Some of us start the night shift early. The dome is opened well before sunset to help equalise temperatures inside and outside the dome. Sometimes the IRTF, pictured above, is opened early as it's observing a planet that's only visible during the day.
Other observatories tend to wait until after the sun has set, the Kecks being an example. Although I'm not familiar with the Keck's policies, one thing you must avoid doing if you open early is having direct sunlight on any of the telescope structure. Even at sunset or sunrise when the sun is low in the sky, the light from the sun will warm the structure it strikes and some expansion will take place. It'll be tiny, a few microns perhaps, but enough to effect image quality.Shortly after I took the top picture of the IRTF, I saw them rotate the dome so the aperture was pointed directly at the sun. There may have been a good reason for this, but it's certainly not something I'd feel comfortable doing. Then again, the IRTF was designed to observe planets in the infrared and sometimes that means pointing close to the sun.
3 comments:
At Keck the rule is not to open unless the Sun is below the horizon. We have no cover for the ten meter mirror like the smaller scopes do. A ten meter magnifying glass would do more than burn a few ants, more like melt steel and start fires.
We can open in the day for shutter maintenance, but only with a sign-off from summit lead and safeguards like proper position of the telescope and personnel watching for sunlight near the mirror.
When we commissioned MICHELLE back in 2001 we came up with what I thought was a very clever plan to get the basic stuff done done without affecting night time operations. It meant observing after sunrise for a few hours until it became impossible to protect the telescope from the sun.
The commissioning team at that point was small, two of us plus a
telescope operator. One of us would be out in the dome almost continuously watching out for the thing you mention - having sunlight
concentrated by the primary. It was a big worry and UKIRT is susceptible to this more than most since there's little clearance between the top-end and the dome unlike most observatories.
We commissioned all modes of MICHELLE successfully and then it went to Gemini. That's another story...
Tom
There was a submm telescope that fried its secondary, I believe.
As heating of structure goes: when I was working at the CSO I found that depending on which direction the telescope was parked at in the afternoon the pointing would be off either in azimuth (if the telescope was pointed south or north) or in elevation (if the telescope was pointed east). We never did point it west during the day when there was daywork being done.
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