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What is the Best Planetary Telescope
You will get dozens of different opinions on what is best, but I will tell you that for any given type of scope
configuration, be it Newtonian, Cassegrain or Refractor, there will be excellent, just acceptable and rather poor
examples for each type. No one type can lay claim to being the absolute best. If you are looking for an outstanding
planetary scope you should consider the following:
In a Newtonian 8" to 12" with small central obstruction under 20%, very thin spider, longish F ratio above F6, excellent tube construction with well ventilated mirror and a decent flotation mirror cell (no mirror glued to plywood). Shorter F ratios require that the object be exactly centered in the field to avoid comatic aberrations. Also, the shorter the mirror, the more you will have to fiddle with the collimation. The mirror should have the best coating you can afford, avoid cheap coatings that lose contrast over time. Get a coating that you can clean without introducing pinholes. Add to that a smooth functioning focuser and you will have a very effective planetary instrument.
In a Cassegrain (this includes open and closed tube systems, Dall Kirkam, Maksutov, classical, Schmidt Cass. etc) 8 to 10" minimum diameter. I would aim for a system with less than 30% obstruction by diameter, F12 or longer. The tube should have good ventilation for the mirror so it is useable during early evening when the air cools down. Expect to pay more for the tube assembly than for a Newtonian. If it's cheap, it probably has too many compromises. Look for a system from a precision builder with a top reputation.
Refractor, 7" to 9" Apo, F8 to F11, or 8" to 12" achromat, F12 or longer. These scopes will cost the most, although excellent achromats can be gotten at reasonable prices from builders like D&G Optical. Inch for inch, refractors will have more light grasp, and in my own experience, an edge on contrast which is important in making out planetary detail. Achromats will have chromatic aberration effects that turn some people off but others have learned to ignore.
You can minimize this by use of filters, or perhaps with the introduction of color correcting lenses such as the Chromacorr (made by Aries) this defect can be corrected at a reasonable cost. Apochromats solve this problem entirely, but at a high cost. There are other advantages such as astrophotography that make Apos the first choice among some amateurs. Be aware, just because a manufacturer claims to use an ED element, it is not necessarily an Apochomat with full correction of chromatic aberration. Avoid cheap Apos, rather buy a good achromat if your budget does not allow a real Apo.
Whatever system you choose, you might want to consider your local viewing conditions. For planetary, light pollution has zero effect, so you can observe right from your backyard in a downtown area. The most important thing is the stability of the air above. The better your seeing i.e. steadiness of the image, the larger the instrument I would install. The farther south you live, the larger the scope that will be most effective. If you can only afford a 6"or 7" instrument, don't despair that you will not see anything. I know some top planetary amateurs who regularly observe with those apertures and have seen amazing detail on the planets.
Good luck in your choice.
- Jon Isaacs, RobM, paul m schofield and 9 others like this