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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1265336" data-attributes="member: 68323">Even a matte black painted surface will reflect around 1% of the light hitting it. At specular it might be even more.So if you need a black target you need a "black hole" -- not a supermassive star but a black box with a hole in it and light absorbent (scattering) material inside away from the hole as a real "black" target. See the link I posted on measuring veiling glare above.Usually they do this by modifying the inside surface of the optical tube so it is no longer flat and so no longer can generate specular reflections that go down the tube. Or they can do as you did with the felt and put in a material that absorbs a lot of light but doesn't generate a specular reflection. The light that isn't absorbed is scattered over a hemisphere (2π steradians).For example, you can cut grooves inside the optical tube that have a sawtooth shape with the long side pointing down the tube. In a telescope design the short side might even be highly reflective (the best way to get rid of stray light is to reflect it out the entrance of the tube. That may or may not work with a lens assembly at the end of the tube. The shape has to be optimized so stray rays can't take an different route to get to the eye.e.g. <a href="http://www.freepatentsonline.com/5189554.html" target="_blank">http://www.freepatentsonline.com/5189554.html</a>You also see this on porro prisms (and roofs I supposed) with a stop at the entrance of the prism to cut down stray light getting beyond that point. Baffling the sides of prisms helps too (so stray light can't bounce into them on a face that's not suposed to get light).A S&T article shows how you can do this on a Newt. Interesting to look at just to see how many different places need some baffling when cleaning up a inexpensive Newt.<a href="http://gmpexpress.net/~tomhole/baffling.pdf" target="_blank">http://gmpexpress.net/~tomhole/baffling.pdf</a>Once again doing the design work properly is the key and is the sort of effort that gets puts into an expensive scope or bins though I think the cheaper ones could do better than they do. But then you wouldn't by the more expensive products.Last night I was testing some bins against stars and Jupiter's moons with a bright moon in the sky just 10° from Jupiter. That test really showed up the difference in stray light handling of the four bins I've been looking at. It's a lot easier to see stray light I suspect the worst of the bins (a Vortex Hurricane 8x28) was suffering exactly the sort of bounce of the optical tube you describe - with a great crescent of stray light on the opposite side of the image from the moon.</blockquote>

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