evidence of astigmatism

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Robin Leadbeater

Attached are examples of before and after spectra when the grating was overtightened after a rebuild, though unfortunately I did not do the zero order test before I cured it which would have conclusively proved astigmatism.  With the spectrum at its narrowest, the star was out of focus on the slit and the throughput was low. Adjusting the telescope so the star was in focus on the slit gave  good throughput but a  wide spectrum. Once the grating clamps were slackened off. Everything returned to normal (The star was in focus on the slit, the spectrum was narrow and the throughput was good)