ATM TDE Mask

[SHANE LAPIERRE <SLAPIERR@bcbsct.com>]

Last evening I was fretting again about imperfect images in my 10" 5.6 homebuilt dob.  I 
purchased the optics several years ago, but assembled the scope myself.  I have never 
made my own mirror, but I keep seeing TDE mentioned on the list a
lot.  I don't know much about testing optics either on a bench or w/ a star, but at high 
power I can definately see diffraction rings (boiling of course w/ the local seeing).  
Last night I noticed a thicker external ring while out of focus
so I assumed TDE???  Anyway, I cut out a 9" hole in a piece of cardboard and covered the 
outer 1/2" of the mirror with the mask.  This drastically improved my view of Saturn. 
Cassini's division very clear, as well as other usually invisible
details, etc.   What is the most direct way for me to "test" this optic to figure if my 
hunch is right (TDE) and determine at what radius it starts?  After that, is there a 
proven masking technique that works?  I don't think I've ever seen
an aperture mask, except for the off-axis ones on dobs.  I'm sure others have done this...

Thanks,
Shane LaPierre