Michael,
Just to reiterate what has already been said well... it's part expectations and part familiarity/understanding your scope.
The C6 is "only" 150mm aperture, and since resolution is proportional to aperture, that will limit the extent of accessible fine features, but also keep in perspective that the atmosphere will mostly limit even a small scope such as a C6 most of the time!
Good nights do come and go though, and a bigger scope will certainly show you more on a better night. But also don't be discouraged by theoretical limitations. Ask Morton about my C6
he may not recall a night at a star party looking at Saturn through my C6, at about 400x IIRC. Large CO and all. Of course, a bigger scope next to me would have shown more under the same conditions. That's just the laws of physics
The things to maximise your C6 have already been touched on... ensure thermal equilibrium, whether that's by letting your scope cool outside before use or by using an insulating jacket.
Collimation of an SCT is fundamental to getting the best out of it, and isn't hard once you've been practicing - certainly easier than a newtonian! Starting with a defocused star looking like a donut... the secondary shadow (centre of the donut) should be centered. Then increase the magnification with a shorter focal length eyepiece. Then progressively start shrinking the donut, collapsing towards focus. Stop regularly and watch... as you get closer to focus you should start seeing rings around a central bright spot. Adjust the collimation screws appropriately to ensure the rings are concentric about the bright spot.
As you get close to focus, the rings should collapse equally in all directions into the focussed star, and at this point you will probably see the energy ripples of the Airy disk. Note that that as you approach good collimation, the adjustments needed on the screws are tiny, and your ability discern will be affected by the atmospheric turbulence (seeing)... if the seeing is not good, the rings will be jumping around all over the place. Try again another night!
Oh, and if you're unsure which screw to turn... the screws are 120 degrees apart...what I do is to stick my finger in front of the scope in the orientation of one of the screws to figure out which is which (this is the beauty of a small scope, it's all within arms reach!). What you're looking for is the direction in which the rings are offset. Once you've found the orientation that matches, adjust the screw a tiny amount. If it makes the offset worse, turn the screw the other way.
Keep in mind that you may need to adjust more than one screw to get the rings concentric...and after _any_ adjustment of the a collimation screw, always centre the star again in the FOV before making a subsequent adjustments. With your final collimation checks/adjustments, you want to be aiming for 300-400x magnification.
It may be hard to visualise, and it's certainly hard to describe, but hands-on I hope it will make more sense.
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Originally Posted by MichaelWB
Hi everybody.
I am surprised and pleased with the number of responses from the IIS clan that my query generated.
There seem to be a number of votes for thermal equilibrium and relative apertures, and collimation gets an honourable mention too. I also like Morton’s thoughts on central obstruction.
The ‘scopes were both outside for about 45 minutes before we noticed the differences. The dob is definitely well collimated according to my laser collimator, though I can’t be sure about the C6, but out of focus stars are circular, and I believe that is a good sign. I haven’t yet got into the finer details of SCT collimation. During the observing session we were able to see the Cassini division and were able to see an eclipse/transit (not sure of the correct nomenclature) on Jupiter's surface by one of its moons with the C6 (one of several astronomical firsts for me with this ‘scope).
Andrew, you might be pleased to know that after comparing ‘scopes we walked back to my place and dragged out my recently completed 14” f7.2 (I picked up the mirror and diagonal on ISS classifieds without any real idea of its quality or provenance, but I got lucky). we had some great views of Saturn, got a good look at the eclipse, but especially seeing detail in Jupiter’s cloud bands for the first time. Yes, it didn’t take long for me to get aperture fever.
My neighbour is quite taken with the C6 and wants one for himself. He regularly works in the country and has access to pristine rural skies, but the dob is a bit too big to cart around and set up after a day's work. He also loves the slow motion controls on the alt/az mount vs. the nudge, nudge of the dob.
Anyway, thanks for the feedback.
Michael
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