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Posted by Kip Crawford on December 14, 2001 01:56:55 UTC

Being in ATM I have seen this before. You may have too thin of coatings on either the primary or diagonal. I'll explain.

Heat expansion in the tube will cause a blurry image, especially planets and bright stars. However this subsides as soon as the tube reaches ambient temps. These problems are more noticable on planets because they are small disks that reflect sunlight. The moon is brighter, but larger, so blurring isn't very noticable. Deep Sky objects are too dim and the color spectrum is ("uneven", so blurring isn't too bad.

If you have a cooled tube and you are experiencing "double vision" it's two main things: 1. optical accessories (EP's) or, 2. The main optics (lenses, mirrors).

First...EP's. Most newer types have coatings that help in diminishing reflection off our eyeballs. When we look at a bright image through the EP, out brain picks up the image, but the excessive light reflects off our eyeballs and back on the lens giving you either a "light shadowed" reflection or a double image. When looking at bright objects, cup your hand around the EP and your eye to keep other scattered light away. If you get a double image still, just standby for part two.

Part two. The main opticals of your scope may have too thin of coatings. You will need to remove the mirrors (both) to do a test.
The reason for this test is to see if you are getting two reflections, the aluminum coating reflection and the glass reflection. If the coatings are too thin (lets say on one side) and OK on the other, what happens is that you get the proper reflection and another from the glass. These two sides may not line up correctly at the focal point. It shouldn't really...not like that. The same might be that the secondary has too thin of coating also. This too can produce a double image, even if the primary is good.

The test: Take the mirrors out. If you have a 8" f/6 then the reflectoin will be 96" (48x2)away. Use a light bulb or flashligh next to a wall. Stand with the mirror (about 96") and try to get a reflection from the flashlight to bounce on the wall next to the flashlight. You should see a detailed image of the bulb. You'll need help with that too...if the image isn't doubled at all the primary is OK. If you have a "ghost" image right next to the real image or the "ghost" dances around, you have bad coatings. This is the glass and reflective coatings giving a double image. On the secondary, just hold it to a light closer to a wall. If you get a ghost image or it dances around in a "blurry" way, it needs coating too.

If these test pass, try each eyepiece to make sure they are coated. The inside of these barrels should be black or dark non-reflected color. I'm assuming your telescope tube is black inside. Anyway...give it a try. I suspect the coatings could be bad.

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