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. |
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17th
April |
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5th
May |
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3rd
June |
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16th
June |
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17th
June |
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1st
September |
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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? |
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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. |
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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).
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Update: 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.
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