When I left Laramie, Wyoming in May 1999 for my post-doc position in Boulder, Colorado, a grad student friend of mine offered to sell me his 1980-vintage Criterion Dynascope RV-6. This is a 6" f/8 Newtonian on a German Equatorial mount. These scopes have a reputation of having good optics; perhaps better than the current crop of inexpensive 6" Newts sold by Orion, Meade, etc. He was really just looking to get rid of it and sold it to me for $50, less than the cost of the 60mm refractor I had previously used!
Cut to October 2002. I kept seeing the telescope in pieces in my apartment, and finally got the urge to put it together and see what it could do.
The optics are indeed quite good, although the seeing is so poor most of the time that it is hard to assess just how close to the diffraction limit I can see. I've used up to 174x or even 203x (29-34x per inch) to good effect on double stars and planets and have split double stars down to 1.2" separation on the rare nights when I have good seeing conditions.
The eyepieces that came with it were a couple of Kellner-types (an 18mm and a 9mm), a 16.3mm Erfle, and a 4mm Ortho. The floaters in my eyes are too severe for anything less than about 6mm to be useful with this scope. However, the Erfle is a resonable eyepiece. To supplement that, I bought a 32mm Orion Sirius Plossl, 12.5, 9, and 7mm Orthos from University Optics, and a 10mm Siebert "Standard". More recently, I bought a set of Orion Expanse "Wide-field" eyepieces, which are working out well so far (20, 15, 9, 6mm).
The telescope has a few flaws. First, with a 1.25" focuser and working at f/8, my largest field of view is only a little over a degree. Typical 8-10" telescopes have the same problem, but they also have the greater light-gathering power, and a 2" focuser makes a lot more sense, giving a much wider FOV. A 6" scope isn't a very large scope, nor does it have a wide FOV. The 6x30 finderscope is adequate, with about a 4 degree field, but the mounting is not great and it is hard to keep in good adjustment. The focuser holds the eyepiece by friction alone. The friction itself is plenty to avoid an accident, but one has to be extra careful when changing eyepieces without moving the scope. This problem was exacerbated by the fact that it did not have a way to lock the scope in RA. The GEM was designed to be used with a motor drive, but in my case it does not work and I really don't like messing with AC power out in the field, especially with all the cold-weather observing I do. I finally had someone drill and tap a hole in the housing for the polar shaft and that works well. The mount itself is a little shaky, but isn't too bad when there is no wind.
Because of the relatively high focal ratio, the tube is pretty long relative to it's weight. Thus, adding a "red-dot" type of finder like a Telrad causes all sorts of balance problems. Luckily, GEMs are pretty easy to adjust, so I was able to take care of that. I had to make a slight adjustment to the focusing mechanism itself, but now it is tight and smooth.
The really good thing about a scope with this small of aperture is that it is lightweight, even with a 4-foot focal length. The OTA, including mounting rings, and an eyepiece, weighs a mere 11 pounds. The entire mount, with pier and legs, comes in at 33 pounds, so that's the hard part to move around, and it is much heavier than a Dobsonian mount. But, even with crude polar alignment the equatorial mount has some significant potential advantages. With the tube rings you can rotate the entire OTA to keep the eyepiece at an appropriate place for an observing chair (which I do not have yet; I just stand to observe unless the object is low in the sky to use a normal folding chair). It is possible to make a Dob mount to accomodate a rotating OTA, but that's not the usual design. You can also move across the sky in RA or Dec, which can be an advantage for certain types of jumps between objects, especially once you get used to the technique.
The main disadvantage of a 6" scope is that it is somewhat limited in the amount of detail it shows on relatively faint objects, and of course in the number of faint objects visible in the first place. This is, of course, relative, because it does a hell of a lot better than a 60mm refractor! But, the jump up to 6" does not give you detail in many more galaxies, for instance. It does let you resolve quite a few globular clusters, and is large enough to see a lot of planetary detail. In fact, a 6" is diffraction limited to about 0.8 arcsec, and most people rarely get seeing much better than that, although a 10-12" scope is closer to the true limit for an excellent spot having an unusually stable atmosphere.
Still, I didn't have any serious problems seeing all of the Herschel 400 objects from my dark foothills sites. Plus, I have seen quite a few objects not on the H400 list. There are probably more than 2000 DSOs visible with a 6" from dark skies. Keep in mind that one has to jump up to a 10" telescope to get just one magnitude more light gathering power, and the next jump is up to 16"! For comparison, the difference between 60mm and 6" is 2 magnitudes. For what it's worth, my next telescope (on indefinite hold at the moment) will likely be a 10" f/5-6 Newtonian.
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File last modified: 17 December 2004