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I have obtained an old telephoto lens that was adapted for astrophotography by the previous owner.

I have found a focal plane and I have placed a small webcam (without optics) there. There was a bit blurred and dark image.

Later I have found that the full image is like 2'' large, while my webcam sensor size is way smaller.

Is there a simple and cheap way to make the final image smaller to fit into 5mm x 5mm sensor? Cheaper than to buy a new astrophotography webcam?

When looking for answers I found How to make the projected image smaller by adding one ore more lenses in front of the built in projector lense? , which warns about achromatic troubles.

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You are right- although you can add a magnifying lens between the telephoto and the image sensor to focus the image field, figuring out the details, procuring the necessary lens(es) and making the mounts is probably more trouble than it is worth.

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Decrease the focal length. Not image height/image position = object height/object position. To obatin a small image height you need a smaller image position, hence shorter focal length.

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  • $\begingroup$ Unless it was a zoom lens, the focal length is fixed, for example 135 mm. $\endgroup$ – Pieter Jan 31 at 16:26
  • $\begingroup$ Or add a small telescope in reverse though it could deteriorate the field of view. the telescope has to be set for parallel in parallel out. $\endgroup$ – SAKhan Feb 1 at 18:14

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