I'm in the process of building my own reflector telescope; I have an 8" primary mirror with a focal length of 1200mm.

Of course a telescope has a focuser that lets the eyepiece move up and down until the image of whatever you're observing is perfectly in focus. My question is - where on a ray diagram is this 'perfect focus' found, and what determines it?

For an object at infinity, reflected by a concave mirror, the image is formed at the focal point. Therefore, I originally assumed that focus is achieved when the eyepiece is at the focal point, and the image is magnified by the eyepiece so that it's visible to a human eye.

However I've seen several ray diagrams that depict the eyepiece as being positioned some distance beyond the focal point, where the rays have started to diverge, and the eyepiece then 'straightens out' the rays so that they're parallel. If this is the case, what determines this distance?

Example ray diagram with eyepiece beyond focal point


The mirror forms a real image at its focal point, the primary image. The eyepiece forms a virtual image of that real image. To do so it must be at a distance from it, as shown in your diagram. If it's one focal length of the eyepiece away, the eyepiece forms an image at infinity. But people have different ability to focus at different distances (they're maybe near-sighted or far-sighted) so there has to be some ability to focus the eyepiece from a focal length away from the primary image to a closer distance from it. There is no "perfect focus."

  • $\begingroup$ May I add to this good enough answer that what you want, OP, is that your eye, makes the same focus (the crossed beams) on your retina. For that, light incoming into your eyes, the rays need to be somewhat parallel, and hence why you collimate the light that was gathered by the main reflector with the eyepiece. $\endgroup$ – José Andrade Feb 10 at 14:53
  • $\begingroup$ Thank you both for your input. So does this mean that, wherever the eyepiece is positioned (at X distance from the real image produced by the mirror), the rays will be parallel upon exiting the lens? Essentially, the position of the lens makes no difference to the parallelism of the rays that exit it? (Sorry if this is a very basic question.) $\endgroup$ – UpDownStrange Feb 10 at 18:39
  • $\begingroup$ The rays exiting the eyepiece will be parallel only if X = the focal length of the eyepiece. If the eyepiece distance is < X, rays will diverge from the eyepiece i.e. a virtual image at a distance less than infinity. Unless you have perfect far-sighted vision, you will not be using the eyepiece a focal length away from the primary image. You need some adjustability in its position to get your best focus. $\endgroup$ – Not_Einstein Feb 11 at 19:18

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