Mirror testing

I am lucky to still have connections with my old employer who has a Zygo interferometer. Here is a summary of the results and links to the full test plots

The Primary mirror is excellent and a credit to WFA Ellison and underwrites his reputation. This was one of his earliest mirrors, and no wonder he kept it for himself!

The secondary is not so good having significant astigmatism, and on researching his book and many letters he later acknowledged the difficulty in producing flats and the importance of having a good one. He never polished flats himself, preferring to select good sections of stock plate glass and then cutting a flat from that section. The process he used to cut out the flat we now know to introduce stress and deformities.

The flat lets the whole telescope down and in practice its influence is visible under star testing with a modern eyepiece. With the eyepieces available to Ellison at the time it would not have been so serious a flaw though.

Primary mirror(click for more detail)

PV 0.163 wave
rms 0.031 wave
Strehl ratio 0.962

Fold (secondary) mirror

normal incidence & full aperture(click for more detail)

PV 0.85 wave
rms 0.144 wave
Strehl ratio 0.445

normal incidence & 5mm eyepiece entrance pupil

PV 0.521 wave
rms 0.084 wave
Strehl ratio 0.765

Full System(click for more details)

(2mm eyepiece entrance pupil)

PV 0.484 wave
rms 0.084 wave
Strehl ratio 0.757

AstroKeith 2014