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Beware of  the flashlight test

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#1 Michael Lomb

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Posted 20 April 2012 - 07:38 AM


My 1978 Questar had a new broad band mirror installed as part of a service last year as the mirror was showing clear signs of the coating reaching the end of it's life, mentioned on a previous post. I noted recently if I looked at the new mirror with a bright light at "just the right angle," that what looked like imperfections showed up. It turns out that this is just an artifact. This was mentioned in the Meade ETX online manual, copied here. If there imperfections in the coatings, it is clearly present in any lighting.

 Inspecting the Optics

A Note About the "Flashlight" Test: If a flashlight or other high-intensity light source is pointed down the main telescope tube, the view (depending upon the observer's line of sight and the angle of the light) may reveal what appears to be scratches, dark or bright spots, or uneven coatings, giving the appearance of poor quality optics. These effects are only seen when a high intensity light is transmitted through lenses or reflected off the mirrors, and can be seen on any high quality optical system, including giant research telescopes.

#2 Brian Risley

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Posted 20 April 2012 - 08:42 AM

Probably the worst thing to do! Makes you see all sort of phantom problems! (And any attempt to fix or clean is probably even more damaging to the optics than what you see!)
Brian

#3 justfred

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Posted 20 April 2012 - 02:43 PM

Thanks, Michael!

This forum and the other topical forums never cease to amaze me by providing information on things you didn't even know you needed to know.

True sense of community. Nice.

Fred

#4 Les

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Posted 20 April 2012 - 02:58 PM

These effects are only seen when a high intensity light is transmitted through lenses or reflected off the mirrors, and can be seen on any high quality optical system, including giant research telescopes.


Giant research telescopes can be quite dirty depending on the their use. I once took a tour of the giant reflector on Mt. Hopkins. Since it was a large tour group, the operators were nice enough to tilt the mirror down so that we could all get a better look at the mirror. It was totally caked in dust/dirt with condensation rivulets running through it. Since it is not used for imaging but only sky mapping it doesn't matter. The guide said that when the mirror gets "really dirty" they clean it off with fire extinguishers!

Quite agree that the flashlight test tends to get one worried about nothing of consequence.

#5 VanJan

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Posted 21 April 2012 - 12:40 PM

The guide said that when the mirror gets "really dirty" they clean it off with fire extinguishers!


:scared: :scared: :scared:

Then again, the "logical" solution to a smoldering unease caused by an incendiary use of a flashlight. :brick:

#6 ColoHank

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Posted 21 April 2012 - 01:12 PM

That's the trouble with those large telescope mirrors; it takes more than one fire extinguisher to clean them. Fortunately, when the Q's mirror gets dirty you can just throw the whole scope into the dishwasher (but please don't try that at home). :lol:

#7 la200o

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Posted 24 April 2012 - 11:21 AM

I've always assumed that all those high intensity photons from the flashlight hitting the mirror/lens must be causing the horrible things I see! :cool:

Bill


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