From: "Richard F.L.R. Snashall" 
To: ATM 
Reply-To: "Richard F.L.R. Snashall" 




Jim Burrows wrote:
>
> At 12:11 2003-04-01 +0200, Vladimir Galogaza wrote:
>
>> Reading this and subsequent discussion I wonder if, taking into account
>> statistical nature of surface error  numbers ( Strehl, RMS, PV)
>> these are linearly additive for two or more subsequent mirrors or some
>> other
>> relationship has to be used to determine  final   error resulting from
>> individual errors.
>
>
> If x and y are uncorrelated random errors, then statistics says
> variance(x + y)= variance(x)+variance(y).  So if the errors are
> uncorrelated, RMSı(system) = RMSı(primary)+RMSı(secondary).  BTW, PV
> isn't statistical and would be nearly impossible to evaluate for
> multiple-mirror systems.  The RMS result is for straight-on systems like
> casses.  For diagonals, hmm...
>

I have to question whether the errors in telescope systems can be
considered uncorrelated.  If the error were "lumpiness", that
might be the case;  but, from listening to this group, would that be the
normal source of imperfection?

        Rick S.

>
>         -- Jim Burrows
>         -- mailto://burrjaw{at}earthlink.net
>         -- http://home.earthlink.net/~burrjaw
>         -- Seattle N47.4723 W122.3662 (WGS84)
>
>

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