Success on the digital azimuth for under $20 total. Below is information how to do it on an Orion XT8. The general idea should be adaptable to many DOBs. I did it in a way to make it as accessible to as many DIYers as possible, and getting repeatable measurements within .06 degrees!
I started with this plastic rule digital protractor. I chose a plastic ruled one as opposed to stainless because the plastic is easily cut with tin snips or heavy scissors. I cut the upper rule (the one the measuring module attaches to) on the 60mm mark. The lower rule cut much shorter on the 230mm mark. Neither measurement is critical and can be off a few mm.
A bit about the protractor. I disassembled mine in order to see if the module can be used by itself by simply unscrewing the rules from it. It turns out that this is not possible because the lower rule has a copper encoding circle printed on it, and the upper rule serves as the bottom cover of the encoder module. But all of this plays into making this project super simple since the solution is simply cut the rules to the desired length and be done with it.
Next I removed the center locking knob, which when unscrewed, exposes the threaded brass hub of the device. I used a 1/8" drill (which does not cut the threads) to drill through the lower rule so the device center axis is clearly identifiable looking at the back of the device. I covered the opening where the knob screwed in with a piece of tape since it provides better sealing and protection.
Then I used a bit of JB Weld epoxy to glue a metric 10x1.5 nut to the bottom of the device, centered over the hole drilled above. You want to get the hole in the nut as concentric as possible with the drilled hole, but if its slightly off, its no problem.
After the epoxy set for 24hrs I threaded a second nut onto the protruding axle bolt of the XT8 base (after adjusting the plasti-nut for the correct platter tension). This nut will serve as a lock nut for the protractor/epoxied nut assembly.
Next, I flipped the DOB base upside down and rotated the triangular lower base in order to thread the protractor/epoxied nut assembly into place. Then flipped it back over, and secured it in place with the lock nut.
Note that if you cut the upper rule a bit shorter so it can clear the side uprights of the DOB base while threading the protractor in place, you don't have to flip the base. I would cut it shorter to avoid this hassle if I ever do this project again.
The final part is to secure the upper rule from rotating. For this I used a couple of right angle braces from the deck building hardware section of Home Depot. Just screw them in place with a couple of #10 x 1/2" pan head screws. Make sure there is no side to side slop between the braces and the rule. I chose this method to allow for the slop in the axle of the stock DOB base to not stress the module. Plus if your epoxy nut is a bit off center, this method will accommodate that as well.
Note that its a good idea to mark the drill with some tape at a little less than a 1/2" so you can avoid drilling all the way through the base when drilling the holes for the #10 x 1/2" screws to secure the brackets.
Super easy project for very little $ that gives great results. That said, if you already have a degree wheel installed, this is probably not worth retrofitting, but is instead an alternative to the degree wheel for people looking to add an azimuth locator.
1) Easy to build and install.
1) Very accurate, and easy to situate in the field. Simply sight in the North Star and press the zero button.
1) After zeroing, must always rotate the scope CW to get the correct degree readings (the protractor starts at zero and increments numerically upward no matter which rotational direction, and will also continue to increment past 360 degrees if continuing past one revolution).
2) The auto-off feature cannot be disabled. This means the device will likely shut off at some point, and must be re-zeroed. The work around is to place a piece of tape across the upper and lower platter when sighting the North Star. Then slice it between the platters so they can move freely. This creates a reference of the North Star until the telescope is moved, and simply lining up the tape marks and pressing the zero button again is all that is needed if the auto-off comes into play.
In practice, the cons are worth it to me to have the luxury of simply sighting in the North Star and pressing the zero button and being good to go.
These photos are pretty bad, but you can get the idea. I'll try to take some better ones and repost....
This shows the locking nut and the scope tube clearance:
...and a pic of the reference tape to be placed while sighting the North Star. Just align these marks and re-zero the protractor if the auto-off gets you:
Edited by skitron, 26 November 2019 - 04:55 PM.