System Testing

Here is the result for the telescope as a system. The fold mirror was supported on axis at 45 degrees(giving rise to the shadow on the map plots) and the test reference sphere was mounted at the focus, outside the field of view of the Zygo.

From the size of the test sphere, I calculate an entrance pupil diameter of 2mm at the eyepiece, so this result is only relevant for a high power eyepiece. A larger pupil would use more of the fold mirror and performance would degrade.

Under the test conditions the full system Strehl ratio is calculated to be 0.757

In practice, with contemporary eyepieces, a much smaller area in the centre would have been actually used. Plus being used at 45deg incidence the results would have been x(sq.root 2) better.

A plot for the central region equivalent to a 5mm pupil at the eyepiece is shown below

The calculated Strehl ratio is 0.765 and using a modern eyepiece (Televue), the astigmatism is easly visible in star testing, but barely noticeable in regular use

AstroKeith 2014