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Joe,
I also use the STI knife edge focuser on an SCT
(C8). I was getting soft focus on many of my shots and for a long time
thought it was operator error <g>. On my last roll I shot many shots
with the f/6.3 reducer and every single one was out of focus. Even with
operator error I would have expected to get lucky once in a while so I began to
suspect the calibration of the Stilletto.
Soooo, I shot a roll of black and white to
try and pin this down. What I did was turn the Stilletto one full turn
counter clockwise. Then I focused on Deneb and took a 30 second exposure
with tracking and about 3 minutes more with tracking off. I repeated this
process turning the Stilletto 1/4 turn clockwise taking the same sort of
exposure each time, until the point I was one full turn past 'factory setting'
(a total of nine shots).
What I found was interesting. The 30 second
exposure made it easy to rule out the settings that were way off (nice little
donuts) and the star trailed part of the image helped isolate the correct
setting (the one with the crispest faint trails). The factory setting was
definitely wrong...the correct setting was 3/4 turn clockwise from the factory
setting!!
To make a long post short (oops, too late!), don't
trust the factory setting! especially if you're consistently out of
focus. Sorry if you already know all this, but it was a revelation to
me!
Carpe Nox, Frank
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