reflector/refractor, planetary-wise

I have an f/5 10" dobsonian.

Is there a noticeable difference in quality in longer focal length refractors as opposed to fast dobs? What do you think is the lowest amount of aperture I could get for a refractor to show improvement over a light bucket on moon/planets?

Thanks for any advice.

Evan Hawbaker said:

I have an f/5 10" dobsonian.

Is there a noticeable difference in quality in longer focal length refractors as opposed to fast dobs? What do you think is the lowest amount of aperture I could get for a refractor to show improvement over a light bucket on moon/planets?
Thanks for any advice.

You would need a rather huge refractor just to equal the performance of your 10" assuming the mirror's good and it's well collimated. To get some idea of relative appearances to a smaller refractor, you can make an off-axis aperture mask that stops your scope down to 4" or so (f/12.5), that will give you a bit of a feel for some of the trade-offs between aperture, obstruction and collimation.

If you use inexpensive eyepieces, the f/5 light cone will be quite taxing on their field of view -- f/6 to f/8 scopes are much more forgiving, and this in itself does not necessitate getting a refractor either.

This really depends on the quality of your dobsonian and how well it is set up. A high quality 10 inch f5 dob (such as the Teleport) with a 1.86 inch secondary (or even 2.14) will give excellent planetary views. It has considerably more resolving power than 4 or 5 inch refractor, even an APO refractor.

The closest SxS comparison (to what you're asking) I've done was between a D&G 5 inch f12 achromat, and an 8 inch f 5.7 with 1.56 secondary dob (Mike Spooner optics). So both scopes were of high optical quality. The dob walked all over the D&G in good seeing and thermal equilibrium. A similar comparison was made between high end scopes on Ed Tings web site with similar results. T Scopes gets excellent results at f 4.7.

This does not mean that you shouldn't get a refractor, a smaller apo is very portable, equalizes very quickly, and gives beautiful views.

The answer depends largely on the quality of your reflector. At a recent star party, my 6" apo refractor was set up next to a very fine 10" f/6 Dob which was noticeably superior on planets. I also spent some time with a very nice 15" f/4.5 Dob which gave exquisite planetary images far beyond what my refractor can do. On the other hand, if your 10" is mediocre or worse, it might not take much of a refractor to surpass it on planets.

Hi Evan,

Making the assumption that your mirrors are OK and the scope is collimated and properly thermally managed it would take a refractor of nearly equal aperture to resolve the same detail as your 10" f5. My Orion 12" f4.9 shows more planetary detail than any of my previous telescopes including apos, Mak Cassegrains, Mak-Newts, achros, and a 9.25" SCT. If you check my classified ads you'll see some of the scopes that I've owned and see that I'm not talking theory or conjecture but describing my personal experiences. If you really want to see what your scope is capable of you need to thermally manage it and keep your collimation as close to perfect as possible. I drill holes into my reflector OTAs and install a fan to blow air directly across the primary and out of corresponding vent holes opposite the fan. Alan Adler and Bryan Greer got together and explained this in detail in an issue of Sky & Telescope a few years ago. Your 10" reflector is capable of showing more detail than a 6" apo if it's optimized to do so. Get the most from your reflector!

Clear Skies,

Steve

I have an f/5 10" dobsonian.

Is there a noticeable difference in quality in longer focal length refractors as opposed to fast dobs? What do you think is the lowest amount of aperture I could get for a refractor to show improvement over a light bucket on moon/planets?

Thanks for any advice.
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I think others have given some good advice. Over the years I have owned a variety of scopes including a 120mm F8.3 Orion refractor, DOBs of 8, 10 and 12.5 inches.

If your 10 inch F5 Dob is anything like my OPT Starhunter 10, then if it collimated and cooled, you will have to spend quite a few thousand dollars on a large APO refractor in order to get better views.

Getting good views with a Newtonian does require some extra works and care but I think it is worth it.

I think a good companion to a 10 inch F5 is a smaller, relatively fast refractor, 3-4 inch 480mm-600mm Focal length. This scope will provide you with widefield views, some nice quick higher powered views..

jon

During the close approach of Mars in 2003 I had my absolute best view through a 10" F/5 Portaball. It was one of those exquisite nights when things just went perfectly and the PB gave an image that looked like the cover of Sky & Telescope. I never saw it better through any scope. On most nights my Takahashi FS-152 turns in more consistent images but when the sky co-operates, the 10" is better.

Longer focal length scopes have better depth of focus and seem to stay sharper while focus on short FL scopes sometimes wanders. A longer FL refractor uses more relaxed FL eyepieces with better eye relief. And they generally do not suffer from image degrading tube currents. But they cost a lot more- they FS-152 on an EM-200 mount is over $10,000 and a new 10" PB roughly $4,000.