From: "Dan Chaffee" 
To: 
Reply-To: "Dan Chaffee" 


From: "John Sherman" 

>I dare you to come to Los Angeles and set any 8" Cass next to my 8" f/8
>Newt in excellent seeing.  (primary mirror by Richard Fagin, tube assy
>by Ed Grissom)    Have you ever seen an 8" Cass that shows detail on
>Ganymede, and resolves Titan into a disc? I haven't.

John,

Obstructions, even SCTs with their ORGARISH 37% secondaries, do in fact,
allow for some superb hi resonlution work. Doesn't seem to phase Thierry
Legault's 12.5" SCT for his  astonishing ccd work, including seeing
Enke during  visual obs. Contrast is affected as MTF predicts, but
resolution of not-so-low contrast things like Regio Galileo on Ganymede are
_not_ out of reach for an 8" SCT
*if the optics are very good*. Ask Mel about his 30% (or so)
obstructed 6" F/4 newtonain and tight Jovan disks at 300X. A year and
a half ago I could detect the northern edge of Saturn's limb  through the
Cassini division in a 35% obstructed 5" Newtonian on a night of in
P-6,7 seeing. High resolution imager (and brash SAA defender of mak
varients) Andea Tasselli saw the lunar Alpine Valley rille with an 8"
(IIRC) instrument with 50%(!!) obstruction.

It's true that we want everything going for low contrast high resolution
work that we, including not making obstructions larger than necessary, but
this issue is almost always exaggerated. If you haven't, take a look at the
Mar. '95 issue of S&T article on the effect of obstructions and
aberrations on Jupiter images. Rather illuminating I think, and in my
experience, right on the money. It clearly shows that quality has a
substantially higher impact on detail than obstruction.

Dan Chaffee

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