[ATM] Hartmann Mask

[Mel Bartels]

>   http://www.oldham-optical.co.uk/
>   at ''Optical standards'' : ''Minimum spot size and minimum Wavefront
> error are actually mutually exclusive''.This is supported by my
> experience.For years I was focusing the scope on a close double star but
> than I had to refocus for Moon and planets.My questions are :
>   -Is this peculiar to my eyes or others have the same experience ?


This is common for me:

1) Focusing on stars is never as good as focusing on planetary detail
2) I don't mind enlarging the Moon a bit by inward focusing
3) Best focus position for extended object is often different than point
source because of focus differences from the eyepiece's center to edge
4) Color of star can influence focusing
5) Poor optical alignment precludes precision focus and leads to a different
compromised focus position: this is very common in amateur reflectors.


>   -How can we find the focus ensuring the scope is operating at minimum
> wavefront error?Focusing on stars it is a rough focusing,in my
> understanding now,the focus position showing the best planetary details is
> closest to the optimum.But if ,at the moment of observation no planets are
> visible?''Observe more ,worry less'' ?Mircea


Then by definition it doesn't matter because you cannot observe it.  Don't
focus (sorry for pun) solely on the optics: the changing night temperature
affects seeing, affects tube length and affects you!  I am constantly
refocusing.  That implies that quality of focuser is critical.

Mel Bartels