Finessing the "VolksDob"
At one time, building your own telescope was the cost-effective way to get a larger scope, but nowadays there are good quality mass-produced Dobsonians available at prices so reasonable that making your own may not be worth the effort. The Orion XT series, Meade LightBridge, Celestron StarHopper, Anttlers DSO and the like make it possible for just about anyone to afford a decent sized scope. They are the telescopes for everyman... what might be called the "VolksDob". With the exception of the LightBridge, these are almost identical, originating from Synta in China and/or Guan Sheng in Taiwan. In some cases, a few parts are made in the U.S. A premium Dob will cost two to three times as much, so you can still save money if you're able to build something like a Starmaster or Obsession with your own hands. And until the rumored 16-inchers hit the market, if you want anything larger than 12-inches, then you'll need to either build your own or go the made-in-America route.Despite the declining percentage of telescope builders, the tinkerer is alive and well. Tune into the on-line telescope forums or any bunch of Dob users and you'll find that everyone seems to be customizing their Chinese-made Dobs. That's because the VolksDobs have some warts. The optics are quite good, but they're not premium scopes, so a few components leave something to be desired. Last year I got an Orion XT12i and since then have been attuned to what other owners are doing to add finesse to these telescopes.
Focuser - It seems that all Chinese-made rack-and-pinion focusers have a common, but easily corrected, malady - the mechanisms are packed with a grease that is so thick that it's more like a glue that hasn't quite dried. When I received my XT, I noticed a "crackling" sound when the focuser was racked in and out. It was the so-called grease. The focuser is easily disassembled and the original grease can be removed with an old toothbrush and mineral spirits. I replaced it with dielectric tune-up grease available at auto parts stores. After adjusting the tension, I have a smooth (and quiet) focuser.
A lot of owners replace the stock focuser altogether with a Crayford focuser. Some have base plates designed so that no re-drilling of the tube is necessary. A Crayford makes focusing smoother with less "slop", especially at high magnification. Many of the Chinese Dobs now come standard with Crayfords. I use a different scope for the moon and planets; the XT12i is reserved for deep sky objects on which low to medium magnification is the rule. So for now the stock R&P focuser has proved acceptable for my purposes.
Spider - The secondary holder (spider) is the classical four-vane type with three adjustment screws to tilt the secondary mirror. The three screws bear against a plastic hub that holds the secondary mirror. If readjustments are necessary, the repeated turning of the screws digs into the plastic hub, gouging out "divots". As a result, the screws always slip into these little pits, making it almost impossible to accurately realign the secondary mirror. To solve this, I added a large washer between the hub and the screws. The screws now bear against the washer, which makes readjustment easier and more precise.
Mirror Cell - The stock flotation mirror cell is pretty good, and if the clips are set so they barely touch the primary, there should be no distortion of the mirror due to pinching. But even with the locking screws, periodic re-collimation is necessary. Next time I clean the mirror, I plan to do two things to better retain collimation: replace the collimation knob springs with stiffer (heavier-duty) ones and put a thin coat of silicon sealer (RTV) adhesive on the inside of the mirror clips. I did both of these on my old homemade 10-inch Newtonian, and hardly ever had to re-collimate. The rubbery silicon adhesive helps keep the mirror from shifting. (Allow the silicon to cure for a few days before re-installation so there's no out-gassing that might damage the mirror coating.)
Balance Issues - If you have a large eyepiece or change to a heavier finder, you may find that the front end of the tube wants to point at the ground rather than the sky. Others have found just the opposite - on some scopes, the rear end is too heavy to achieve balance. A common solution is to use a heavy magnet strategically placed along the tube. Others have used arm/ankle weights with Velcro straps available from sporting goods stores. In my case, the tube was just a bit rear-end heavy, so I replaced the little handle at the front of the tube with a small counterweight. The XT Intelliscope series has a tension adjustment knob and other Chinese scopes come with tensioning springs attached to the trunnions. These work well enough, but still can't fully compensate for a tube that's severely out of balance.
Azimuth Axis Motion - The scope should move smoothly about the azimuth axis, but not too easily. On some scopes, there's a ball-bearing assembly on which the rocker box rotates. This sounds good in theory, but users have found that the scopes move so freely that you lose your target with the slightest touch of the scope. One supplier solves this by including thick Velcro pads that add a slight amount of friction. Many users have simply discarded the ball bearing assembly and replaced it with the usual Teflon pads and Ebony Star sheet stock. Ebony Star is a Formica brand counter top material with a slightly bumpy texture that keeps friction fairly low.
For scopes that use pads, there is often the problem of "stiction" - the tendency of the telescope to need too much force to start moving in azimuth, resulting in jerky motion. If a scope has plain white plastic pads, they can be replaced with furniture glides called Magic Sliders that use real Teflon. The optimal load for Teflon pads is 11 to 15 pounds per square inch. The Intelliscope series uses real Teflon, but the pads bear against the bottom of the rocker box, which is nothing more than the plastic coated particleboard. So there was some stiction. I had tried applying Silicon spray to the bottom of the rocker box, but this made the stiction problem worse. I also tried graphite lubricant, with no noticeable effect. One Internet correspondent reported good results with a very light coating of carnauba car wax. A common solution has been to place a layer of Ebony Star on the bottom of the rocker box. An alternative is the use of milk jug washers. These are large washers cut out of plastic milk jugs. Several of them are stacked around the rocker box central screw to take a slight bit of pressure off the pads, thereby reducing the starting torque needed to move the scope about the azimuth axis. The milk jug washers solved the stiction problem for me. The Intelliscope version of the XT has a computerized object locator option that uses an encoder disk around the central screw of the rocker box. As far as I know, you can't use the milk jug washers with the encoder disk. But since I don't use an object locator, I simply removed the encoder disk.
Portability - The Meade LightBridge is a truss tube design that disassembles and is easy to carry even if left assembled. All the others have solid tubes made of steel. While these aren't terribly heavy for their size, they are awkward to carry, especially in the 10 and 12-inch sizes. The upside is that you don't need a black shroud to keep out stray light. A carry sling for these scopes is available commercially and some users have made slings of their own. I installed a metal strap handle atop the tube between the trunnions, which is close to the balance point. This makes it much easier to carry, plus it provides a convenient handhold when maneuvering the tube up near "Dobson's hole" (toward the zenith).
Anti-Reflection - Although the insides of the tubes are painted a very flat black, some incident light still can be scattered, thereby reducing contrast when viewing dim objects. In order to prevent scatter, the light rays hitting the inside of the tube must be interrupted. The usual solution is to cover the inside of the tube, or at least the part opposite the focuser, with a rough material such as flocking paper, available at many craft stores. I used a self-adhesive black felt opposite the focuser. Under the night sky I can just barely see the inside of the tube, but the part with the felt is like looking into a black hole ... totally dark! Baffling can also be used, one method being to place strips of self-adhesive sponge rubber weather-stripping material around the inside circumference of the tube at strategic points.
Finder - The stock 8x50 finder works okay, though the edge of the field shows distortion. The biggest complaint is that there isn't a real lens hood, so the objective is pretty well exposed and quickly dews up. I replaced it with an Antares 8x50 straight-through finder, which is optically better and fits well in the XT finder bracket. Quite a few owners add a 1x finder such as a Telrad. The Meade LightBridge comes standard with a 1x finder instead of an optical finder. Since the LightBridge tends to be front-end heavy, if you want an optical finder, the rear end will have to be well weighted to compensate.
Mount - Unlike premium scopes, the VolksDob mounts are made of plastic-coated particleboard. There is some lateral flexure, particularly on the Intelliscope series with its higher side walls (most prevalent in the 12-inch version). The issue can be dealt with by adding braces on the outside of the rocker box to stiffen it. I made braces of one-inch thick oak, and that virtually eliminated the flexure. Another advantage of wood braces is that they add structural integrity; screws simply do not hold as well in particleboard as they do in solid wood. Rather than mess around with the existing rocker box and ground board, some owners build or buy a whole new mount made of marine plywood. Others keep the existing ground board but make a new rocker box.
Conclusion - Some people who might have built their own telescopes are instead now turning to the mass-produced Chinese scopes that are steadily improving in quality and features. They're certainly usable right out of the box; however, they can be improved upon. A fully tricked-out VolksDob isn't quite the same as a premium Dobsonian, but it comes darn close!Published in the March 2007 issue of the NightTimes