Digital Setting Circles


The AstroSystems pivot bolt kit comes with a predrilled hole in the pivot bolt. This makes mounting the Azimuth encoder easy. The only two things I had to worry about both had to do with clearance.

  1. Make sure that the mirror cell clears the encoder when it swings up and down. This wasn't hard to deal with, I just added a little extra to the clearance calculations (which are covered in the design spreadsheet).
  2. Make sure that the mirror cell doesn't crush the encoder when it is placed in the rocker box for storage and transport. I did this by placing spacer blocks along the edges of the rocker box to keep the mirror cell elevated a bit.


For the altitude encoders, there were three issues to worry about. At the top, there is the question of how to attach the encoder to the side bearing. In the middle is the need to make sure the encoder's arm clears the knobs holding the side bearings to the mirror cell. At the bottom there is the problem of how to attach the arm loosely to the rocker box.

Digital setting circle
protector.To attach the encoder, I cut a "C" shaped piece of wood and glued it to the side bearing. The inside of the "C" block is the same size of as the encoder. My original plan was to hold the encoder in place with friction. This system has two advantages. First it allows for easy installation and removal of the encoders. Second, the "C" block offers some protection for the encoder. Alas, I am having some problems with precision when using the digital setting circles.

setting circle in the protector.A helpful soul at Astrofest pointed out one problem. He pointed out that my encoder could rotate a bit within the "C" block. Even a tiny bit of rotation results in big errors in where the scope is pointed. I tested to see if this was indeed a problem by using some masking tape to hold the digital setting circle in place and found that they became more accurate.

I still need to find a permanent fix to this problem. My current plan is to use a bit of Velcro to prevent this rotation. I am concerned that this will still allow too much rotation. So I have also considered using a stop of some form to keep things in place. An alternative idea is to just glue or double stick the encoder permanently in place. The hope being that the "C" block will offer sufficient protection to prevent damage to the encoder during transport.

Even with this fix though the digital setting circles are not as precise as they should be. I have a couple ideas of what might still be the problem. The first is that perhaps I should use a cross hair eyepiece when doing the initial alignment. The second idea is that perhaps the bearings are slipping as much as 1/8" between the sides of the rocker box.

As mentioned above, I also kept an eye on the clearance of the knobs for the bolts holding the side bearings to the mirror cell. Through a miracle of fate though, it turned out that the 3/4" tall knobs just barely clear arm. No need to have even worried.

I use a pin to hold the encoder arm in place on the bottom end. The pin pops into a hole I drilled into the rocker box. The pin works with the slot in the arm to prevent the arm from rotating. The pin comes out when the scope is being transported.

Nylon spacer for digital setting circle altitude arm.In the field I noticed that the arm wobbles a bit and occasionally does hit the side bearing knobs. So I put a 3/4" long nylon spacer on the bottom end pin to prevent the wobble from causing problems.

Things to be done with the digital setting circles:

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Last updated 1/12/06