I appreciate that this is an issue that has been extensively discussed over a number of years, but having combed the forums and help files I have been unable to locate a definitive answer.
I am attempting to use ‘Slew and Centre’ in SGP with Plate Solve 2 on an older Losmandy G11 mount with Gemini I.
The problem is that after the first Plate Solve the Dec px error reduces to within an acceptable pixel tolerance but whilst the RA px error also reduces on the first plate solve, all further plate solves make no difference to the RA px error. The result is that centring ultimately fails and the sequence aborts.
I have read that this might be the result of SGP fighting the pointing model in the Gemini I. In any event, is there a solution / set of settings that will resolve this issue?
You might want to check the adjustment of your worm gear, particularly if you have the older two-piece worm blocks. I’ve had similar problems with my older G11 with G1 where it would respond to a slew command but would take multiple centering commands to see any movement in either PHD2 or SGP, and would fail centering in SGP for this reason. Every time that happened, readjusting the worm blocks solved the problem. I have not had that problem since upgrading to spring-loaded single piece worm blocks.
Thanks Larry, the worm gears are properly adjusted as far as I can tell. The problem does not appear with PHD2, only with SGP and only when attempting to ‘Slew and Centre’ using Plate Solver 2.
Check your G11 telescope configuration (ASCOM settings or Gemini ASCOM interface). Make sure “Receives coordinates in J2000” is unchecked. I had this exact problem with my G11 when this setting became inadvertently checked.
I believe that SGP converts from J2000 to JNOW when sending coordinates to the G11. At least that’s what my version (3.2.0.609) does - I can see it in my SGP logs.
Many thanks for this Jim. The box was indeed checked and I have unchecked it.
It makes perfect sense now I think about it. If the box is left checked the G11 will think it has been sent J2000 co-ordinates by SGP and will centre at what it thinks is the correct J2000 position. When the second plate-solve happens it will see that the G11 is still off by reference to JNOW and send the same JNOW solution co-ordinates again. The G11 however, will again read them as J2000 and think it has already moved to those co-ordinates and will not move again. In short, unless the box is unchecked, SGP keeps saying “move” and the G11 keeps replying “why, I am already there.”
I wonder if it is possible that the same problem can occur if the G11 has a detailed pointing model active. SGP will send the G11 co-ordinates based on the first plate-solve and the G11 will move having regard to its active pointing model. If the model cannot match the accuracy of a plate-solve (as it generally will not) then when the second plate-solve happens it will see that the G11 is still off (by reason of defects in it’s pointing model) and send the same solution co-ordinates again. The G11 however, will, by reference to its pointing model, think it has already moved to those co-ordinates and will not move again. Once more, in short the presence of the detailed Pointing model means that SGP keeps saying “move” and the G11 keeps replying “why, I am already there.”
As soon as these interminable clouds clear, I will give it a try without the box checked.