Periodic error in DEC guiding
Thomas Fowler
Bruce Waddington
It’s always a little hard to diagnose these things just from guiding data, but I think your problem is that you’ve got some serious stiction (static resistance) in the Dec drive system (Dec in green):
As you can see, there is a considerable delay in getting the Dec axis to reverse direction and actually start moving. This is caused by some combination of simple backlash and stiction. Let’s start by looking at the start of a guide-pulse sequence that requires a direction reversal (red arrow). Starting at the arrow, there are 14 south guide commands issued, each averaging over 300 milliseconds – yet the axis doesn’t start moving. So what is happening with the guide pulses? There are two likely answers. First, there is some amount of backlash in the gear system, in which case a guide command will cause the worm to move but not the worm wheel because there is a small air gap between the faces of the two gears. The second possibility is that the gears are fully meshed but there is enough binding, load imbalance, or other static resistance on the axis that it won’t start turning. In this case, the worm is turning, it is engaged with the worm wheel, but the worm wheel isn’t moving and the energy delivered by the motor is going elsewhere – e.g. deforming various parts of the drive assembly or mounting blocks, causing a bearing to rock, whatever. So the energy is being temporarily stored in places we don’t want it. This continues until the static resistance is finally overcome, the axis breaks free and starts moving, and all that stored energy is then released. That creates the over-shoot. Because the size of the over-shoot seems to be nearly as large as the original guide star deflection, I would say you have a lot of stiction in the Dec drive system.
This kind of problem can be easily caused by over-tightening the mesh on the Dec drive system, but it could probably be caused by other problems in that area. It’s probably not a good idea to talk about this as periodic error because, as you already know, that term doesn’t make any sense for a drive system that isn’t running continuously. I think you should get some help from Losmandy, show them this graph, and talk about direction reversal problems and apparent stiction. I’ll bet they can help you out.
Good luck,
Bruce
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Brian Valente
I have a Losmandy G11, and when guiding I get large periodic errors in DEC. RA seems OK. Since the DEC motor isn't on all the time, it can't be the kind of periodic error that occurs with RA. I've also used MGEN-3 with the mount, which also shows a periodic error in DEC. I use ST-4 guiding, but I've swapped ST-4 cables and no change. I use a 60mm Orion guidescope, with an ASI 120MC for PHD2 (the MGEN unit has its own camera). Any suggestions are appreciated. A Fourier analysis shows sharp peaks in the 70s and 104s range. Thanks in advance! I've attached a guide log.
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