Determining Declination Backlash Using a CCD Camera

By using a 4mm eyepiece, speed 3 (8x sidereal) in ALT-AZ mode I found that my DEC axis backlash can be compensated by entering a value of 30 . This is the value I have been using until now. But, most of the time

• The telescope is used in a completely different configuration while imaging (heavy camera, counter weights, etc);
  
• I am using polar mode, not ALT-AZ;
  
• I am using a guiding speed of 66% sidereal (sidereal is 15 arcsec/sec, so I am guiding at 10 arcsec/sec), not 8x sidereal while imaging;
  
• For any speed other than guiding speed, the UP/DOWN buttons behave different. For guiding speed UP/DOWN moves the Declination axis only, but for any other speed, this moves the telescope Altitude. In ALT-AZ mode, Dec movements cause both axis to move, while in Polar mode, an Altitude movement causes both axis to move.
  

So, for a proper backlash compensation, the amount of backlash should really be determined in the same configuration as that will be used for imaging.

A more accurate approach to determine the DEC backlash compensation in polar mode is:

1. Set up the telescope with the camera attached as usual (but not with focal reducer for increased accuracy).
   
2. Turn off tracking (SETTINGS - TELESCOPE - ...).
   
3. Point the telescope to east.
   
4. Set the guiding speed to 66% (or whatever what you usually use).
   
5. Select guiding speed (MODE - 1)
   
6. Take an image of 60 second, while doing that, do the following:
   
   
   
   1. Press UP key for 10 seconds.
      
   2. Wait 10 seconds
      
   3. Press DOWN key for 10 seconds
      
   4. Wait 10 seconds
      
   5. Press UP key for 10 seconds
      
   
   
7. TODO
   

This is how I came to use the value 100 instead.

Of course, backlash is only a problem if the the guiding direction is changed, so, with a proper polar alignment and an AO7 all this is not really a problem. Also, as long as the direction is not change, the issue of retrograde motion also doesn't appear (this is the problem of a drive, usually the Dec drive, to move a bit in the opposite direction selected by the keypad, before resuming in the correct direction).

On the night of 2004-02-07 I set out to improve my DEC backlash correction using the Single Axis Mount Dynamics command of CCDOps. The telescope was mounted in polar mode.

TODO

No backlash compensation Backlash 8s, Y [-7.87,34.56]	Backlash compensation 25 Backlash 6s, Y [-7.33,48.56]
Backlash compensation 50 Backlash 4.5s, Y [-10.57,45.69]	Backlash compensation 75 Backlash 2.5s, Y [-33.86,33.80]
Backlash compensation 100 Backlash 1.5s, Y [-25.32,38.82]	Backlash compensation 150