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Old 26-06-2020, 01:58 AM
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ngcles
The Observologist

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Join Date: Dec 2006
Location: Billimari, NSW Central West
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Dedicated scope for planetary viewing ... only?

Hi Peter,

The things that will make for a good "planetary viewing only" telescope are (not in any particular order):

(1) Good to long focal length (ie more than 1,500mm f.l) because you can use longer focal length eyepieces with bigger eye lenses and comfortable eye relief resulting in less eye-strain from peering through a pinhole with your eye pressed up hard against the eye lens. Planetary observing takes time and patience until the right moment(s) come along when the seeing settles and everything that can be seen in a particular aperture, becomes visible. Sometimes it can mean many, many minutes before that moment or two comes along. That comfort level is a nice thing to have.

(2) High accuracy, well collimated optic set. Newtonians do tend to need more frequent "tuning" in this department. Refractors and catadioptric telescopes generally hold collimation better. Collimating a telescope is a basic skill and once you are used to it, isn't difficult -- but it is critical to getting the best planetary views particularly with an f/4.4 like the Skywatcher where the difference between "close" and "perfect" is a very small margin and makes a substantial difference on eyepiece image quality.

(3) No central obstruction or at worst a small one. This is why, inch-for-inch, refractors rule. The problem is that they aren't readily available above about 150mm aperture. If the aperture contemplated is above 150mm, a Newtonian with a small secondary is the next best performer. If well made, collimated etc it will normally produce the best and most contrasty visual image. 9/10 nights when the seeing is okay, good or even very good you won't notice a difference between a 'scope with a small secondary and one of equal aperture using a larger secondary. It's on night 10 the difference will become apparent in the eyepiece. It is for this reason catadioptric 'scopes like Maksutov Cassegrainians and Schmidt-Cassegrainians are generally not the 'scope of choice because their secondary mirrors are so large (between 30-40% of the aperture by diameter). Large central obstructions transfer light from the central dot of an airy disc to the surrounding diffraction rings. This is a contrast killer on that 10th/10 night(s). Both these designs have good to long native focal lengths which is nice but large obstructions, which aren't. The secondary in the Skywatcher 16" f/4.4 is 25% which is not ideal either, but somewhat better than all commercial catadioptric telescopes. Kept under 20% by diameter, the secondary size will have very, very little effect on visual image contrast. One thing about Skywatcher telescopes: I have used more than two dozen over the last twenty years and I'm yet to see a "dud" optic set. Routinely they are good to very good for a mass-produced telescope.

(4) Aperture rules. A good big telescope (assuming equal quality optics) will always outperform a good little telescope on planets. You will see finer detail due to better resolving power, more easily and there will be a considerable improvement in colour saturation. In this last department, using a couple of examples I happen to own -- an 18" f/4.9 Dobson-mounted Newtonian -v- a Celestron C8 is a humbling experience for the C8. Viewing Jupiter at similar magnifications where the colours are pretty subtle, the 8" eyepiece image almost looks black & white while the 18" is filled with abundant subtle colour. More light equals more for your cone-cells to work on and a more pleasing, more detailed view.

(5) As with all types of observing, a good telescope is the one you'll be inclined to use frequently. As for this aspect, your personal circumstances are best known to you. It's all well and good to own a big 'scope but no point if the trouble of getting it out and deployed means it sits sulking in the corner of the shed all the time.

The Skywatcher 16" isn't a bad choice at all provided you have some good eyepieces and it is carefully collimated. Lots of aperture = lots of detail + colour. The tracking is a decided bonus meaning the target can be left centred in the eyepiece at pretty high magnification where the image quality is optimal. The size of the secondary mirror (25%) isn't optimal, but there are worse out there. The thing the Maksutov's and Schmidt-Cassegrainians have going for them is good aperture with a relatively long focal length and in a compact, portable package. The con to that is the very large secondary mirror and high cost compared to a Newtonian of the same aperture. As I said, inch-for-inch, refractors rule but generally speaking, they don't have a lot of inches and every one of those inches is mighty expensive for a quality instrument.

The things that make for a good deep sky telescope aren't that different to a good planetary telescope: Quality optics, good contrast elements and ... aperture rules.

Best of luck with your choice,

L.
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