Space Forum / Amateur Astronomy / March 2006

Since I messed with (and improved, I think) the collimation of my 8 inch
CAT this is the first night of good (3/5 according to the Clear Sky
Clock)seeing.

I was able to find Saturn and see that the Cassini division was visible,
but only intermittently.  I was using a 17mm Orion eyepiece
with a yellow filter and a 2x Barlow.  I could clearly see the shadow of
the planet on the rings.  This was the best demonstration I've had yet of
exactly what "seeing" is and means.  For other newbies, the affect is not
so much like the image is moving in and out of focus, but more like the
difference between different computer monitor resolutions.  At the poor
moments it's not "wrong", but at the better moments, it's *better*.

A look at Aldebaran showed me that the the seeing near the horizon was
much worse than near zenith, and that my collimation still needs
improvement.  Or maybe that my telescope does not hold its collimation
well.  I certainly did not follow the advice that for every screw
tightened during collimation, two must be loosened and vice versa.  I
suspect that the point of that advice is to keep everything snug and help
the scope hold its collimation.  I intend to revisit this.

Removing the yellow filter, revealed (again only intermittantly) slight
glimpses of the sort of features one sees in time exposure photographs of
Saturn, meaning that the planet's disk did not appear to be perfectly a
single color/brightness.  Something was going on there, but nothing like
any photograph I've ever seen.  But since tonight was an exceptional
night, it was also nothing like I've ever seen at the eyepiece.

I'd call it a successful night, and Jupiter won't be up over the trees for
another hour and a half.