Encoder resolution
There is some uncertainty as to the RA encoder resolution. Some DDM60 owners have suggested that the RA encoder resolution as calculated by Autoslew is incorrect. For example, my RA resolution is 1.0012981.
It is possible that the correct value is the reciprocal which is 0.998703.
The following charts show the star distribution errors as calculated, normal on the left, and with the inverse value on the right.
17th April

5th May
3rd June
16th June
17th June
1st September

In most cases the right hand image seems to give a more sensible distribution. Stars in the North-West have an error to the right. Those in the South-West have errors to the left. As they approach the 1st star in the sequence (most likely the star that was 'synched') the errors decrease as expected.
The left hand images show a more chaotic distribution which is difficult to explain. Sometimes stars which are close together have an error in opposite directions which seems unlikely to occur.

Is there a bug in Autoslew?
Update: Philipp Keller (who wrote the Autoslew software) has pointed out that the star distribution charts are meaningless if you do not also take into consideration the scaling of the vectors. The length of the largest vector is always set to a standard length. For example, the 1st September chart using RA value 1.0012981 has a scale of 0.07 arc min.
Using 0.998703, which gave 'sensible' vectors, the scale was 3.52 arc min - some 50x larger.
  However, some DDM60 users have obtained perfect 15minute unguided tracking after adjusting the RA value.

With guiding it appears that my mount moves slightly too fast in RA, and the average RA guiding correction is negative.

(The DDM85 mounts have fixed encoders with known resolution so there is no confusion).


Continued experimentation shows that inverting the resolutions is not the thing to do. Furthermore, there is now a method provided by Autoslew for accurately determining the encoder resolutions once and for all by slewing between 3 stars - start at star 1, slew to star 2 with approximately the same DEC, back to star 1, then slew in RA to star 3 and back to star 1. Calculate only the encoder resolutions and you are done.