Re: SV: ATM Diag. size and Focuser extra travel

[astillma@nit.AirTouch.COM (Anthony Stillman)]

An old quote:  "Its not how big it is, its whether or not you have one."

I've been able to see the central star of the ring nebula through a 7 inch
un-obstructed refractor.  I can't say the same for any obstructed reflector.  
Your milage may very, but my experience has been that un-obstructed light paths
are far better than even the smallest obstruction.  Of course I'm not talking
about huge improvements, but after you look through enough scopes, you begain 
to realise that that little extra bit of perfromance is hard won.  Is it worth 
it?  Yes and no.  For city observing, no.  For dark steady skies, definetely.
Also, the eye comes into the balance, as it has its own spacial frequency
responce function, which alas seems to degrade with time and veries from person to 
person.  If you are using a CCD or grain film, you should consider wheather
or not an obstruction will significantly effect the image.  Also your mount may
not be stable enough for the picture to benefite from an un-obstructed light
path.

What I find most interesting about obstructions and for that matter appodising
masks, is that they can improve performance at certain spacial frequencies.  
While the overall image is degradded, certain details are improved.  This detail
can and has been used to split double stars and improve the contrast of highish
frequency detail on distrubuted sources.  

Anthony