Horizon Avoidance During Slew
The IDS treatment of below-the-horizon slews varies by firmware version. Depending on which version you have, it may:
- Stop a slew in progress when it bisects the horizon
- Continue the slew and allow the telescope to point downwards.
Neither solution is optimal. If the telescope does not have a secured mirror then this could fall forward when pointing below the horizon. The alternative interrupts a slew in progress and can be tricky to recover from because the drive system is reluctant to allow a new slew to start when already below the horizon.
A better solution would be to never allow pointing below the horizon by slewing to an intermediate point and then continuing towards the final destination.
We know the slew rate (in degrees per second) for each axis. Therefore, we should be able to predict when any slew is likely to approach the horizon. When this condition is detected, it should be possible to choose an intermediate location away from the horizon that will allow the slew to complete in two segments.