The title of this page suggests my 22-Inch Dob is finished, but that is misleading. Large scopes are always works-in-progress. Still, in June I did some work balancing the secondary cage — which was too light even with the finderscope and everything else believe it or not — and treated the bearing surfaces with Turtle Wax. After these changes, the scope balances at all elevations with or without an eyepiece and moves smoothly with finger pressure. That is enough to call if finished, I think. It certainly is in Elly’s opinion!
The picture above was taken at Ron Abbott’s dark sky site on April 16. That was my first trip to a dark sky site with the 22-inch Dob.
This is a picture of the scope without the shroud in place showing the truss assembly. I worried for quite a while about what to cover the tubes with. They are 1.25 inches in diameter, which is larger than readily available pipe insulation. One day at Home Depot, though, I noticed some pipe insulation rated for 1-inch galvanized (which is thicker than copper pipe) and decided to try that. It easily stretched over the 1.25-inch tubes. And the price was right: .78 cents each. Note that in this picture, which was taken in April, the keeper rails had not been added to the rocker sides. They are shown below.
Here is a picture of the fully loaded secondary cage. The finder mounts are drilled so that the Telrad and finder scope can be switched to either side of the focuser board and positioned higher or lower on the secondary cage. I ended up adding six pounds of counter-balance weight, even to this! (Must have miscalculated the center of rotation on my mount.) The cage doesn't look any different with the weights, though, because they fit inside the ends of the truss poles. I made them with metal tubing and #9 lead shot. The black box controls the remote collimation motors in the mirror box. These make the scope a breeze to collimate.
Here is a picture of the backside of the mirror box. The collimation motors can’t be seen, but the drive belts and the edge of one motor mount are visible. The cutouts at each corner are to increase air circulation. Also, the control box shown in this picture is not the final part. I rebuilt the box in a longer format to accommodate PWM cicuits used to adjust the collimation motor speed, cooling fan speed, and dew-control components on the secondary cage.
Here is a picture of the base from the front, showing the 22-inch mirror, truss-pole clamps, etc. This was taken before the keeper rails were added. The battery holder can be seen on the front of the rocker base. It is large enough to hold four 7.5-amp hour AGM batteries. I have been running the scope with two of these, which will probably be all that is ever needed. The additional space is used to store the coiled power cable that connects the mirror box to the rocker base and the battery charger.
And here is a picture of the finished base. One of the keeper rails can be seen, along with the completed control box mounted on the back of the mirror box. As it happens, the cooling fans did not work out. They introduced far too much vibration. I could dial them down to a low setting with the PWM controller, but then I could not see that they made much difference. What I have settled on for cooling is to position a regular house fan behind the scope in the evening, after the sun has set, blowing air through the mirror box and out through the front end of the truss assembly. Running the fan for 45-minutes to an hour makes the scope razor sharp as soon as it is dark. This is the same cooling approach Steve Kennedy uses for his scope. He has figured the mirror for optimal performance while it cools during the course of the night. I am very satisfied with this approach.
I might do some further experiments with cooling fans in the future, but it is not an issue that concerns me much. Frankly, I put a lot of energy and time into the cooling fans that I could have saved. I would advise other large dob builders not to put much effort into cooling fans. If you feel you really must do something, just mount a fan on the back of the mirror cell as is often recommended. I am skeptical they make much difference for a large Dob compared with what can be accomplished using a house fan as described above. Fans can make a significant difference on smaller, closed-tube scopes, but that is a rather different situation. Anyway, mounting a PC fan behind the mirror is is easy to do, is not costly, and can be turned off or removed without trouble.
Here is the mirror box, with the altitude bearings and truss clamps removed, in the storage compartment of our 22-foot RV. All of the scope components fit into the RV so no trailer is required. It takes about 30 minutes to set the scope up or put it away. This can all be done single-handed, except for setting the mirror box on the rocker base. That is a two-person job at present. I plan to build a wheeled dolly that will enable me to set or remove the mirror box on the rocker base single-handed.
And here is a beautiful sunset at the ASKC dark sky site with the 22-Inch Dob set up and ready for a night of observing.
