hopelessly blurry images much like you see when looking down a roadway on a warm summer day.
Last November during an interview for a TV documentary lights were set up to aid in the filming. The lights were both bright and hot. One of the lights was shining toward another one which happened to be close to the inside of the dome. The result is that the hot, rising turbulent air actually became visible as its shadow was cast on the inside of the dome.
Here is a short video clip showing turbulence caused by warm rising air inside our big dome.
Look just above the shadow of the light fixture to see the effect of the warm rising air.
That's exactly what you don't want for astronomy. Of course the interview was held many hours before that night's observing and didn't impact operations much at all because a couple of lights doesn't really change the overall temperature inside the dome.
One in a while it is actually a good thing to heat the inside of the dome.
Right now we are just a few days into an engineering run where the Palomar day crew and support engineers from Caltech will be disassembling the Hale Telescope's mirror supports. Each of the 35 mirror supports to be worked on during the next several weeks contains over 1,000 parts. Many of the parts are tiny and difficult to manage.
For this engineering run, since we will not be performing astronomical observations with the telescope, we will make an exception to the no heat in the dome rule. The heat is on.
Yesterday morning we hooked up two big electric heaters, one of which is seen below.
We also recently modified the dome so that we could run two big propane heaters to heat the interior for the crew. So to safely vent away the exhaust gasses two dual exhaust vents were added. Have a look for them the next time you are visiting the observatory.
The temperature inside the dome has climbed all the way up to 40F, making it not quite pleasant for working yet, but it is improving.
As the engineering run is completed we will open the dome and vent out the warm air and return to nighttime temperatures and astronomical observations.