Eyepiece projection focus??

I am trying to use a Televue eyepiece projection adapter with my meade 127ed f/9. I am having no luck. I can use prime focus OK, but no matter how I set this thing up, I cannot seem to come into focus using the eyepiece adapter. Are there formulas I can use to determine the approximate distance to the eyepiece, and to the camera to find the focus point? I have tried it with and without the diagonal.

Regards;
Keith Garrett
Wolfeboro NH

Is the eyepiece itself in focus? I know it is hard to tell, since the eyepiece is buried in the projection adapter.

Is it that you look like you are getting closer and closer to focus, and then it starts coming out of focus on the other side (without ever looking like it got to focus)? Or as it is coming into focus you run out of travel one way or another?

Alex

Keith,

Eyepiece projection always requires that the eyepiece be moved outwards from the position of focus for visual observation. Just how far out depends on a couple of factors but it will never be more than the focal length of the eyepiece used. For example, a 25mm eyepiece used for projection will never require more than 25mm of out focus.

There are formulas but because eyepieces are typically not simple lenses, it is difficult to tell exactly where to measure from. However, you can get a very good idea of the mechanical setup required by projecting the image of the moon, (or a distant street light), onto a card held up to the eyepiece. Hold the card about three inches back and adjust the focus for best projected image.

Eyepiece projection results in an magnified image. The distance to the image is determined by the magnification and the focal length of the eyepiece used for projection. M = (distance to image - focal length)/ focal length. A special case occurs when the primary image is exactly two focal lengths from the lens since this puts the projected image at exactly the same distance and magnification = 1. I have attempted to illustrate this in the simple sketch attached.

have fun!

dan

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