calculating focal length of extender?

Hello -- I am hoping someone here can help me. I have what looks like
simple optical problem but lack ray-tracing software or enough
knowledge of optics to solve it.

I have a telescope whose focal plane is 1/2" outside of the most
retracted position of the focuser tube. I need more room to attach
some instrumentation (a spectrophotometer), so I would like the focal
plane to be further outside the tube. I think I can do this by adding
a concave lens ahead of the focal plane, so that it causes the
converging cone of image light from a point source subject (a star,
say) to converge less strongly, and hence come to a focus further
outside the tube.

I can calculate the angle "a" of the converging cone of light (knowing
the size of the telescope mirror and its focal length). I also know
the distance "d" that I want to move the focal plane. Is there a
formula I can use to calculate the required focal length for the
concave lens I need to do the job?

Thanks for any advice or references,
Robert
.

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