Problems with the DEC driveon my CG5
I have an 8" Celestron Newtonian (C8-NGT) on a CG5 mount with a Nexstar Advanced Hand Control (with HC 3.04, MC 5.06 5.06). I have had it for about 6 years and used it in various configurations with cameras and tracking scopes. In all this time I have been able (even though at times the mount has been somewhat overloaded) to do a 'Quick Align' and be confident that if I enter a 'named star' or 'named object' the object would be in the finder. Until Now! At the beginning (and end in the UK Lake district) of the clear frosty night I dialled in 'Andromeda Nebula' pressed go, and watched in frustration as the 'scope aligns to gaze at the wall of the house. With time being of the essence I manually located M31 and got 30 of the best 1min images I have ever taken, so Polar alignment spot on!
I assumed that the mount for some reason had lost its way and so (after a number of fruitless attempts at auto and three star alignment) I did a factory reset and then again did auto and three star alignments. Always the same result - 'scope drives and ends up nowhere near the required object.
But not quite nowhere - the RA is correct but the DEC has a problem. I attached the mount to Stellarium and could drive the 'scope icons around using the 'NexStar' hand control buttons, the 'NexStar' hand control 'goto' and the Stellarium 'goto' function. The RA and DEC values from the two systems were identical.
Back in the real world the scope will drive to the Home position 21° 34m 5s / 90°perfectly. But told to drive to Castor (true = DEC 31° 53m) the display on Stellarium shows the 'scope driving to the correct position but in reality the scope will drive to a position some 17° beyond Castor (towards the horizon). If you manually align the 'scope with Castor the DEC is 48° 49m (taken from Stellarium).
The situation is the same for Capella (DEC is 45° 59) given the command 'goto Capella', Capella on Stellarium stops at the right place. The 'scope drives down nearer to the horizon and if you manually align it on Capella the
DEC is 63° 21m.
Interestingly the error in both of these is around 17°
The mount drives quite smoothly, especially so since I have stripped the mount down and removed Chinese solid grease (of which there was very little). I have looked at the power supply, I use a computer power supply which produces (and always has) 11.3V, nevertheless I have stuck a DC booster on the output and I am now using 12.9V. Needless to say it makes not a jot of difference!
Since the 'scope hand set buttons and commands exactly duplicate the movements of Stellarium I conclude that the data being sent to the DEC drive board is OK.
Since the scope will drive to the home position (in reality and in the Stellarium simulation) is held in a different location to that which the target co-ordinates are held.
There is a gremlin on the DEC board that modifies the 'scope stop position. Just for a laugh I now intend to plot the difference between the intended and actual DEC values against the intended value.
Sorry to blat on at such length - hope somebody can help.
Any ideas gladly welcomed!
Probably the DEC board?
Bought a red dot finder!