Quote:
Originally Posted by Aster
So, can someone tell me what 1/16 wave RMS or better means in plain total wave front???
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RMS figure is a better indicator of mirror quality as it factors in the relative surface area of an error , although doesn't tell you necessarily of how well the mirror will perform. If most of the error consists pure spherical aberration then there will be a smooth sinusoidal error of around 1/4 wavelength at the 70% zone. Such a mirror can perform well but planetary contrast will be a little washed out.
If the mirror were pretty good spherical aberration wise but say a rolled up or down zone but only effecting the edge, the mirror may still have 1/16 wave RMS, but give a very poor looking star test, and even worse planetary performance. The golden rule for optics is that errors if any want to be smooth and gentle slope. The only optical claims I've seen on the GSO website say 1/16 RMS ( roughly equivelent to the old 1/4 wave Rayleigh criterion) and seems to be a very reasonable and safe claim on the basis of the handful I've seen.
'Numbers' don't mean a lot without seeing what kind of surface shape is involved. Two different 0.95 Strehl mirrors can have vastly different planetary performance. The optician who understands this and why and how will make consistantly better performing mirrors for planetary observation.
In the case of mass produced mirrors you have no real guarantee about what you get so its a kind of lucky dip that you don't encounter in other hobbies such as Hi Fi for instance. It seems to me that people have much higher expectations of mass produced optics than they used to which is neverthless a good thing.
Mark