Vixen makes a very nice Alt-Az telescope mount called the Porta II. Its main weakness is the tripod that it is sold with, but fortunately the mount is sold by itself and can be paired with a better tripod. The mount comes with an adapter that works with its paired tripod with an 10 mm bolt, but not most other Vixen tripods with an north index finger. If you remove the Vixen mount adapter you will find 3 standard 1/4 x 20 tpi camera standard threaded sockets used to attach the adapter and also a 3/8 x 16 tpi standard tripod socket. These allow the mount to be used with almost any standard camera tripod, but the off center mounting is undesirable. Vixen sells an adapter with a 1/4 x 20 camera thread socket in the center, but this isn't useable with most heavy duty tripods which use 3/8 x 16!
I decided to make my own adapter to pair the Porta II with a Manfrotto 046B (also Bogen 3036 or 3236). These 15 lb tripods are heavy duty, rated for a 25 lb load. Adjustable cross braces at mid-leg make them very adjustable on uneven ground and very rigid. Like most heavy duty camera tripods, it uses a 3/8 x 16 tpi screw to mount the head. They are easy to find 2nd hand for less than $100. The bottom of the Porta II mount, with the Vixen tripod adapter removed, is shown below:
I used the following supplies for the adapter: 3 flat head 1/4 x 20 tpi, 1 inch long, stainless steel screws and a 4" x 4" x 1/2" aluminum plate. I also made one with 1/4" aluminum plate, which works well with the Manfrotto, but might require some adjustment with another tripod if the 3/8 x 16 mounting screw extends more than 1/4" above the tripod. My main tools were a 3/8 x 16 tpi tap, drill press, miter saw with blade for cutting aluminum, and a metal file.
First I marked the center of the aluminum square and drilled a 5/16" hole in the center for the tap. I used the drill press (un-powered) to start the tap square to the surface. Once the tap was started I needed a big wrench. If you haven't tapped a screw thread before the process is to use lots of oil, advance 1/4 turn, back off a turn and repeat with patience. Once done you can line up the Vixen adapter as a template with a 3/8" screws and mark the location for the 1/4 x 20 through holes.
I used a 9/32" drill for the trough holes for the 1/4 flat head screws and a 1/2" drill to countersink the holes so that the screw heads don't protrude about the surface.
Once all the holes are drilled, I made a simple jig with a flat board drilled with a 3/8" hole. A 1" 3/8 bolt through the adapter place allows it to rotate for 16 tangent line cuts to rough out the round shape.
Before proceeding I inventoried my fingers and eyes and made sure I was proceeding in a safe manner. Next I made my 16 tangent cuts. Afterwards I removed my eye protection and gloves, then checked that my body part inventory matched the numbers at the start. Here is the roughed out adapter:
I removed the adapter from the jig and bolt and placed it in a vice. A few minutes of work with a metal file left it nice and round. Using a 3/16 bolt, I placed it in the drill press chuck so that I could smooth the edge with emery paper. I left the mating surfaces rough for a good grip.
Here it is mounted on the Porta II with the 1/4 x 20 flat head screws.
Finally the Vixen Porta II on my Manfrotto tripod:
The Porta II mount can now easily be used with any tripod that uses a standard 3/16 x 16 tpi screw mount or with a thread adapter a 1/4 x 20 mount. This combination should work well for any telescope less than about 12 lbs or 4" in aperture, with an 11 lb Explore Scientific AR102 4" refractor in place the tap setting time is 2 seconds.
If you are not comfortable working in metal, a version in 3/4" hardwood should work nearly as well. You will need a 3/8x16 t.p.i. stainless steel T-nut for the tripod mount screw. A 5/16 or 3/4" long one should do the job.
Content created: 2020-05-22 and last modified: 2021-05-20
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