Amateur Astronomy Notes

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sep 19, 2010; updated dec 13, 2010

Review of Vixen 5863 Porta II Mount


I  didn't have very high expectations when I bought this Vixen Porta II alt-az mount. I needed a portable mount for my 8-inch SCT for those occasions when it's too inconvenient to set up and align a 70-pound equatorial mount. But I expected that, at best, this mount would have to be dismantled and used for parts. I was surprised to find that the Vixen is fairly well made.

Vixen mount
Vixen Porta II mount modified to handle 8-inch SCT. The plastic knobs have been replaced with aluminum handwheels and a larger dovetail clamp has been installed.

For example, unlike cheaper mounts, the Vixen uses worm gears. Inside the aluminum casting, there is an aluminum worm gear driven by a brass worm. There are manual alt-az mounts on the market that are cheaper and support more weight, but few of them have worm gears--most are simple push-to-rotate mounts, maybe at most with a panning handle. Push mounts require locking to prevent unwanted rotation, and are next to useless when using high power. Like the cheaper mounts, the Vixen also has a clutch, but its purpose is different. In the Vixen, the clutch is adjusted by hex screws. With the clutch loosened, the Vixen can be rotated by pushing or by rotating the worm gear knobs. When it's tight, only the worm gears will work. The worm wheel has 120 teeth, which means there is a 1 to 120 gearing reduction. The 1:120 gearing ratio allows for accurate Myths about alt-azimuth vs. equatorial mounts
Although most people associate equatorial mounts with computer control, they're also essential for manual control if you're observing objects near the zenith. When you point an alt-az mount straight up, the azimuth adjustment becomes useless--the only effect is to rotate the view around in the eyepiece. It's also a myth that an equatorial mount requires counterweights. With proper consideration to balance, it's possible to build an equatorial mount that requires no counterweights at all. (I know, because I've built them.)
pointing. Unlike certain cheaper models, the Vixen allows pointing at any angle, including the zenith. The Vixen's worm gears allow the telescope to be rotated 360 degrees in both axes, as long as it stays in balance. I had no problems rotating a 20-pound scope from zero to 90 degrees. It actually went to 120 degrees before the tripod started to tip over.

A certain amount of instability is unavoidable with such a lightweight mount. The whole thing, including the tripod, weighs only about 12 pounds, making it easy to get through the door with a minimum of swear words and broken vases.

There were a few minor problems. On my unit, the azimuth gear had a significant amount of backlash, because the worm gear was too far from the main gear. Luckily the gears are mounted on eccentric cams, so if you have a special tool, you can adjust this by tightening the cams. Each axis also has a slip clutch, which is adjustable to handle heavier loads, provided you balance them. Two hex wrenches are included in a special magnetized compartment for adjusting the clutch tension and disassembling it.

Now the bad news: the Vixen comes with a mini-dovetail clamp that only has a maximum opening of 1.75 inches. Although they call it a "World Standard", in fact there are different incompatible sizes of dovetail clamps. In order to use something like a C8, you have to build your own larger clamp. The old one can be easily removed using the hex wrenches, and there are eight ¼-20 threaded screw holes that make installing a larger one (or whatever else you have) and changing the orientation fairly easy.

Another bad feature is the cheap plastic knobs provided with the unit. The knobs are mounted on flexible plastic stalks. It's difficult to adjust a scope, especially a heavy one, with these little knobs, which are awkwardly placed, hard to turn, and tend to fall off. To get a decent amount of control, I replaced the knobs with 6-inch aluminum handwheels and replaced the plastic stalks with 1/2-inch diameter steel rods. Unfortunately, because of the way it's made, it was necessary to make more modifications to the mount to get the two handwheels to come out in the correct orientation. This meant machining another part and tapping four ¼-20 holes in the appropriate locations. Not hard to do, but all together these modifications (including the handwheels) cost over $50 just in materials. This should not be surprising: these days, if something is made in China, as this one is, you expect to have to re-engineer it before it works properly. When suitably modified, this mount is quite usable with a big scope in spite of the fact that vibrations take several seconds to die out at high power.

The only caveat is, if you tighten up the slip clutch, the aluminum worm gears can be damaged if you manually push the telescope or the "handles" (which are actually part of the worm) too hard. Picking it up by the handles must also be avoided. These handles are made of brass electroplated with what looks like chromium, and they bend very easily. Once bent, the metal is fatigued and will eventually break. But remember this is (or was) a 12-pound tripod with a 20-pound scope mounted on it. A smaller telescope (up to 5 or 6 pounds) has the same caveat, but should have no problem with vibrations.

This brings us to the reason for the three stars: the descriptions on the Vixen website and other online vendors all say the mount is made in Japan and that it can handle 20 pounds. Both of these are wrong: the box and manual clearly say "Made in China." The manual also says the mount is designed for 5 kg (11 pounds) maximum. Vixen may not actually be trying to deceive the customer, but they are clearly mighty confused. Anyway, the construction is good enough that you could upgrade it to handle the 20 pounds that they advertise without stripping the gears. With a 20 pound load, you will see lots of vibrations whenever you touch the scope. But if you have to drag your mount up the side of a mountain in order to use it, your back will thank you for it.

Update (Dec. 13, 2010): After using this mount for three months, I finally dismantled it and used it for parts.