You are da man! With clear skies last evening I proceeded with my
experiment as promised. First collimating to the shape of the
donut. Saturn's rings looked good, but not all that impressive.
Went back to Alderbaran and collimated on the tiny diffraction rings
and centered the dot at high mag. Bingo! Cassini's division was
clearly visible all the way around. I don't remember ever seeing it
so plainly! Seeing was pretty good too, which helped some.
In retrospect here is what I was doing wrong: I started the process
at a moderately high mag, 200x. At this mag the donut looks outta
wack when the tiny rings are concentric. When I went back to low
power, say 77x, the donut looked pretty good. The other thing that
made a *big* difference was fine tuning the collimation at even
higher mag. With my Barlow in I centered that dot dead on at 400x.
If anyone doubts Rod's advice concerning the use of the tiny rings,
try this experiment. Collimate on the tiny rings at high mag. Now
take the focus out till you see the big donut. In my case the donut
is skewed to oneside. Now bring the focus in ever so slowly. What I
see is the big donut collapses in to form a bright ring which is
skewed relative to the concentric tiny rings inside. Now if you
focus in ever so slowly you will see that eventually this bright
outer ring begins to take on the shape of the tiny inner rings, and
just before critical focus you will see a single bright ring
surrounding a central dot (no comet tails). Focus a little more and
you see the airy disk with a faint diffraction ring around it (if
seeing is good). It doesn't get any better than this.
So I guess the answer to my question, is there something wrong with
my scope, is a definite no. At least I'm happy with the quality of
viewing. Still the purist in me wonders why that big donut looks
skewed when the optics are perfectly aligned? Is this an indication
that the optics are mis-shapen? (I've heard reports of C8s
manufactured in the mid 80's having quality problems...I bought mine
in early 85). Is this skewed donut a common thing with properly
If you guys think I have a turkey, give it to me straight, it won't
hurt my feelings.
Thankx to all you guys who chimed in with questions and
recommendations. I appreciate all the help, you are a great group!
Clear skies, Frank
--- In sct-user@y..., RMOLLISE@a... wrote:
> Your words are encouraging, but now I'm getting confused. The
> previous post, from Mr GNowellSct, says to completely ignore the
> rings that 'distract' from proper collimation and you advocate
> the tiny rings whilst forgetting about the fat donut. I've done
> of reading on collimation (even been to your site ;-) and they all
> start out with centering the fat donut, and then focus in with
> mag. Ok, I've centered the fat donut, and now I move to higher mag
> and focus in to see the tiny rings.
> Hi Frank:
> Again, use the diffraction rings! Use the "donut" only as a rough
guide to collimation!