I have a takahashi fc100d on a cg4. I want to get a larger scope to use on nicer nights when I am going to be out for several hours. I care most about lunar and planetary. I was thinking about getting a 10 inch dob or a cat that I can mount on my cg4. What are my best options?
A 4" doublet is small, light, nimble, effortless, and a joy to wield. If that is what you are used to, then be cautious as you go larger as things become larger, heavier, unwieldy, and effort prone very quickly. So you need to be in the mindset for all that, especially thermal management of the optic (which there is none with your Tak). So more work, not effortless, more planning, sometimes frustration, less observing, less fun -- aperture is great, but it is not free. Given you are used to a 4", I would say you should go with 8" instead of 10" as overall it is more ergonomic and less of a hassle for thermal management/acclimation with its lesser mass. An 8" aperture also gets you .57 arcsec resolution, and that is well below what extremely good seeing conditions will show you (the 4" gets you 1.14 arcsec). I tend to like my minimum exit pupil for Jupiter to be around .65mm. Smaller than that and the dimming loses too much of the low contrast details on the planet. So in your 4" that exit pupil gets you to 154x, with the 8" it lets you have more image scale and gets you 313x! A nice jump in capability for when the seeing will support that magnification at your location. With Saturn or Mars, I find these can take a smaller exit pupil well so will routinely go to a .3mm or even smaller with them (350x in the 4", way higher with an 8" but need some really special evenings for a steady view at my location over 400x).
Now at many locations, seeing does not often support sub-arcsec seeing. However, when I lived near D.C. I used to observe most every clear night. Observing that often I would find that in general, I could expect at least 2 to 3 observing sessions a month where I encountered very excellent seeing for steady high magnification planetary (i.e, 350x-450x is rock steady). Now most of my observing is with a 4"...and taking a 4" out every clear evening for a few hours is no chore at all. Hauling my 10" out that often though is something I would never ever do as just too much prep and care and feeding to manage the thermal behavior and collimation and cleanliness of the mirrors, etc. So if all I had was my 10", well then I would not be out as often, and that means I would not encounter those really good seeing times as often. So just food for thought as how much planning and effort you are willing to undertake with your equipment can impact not only how much you observe, but also how likely it will be that you encounter the best seeing. If extra effort, bigger carry loads, etc. are not an issue for you, then you are golden and not a worry.
A premium mirrored 8" Newt/Dob would be nice IMO. I would go with that over an SCT simply because it will also be great as a generalist since it can achieve a lot larger TFOV than an SCT can. IME the Newt is also a little easier to thermally manage than an SCT and the focuser can be had that is much nicer/precise (no mirror flop), and has a smaller CO. But if you don't care about TFOV and want the smaller footprint over other considerations, then I've had some excellent planetary views through an 8 SCT. But if I were to get one, would go through Company 7 or the like so at least you are assured of getting a mirror up to specs. My preference though would be the Newt. Who would make a premium 8" mirror? I'm sure other will chime in as to who.
Edited by BillP, 05 April 2018 - 03:39 PM.