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Dobs Vs Refractors

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Posted by Daniel Johnson on June 19, 2003 00:35:18 UTC

Before I object to anything regarding 5- or 6-inch refractors, let me first say that they are a reasonable choice for city dwellers. A 16-inch Dob is also huge and almost impossible to transport in ordinary passenger cars.
The idea that a small refractor outperforms a much larger Newtonian (Dob) or SCT on planets is myth, and it has been thoroughly debunked in Sky&Telescope, and also in side-by-side comparisions with my own eye. Still, it is a myth based partly on facts. Advantages for the refractor:
1) The refractor is (nearly) always in perfect collimation. Newts and SCTs need frequent attention in this regard, and failure to collimate perfectly degrades high-magnification images a lot.
2) The refractor reaches thermal equilibrium quickly. A 16-inch Dob takes forever, unless you install fans to help (and S&T published a wonderful article on how to do this a couple of years ago). My 12.5-inch Dob had horrible thermal problems before I put in a fan.
3) Because the larger scope has higher resolution, it will show turbulence that a small refractor fails to resolve. So, it may seem to give images that dance more once it has reached equilibrium, but only because it is capable of showing smaller flaws in the image!
4) The central obstruction of Newts and SCTs is often cited as a reason that refractors are superior. And refractors are, in fact, superior in planetary imaging AT THE SAME APERTURE. However, even though the central obstruction scatters more light outside the central circle of the diffraction pattern, the larger scope has a smaller central circle, offsetting the disadvantage. The end result is that a perfect 5-inch refractor roughly matches a perfect 8-inch SCT on planetary work. A larger SCT (or Dob) outperforms the refractor, assuming perfect collimation and thermal equilibrium. This 5-inch = 8-inch figure has been published by S&T and it matches my own observations.
Or ask yourself this: since you can make an off-axis mask to convert the 16-inch into an unobstructed 5-inch, how can the 5-inch possibly be superior?

Still, I think that one of the refractors you are considering, or an 8-inch SCT, is a good choice for a city dweller. And don’t discount the value of motor drives, especially at high power. Since I bought my 10-inch SCT, I have not used my 12.5-inch Dob even once. The convenience of automatic tracking more than makes up for the aperture difference in this case. And you will enjoy the permanent collimation and slightly more rapid thermal equilibrium of the refractor.

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