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Looking through the telescope the wrong way. June 27, 2008

Posted by jcconwell in Observatory, telescopes.
Tags: , ,

Living in East-central Illinois means that when people visit the EIU observatory you have to contend with weather. At night, about half the time, it will be cloud covered and you just get to look at the telescope. If you come in the day its even worse. We can actual still see the football stadium light that are about 3000 feet away in the day.

On these occasions one of the things that I do is to lower the telescope and let people look down the tube and see the 16″ mirror, which acts like a really BIG makeup mirror. Since I have a Schmidt-Cassegrain telescope, with a front corrector plate, one of the thing people comment on is how DIRTY the front plate looks.

Don’t I ever CLEAN it!!!…….

Nope…. well not quite as often as you might think I should… and there’s a reason.

Most telescope lenses , and camera lenses, come with an anti-reflection coating, typically a coating of MgFl. You can tell when you look at it because it gives the lens a slight purple, or sometimes yellow color. The coating is put there to increase the transmission of light, to the eye or camera, by using the wave properties of light to eliminate reflection.

The coating is very thin, quite delicate, and can be easily scratched, and is expensive. Our 16″ telescope cost about $500 to get it recoated. So cleaning it is a very big project … distilled water…lots of care and can take several hours to make sure the dust doesn’t scratch the coating when it’s removed.

Also, you don’t need to do it very often. Even on a very dusty lens the light is only cut by a few percent. The dust on the lens is so much closer than what your looking at, the dust is out of focus, so you don’t notice it with your eye. A digital CCD camera is more sensitive, especially if the dirt is not uniform. We can compensate for that by taking what is called a “flat field” briefly, this is a photo through the telescope of the dirt! This is then digitally subtracted from the from any photo! Electronically cleaning it up.

So the short answer is I don’t clean it often , cause it’s tricky and I don’t need to.



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