The Lake County Astronomical Society

Looking down the Focuser Mystery

Here's a novel question: In a Newtonian telescope, when you look down the focuser tube, you see your eye staring back at you in the reflection of the secondary mirror. But why don't you also see at least a part of your eye when there's an eyepiece in the focuser? Here's the answer from Philip Harrington, author of the book Star Ware:

"Keep in mind that when you look through an eyepiece, you are focusing the light from an object located, for all intents and purposes, at infinity. Anything nearby is still there, but appears very "diffuse" (maybe not a terribly accurate phrase, but good enough). When you look through the eyepiece-less telescope at your reflection, your eye is focusing only inches away. I suppose, in theory, you could focus your eyepiece on your eye's reflected image, as well. Of course, the closer the object is, the greater the backfocus needed. To get the reflection in focus would take quite an extension tube!"