Saturn - Why can't I see Cassini division?

Hi,

I was observing Saturn last night between 8:30 and 9:30 GMT using my 8"
SCT (LX90) and I was hoping to see the Cassini division. According to
several articles I've read this is supposed to be visible even in a
much smaller scope. However, Saturn's disc and rings are both very
bright and I could make out no detail.

Do I need to use a filter to be able to see this sort of detail?

Also, which eyepiece is best for observing Saturn? Everything looked
very well defined using a 20mm eyepiece but when using a 2xbarlow with
that eyepiece or using a 9mm eyepiece on its own the image wasn't so
well defined. I tried to collimate the scope last night as well and I
don't think I've got the hang of that yet so maybe that is the problem.
Or perhaps conditions last night weren't ideal?

Any advice appreciated!

Thanks,

Andy.

Andy Grove wrote:
Hi,

I was observing Saturn last night between 8:30 and 9:30 GMT using my
8" SCT (LX90) and I was hoping to see the Cassini division. According
to several articles I've read this is supposed to be visible even in a
much smaller scope. However, Saturn's disc and rings are both very
bright and I could make out no detail.

Do I need to use a filter to be able to see this sort of detail?

No filter is necessary for the planets. But the rings of Saturn are
'closing', so the Cassini division is not really visible all around the
planet this year, only on the edges.

Also, which eyepiece is best for observing Saturn? Everything looked
very well defined using a 20mm eyepiece but when using a 2xbarlow with
that eyepiece or using a 9mm eyepiece on its own the image wasn't so
well defined. I tried to collimate the scope last night as well and I
don't think I've got the hang of that yet so maybe that is the
problem. Or perhaps conditions last night weren't ideal?

With an 8" SCT, you can use a 4mm eyepiece if the conditions are good, ie
turbulence is low. But even with turbulence, you should be able to use a
9mm.
If the view with a 9mm is not good, I think that your scope is not well
collimated. Do the collimation again when the OTA is at the outside
temperature (wait for at least 2 hours).
If you think that the collimation is OK, may be there was a thin layer of
high altitude clouds, and to be sure you will have to observe again

--
Norbert. (no X for the answer)
======================================
knowing the universe - stellar and galaxies evolution
http://nrumiano.free.fr
images of the sky http://images.ciel.free.fr
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Andy you have probably identified the two of the three main reasons why your
views may not be too good. I was observing Saturn last night at about
midnight with a 12inch Dob at powers up to 300x. The Cassini division was
visible in moments of good seeing but nowhere near as clear as it can be.
My scope was cooled and collimated so I put it down to mediocre seeing down
here in Dorset. So what is the third reason why your views may not be so
good? Your scope needs to be close to ambient temperature. In the UK this
means that it needs to be outside cooling down for at least an hour before
it will perform at its best. Keeping it cool and collimated will allow you
to see Cassini as best as the seeing will allow. There are no need for
filters for this.

Adam

well defined. I tried to collimate the scope last night as well and I
don't think I've got the hang of that yet so maybe that is the problem.
Or perhaps conditions last night weren't ideal?

Hi Andy. You will be able to see the Cassini division in your scope - I
saw it in my TV Pronto (70mm semi-apo refractor) earlier this year at
100x. Three things you need to do to improve your chances a
1) Make sure your scope is perfectly collimated. Get someone who has
experience with SCT's to do your scope and show you how it is done.
Don't rely on laser collimators - the ONLY way with an SCT is star
testing.
2) Use a bog standard plossl or super plossl eyepiece - you won't need
a huge wide field eyepiece like naglers and stuff for seeing fine
planetary detail. The extra glass elements in wide field designs
contribute to increased loss of contrast. That's why you suffered image
breakdown using the barlow. There comes a point when you can
over-magnify things. The secret is in finding out just how much
magnification the seeing will permit.
3) Wait until later in the night. At 9.30 p.m. Saturn is not really
high enough in the sky to give the best image. When objects are near
the horizon, you will be viewing them through a thick air mass, a bit
like the heat haze you see along a road on a warm day. Try again around
midnight when the planet is at it's highest point in the sky.
Don't be discouraged if you still have trouble - it takes practice and
experience to pick out fine details and low contrast features. It may
be that you're actually seeing it, but expecting something more
dramatic. Good luck, Dave.