Setting the Home position of the mount

Setting the Home position of the mount

The home position of the mount is the position where the telescope is pointing at the NCP (north celestial pole) or SCP(south celestial pole). This is accomplished by setting the mount with the balance (counter-weight) bar pointed downwards and the scope aimed north or south (as appropriate).

This is quite difficult to do with any real accuracy and it is not essential except that it helps with GOTO's for the alignment stars. Current versions of EQMOD store alignment data reducing the dependency on accurate home position for alignment GOTO's since the mount will be essentially already aligned at power up (although it may still need small adjustments).

Once the mount has been aligned the accuracy of the home position becomes irrelevant.

Technique 1 for setting the home position (from Mon)

To accurately set your mount to the Home position, you need a leveler (digital or bubble both work adequately).

  1. Position the counterweight shaft horizontally and level it using the bubble level on the counterweight shaft.
  2. Set the RA setting circle to read 6 hours.
  3. Lock the RA setting circle.
  4. While the counterweight shaft is in the horizontal position, level the DEC side (dovetail and scope) horizontally using the leveler on the dovetail (or on the telescope).
  5. Move your DEC setting circle to read the exact degrees as indicated on your mount's altitude indicator (your site's altitude). Choose the DEC degree setting such when you move the scope to 0 Degrees DEC circle (cw shaft still in horizontal position), end of tube should move up).
  6. Lock the DEC setting circle.
  7. Set the mount to the RA Home Position by moving the RA shaft until it points to 0 Hours. (counterweights should now be pointing down).
  8. Set the mount to the DEC Home Position by moving the DEC shaft until the DEC setting circle reads 0 degrees.
  9. Scope should be pointing at NCP/SCP (North Celestial Pole/South Celestial Pole) at this point.

Once you have found this position, you may be able to find places on the mount that are level so that you can reset the mount, if necessary, without repeating the above procedure.

Technique 2 for refining the home position

This particular method may or may not work for you. You may just want to experiment with it. The major difficulty may be the subtle movements of the mount that are required while it is unclamped. It is certainly difficult compared to slewing under fine motor control but is no more difficult for most installations than the initial setting of the home position.


  • You're using a fixed pier installation… although this technique might be useful for portable installations if you have to re-align during the observing session.
  • You have a good polar alignment
  • When the scope is unclamped and moved it does not move the ring counters.
  • Alignment does not change the setting of the home position with respect to the ring counter


  1. set the mount in the home position by eye to the best of your abilities. (no need to use technique 1).
  2. Start alignment and slew to first alignment star (chances are it won't be in the FOV).
  3. Unclamp the telescope and use your finder to align on the star. Moving the scope manually in small increments is difficult but get as close as you can. You might actually be able to get the alignment star onto the CCD or within the FOV of the main scope.
  4. Clamp the scope.
  5. Continue with 1-point or N-point alignment procedures.
  6. Typically future power up first alignments will be much closer to the FOV than rough home positioning of the mount.

To prove out the adjustment:

  1. Park the scope.
  2. Unpark the scope.
  3. Start alignment (you should find that the first alignment star comes very close to your FOV).
/home/welshdra/public_html/eqmod/data/pages/setting_home_position.txt · Last modified: 2009/08/29 20:36 (external edit)
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