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Polar alignment


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To pass away the time during the (mostly) cloudy nights over the Christmas period, I decided to update my SynScan star visibility spread sheet.  The polar scope ‘time’ value given by the spread sheet always lagged behind the value given by the Skywatcher SynScan controller which I presumed to be due to the original version of the spread sheet used fixed J2000.0 coordinates for Polaris whilst the SynScan controller applied precession.


There are several methods out there to calculate the precession of the stars over time, but those that use approximations become increasingly inaccurate as you approach the poles.  I chose the 3 x 3 matrix method using coefficients that I found in a NASA technical note from 1970.  Having applied the precession corrections to the Polaris coordinates, I found that the polar scope ‘time’ value given by the spread sheet was now ahead of the value given by the SynScan controller.  So I decided to do a bit of digging by comparing the J2020.0 Polaris coordinates of the spread sheet and the SynScan controller with those generated by TheSkyX and C2A (the only two planetarium software packages I have installed).


The spread sheet, the SynScan controller, TheSkyX and CA2 all use virtually identical J2000.0 coordinates for Polaris.  These are the generated coordinates of Polaris for J2020.0:


TheSkyX                                    RA 2h 58m 11s   DEC 89° 21’ 10”

CA2                                          RA 2h 57m 05s   DEC 89° 20’ 53”

SynScan Stars V1.2.0                  RA 2h 57m 02s   DEC 89° 20’ 53”

SynScan firmware V4.39.04         RA 2h 50m 24s   DEC 89° 21’ 00”


Whereas all four agree closely on the declination, the SynScan controller differs in RA approximately between 6.5 and 7.5 minutes.  As a result, the polar scope ‘time’ value given by the SynScan controller will currently be between 3 and 4 minutes slow.  Not that it would make a massive difference to the polar alignment since the divisions on the polar scope graticule are only every 10 minutes, but every little helps.

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