Telescope Reviews: A focusing question

Take a look at the photo, it illustrates the light cone coming from a star and what's going on with a knife-edge and Ronchi focuser.

With the knife-edge, you place the edge outside the cone and move it slowly across the star by moving the telescope. Imagine you're inside focus. You're obstructing the light cone with an edge - for a grossly out-of-focus star you'll see a circle of light (or a donut for an obstructed telescope) with a dark edge moving across it. For a very close to in-focus star, you'll see a small dot, but as the edge moves across the dot you'll see it slowly fade out as you cut across the tiny disk. When the knife edge is RIGHT at the focal plane, you'll see the dot just wink out all of a sudden because you're cutting a tiny point of light instead of a small disk. So the method of using the knife edge is: move the edge across the light beam, observe behavior, adjust focus, and repeat until you've found the point where the star just winks out.

The Ronchi screen applies some of the same principles but is very different visually and in use. The Ronchi screen is a series of parallel lines etched into a piece of glass and then blackened. The picture illustrates the "bars" of the screen seen edge on. Imagine you're gross out-of-focus, inside of focus again. You'll see a disk (or donut...) with a bunch of black lines superimposed across. Now as you move closer to focus, the number of "lines" cutting the cone gets smaller, so you see fewer, bigger lines, spaced farther apart (a Ronchi screen focuser uses a magnifying lens system to magnify the cone so that even in focus you see a disk rather than a dot...). As you get closer to focus, the lines will spread apart, until either you've got the focus point completely obscured by one line, or the focus point will land between two lines. So either you'll see a completely obscured disk or a completely clear disk. In practice you rack the focuser in and out and see the lines expand/contract as you move the focuser.

If you think about the light cone falling "between" the bars of the screen, you'll realize that the fundamental accuracy of a Ronchi screen focuser is determined by the spacing of the bars - the tighter the spacing, the more accurate the focuser will be. In the case of the knife-edge there's no such limitation, an edge is an edge.

I personally find the knife edge less ambiguous, you can really tell when you've got focus. It's a little harder to do because you have to move the telescope - with the Ronchi all you have to do is rack the focuser in and out.

Either way is the best way to get precise focus, the downside being that your knife edge/Ronchi focuser has to be calibrated to the specific lens flange/focal plane distance of your camera.

Commercial knife edge focusers: http://www.sciencecenter.net/hutech/mitsub/focuser.htm

Commercial Ronchi focusers: http://www.stellar-international.com

Cheers,
Suk

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Edited by Suk Lee (12/19/04 08:03 PM)