Re: ASCOM questions - MoveAxis() on Mach2


Mike Hanson
 

Hi Shane,

The API you are using is modulating the tracking rate, not really "slewing".  When a custom track rate is commanded it takes effect immediately.  Yet, the mount cannot support infinite acceleration.  So, the speed will "overshoot" to "catch up" to the position represented by "rate times time".  This is helpful when synchronizing to a satellite to avoid the need to account for mount acceleration.  One challenge in synchronizing to a satellite is knowing the exact instant to apply the new rate.  The acceleration is different for different vintage of products (CP1, CP2, CP3, CP4) and can vary across software versions - "it takes too long to speed up and slow down, let's improve the acceleration on the next software version".  Seemingly innocuous design decisions like these can have unintended consequences to those trying to chase satellites.  The best way to solve this problem is to not require the programmer to account for, or overcome, mount acceleration.  However, this means that there will be speed overshoots every time the track rate is changed by a large amount, so the mount can catch up to the new "rate times time".  There are other algorithm-intensive ways programmers can and do solve this problem, but we try to make it as easy as possible.

It sounds like you are not using the API to track non-sidereal objects.  In that case, you might be better off using an API that simulates the N-S-E-W buttons.  Or, provide coordinate slews.  In these cases, the slew status will more accurately affect true slew state. If you experiment with the N-S-E-W buttons in any of the programs that provide them (NINA, APCC, MaximDL, SkyX, etc), you will find behavior more like you expect. I cannot say what ASCOM API calls are being used to do this since I've not used them directly myself.  I need to defer that to others.  However, your work with Python may inspire me to give it a try myself.

Lastly, any system, no matter how rigid it may feel, will have mechanical settling time, and will "wobble" a little at the arc-second level following a slew.  The Mach 2 encoders can help with the settling of the respective axis position, but not with the settling of the optical train, or wobbling orthogonal to an axis of rotation.  So, you'll probably want to wait a couple of seconds after any slew has completed to begin an image.  Even if the system has "logically" and "electrically" settled, it may not be mechanically or optically settled.

Regards,
Mike Hanson

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