From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 



Steve:

        Sounds like an overcorrection type pattern if the outside focus
diffraction
rings are clearer.  This might be due to cooling, as mirrors tend to
overcorrect as the temperature drops.  I think it's because the edge has
more area exposed, both from the front and side of the disk and cools
faster than the rest of the disk.  Sort of like a temporary Turned Down
Edge, or Long Radius Edge. Thermal currents off the mirror can cause
strange effects, often like tube currents which can look like
overcorrection.  The cooler air at the outside edge of the aperture is
denser and refracts light away from the axis.

        Have you tried the secondary shadow method?  The secondary shadow
should
appear at equal distances inside and outside focus.  Mark the focusing knob
so you know how far you are actually turning it. Dick Suiter says that the
distances should not exceed 2:1 for good optics.  It takes many nights to
get a final idea of the figure.

        Important to keep viewing regardless of any abberations you suspect.
Our
club has an old 6" f/15 Askania triplet lens that doesn't star test
well, but the views are super anyway.  Our 16" Meade isn't all that
hot either, but it is a very enjoyable scope! My 6" f/10 Newt is a bit
overcorrected, and I often fuss over refiguring it or spherizing it and
flexing in the correction.  Once I start viewing, though, I forget all that
stuff and just observe.

Colin

--- BBBS/NT v4.00 MP