Re: ATM Effects of an inferior secondary

[astro.electronic@t-online.de (Michael Koch)]

Hi Bill,

> Speaking of secondaries, who can characterize the effects of a secondary
> that is less good than the primary?  What sort of defects does one see with
> a crummy secondary, say on a star test?

The most common defect of a bad secondary is spherical aberation (secondary is 
not flat), which becomes visible in the star test as astigmatism because the 
mirror is tilted 45 degrees.

> Is it possible to sort out
> secondary defects versus primary defects at the eyepiece?

Yes, in case of astimatism you can rotate the main mirror. If the astigmatism 
doesn't rotate, it must be in the secondary or in the eyepiece. Rotate the 
eyepiece to make sure that it comes from the secondary.
In case of other secondary defects it's difficult to see in the eyepiece 
where the defect comes from. 

> Must primary and secondary PV ratings be matched for optimal performance?

No. Both PV values should be independently as small as possible. It's very 
unlikely (but not impossible) that the PV error of the main mirror is 
compensated by an opposite error of same size in the secondary. For error 
compensation, the surface errors of the two mirrors must match over the full 
surface. This is _VERY_ unlikely.
  
> Can the secondary have a somewhat poorer PV rating than the mirror, ...

Yes, the secondary can be poorer by a factor sqrt(2) = 1.414 because it is 
tilted 45 degrees. In this case the main mirror and the secondary produce same 
amounts of image degradation.

> ... without image degradation?

No, every surface error causes image degradation. 

Michael

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 ASTRO ELECTRONIC  Dipl.-Ing. Michael Koch  http://www.astro-electronic.de

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