Re: Tracking and guiding with and without encoders - Part2


Roland Christen
 

The Mach2 mount needs the encoders to be on in order to produce smooth tracking. Without the encoders the error of the microstepper will dominate and you will have elongation of the stars in RA even though the rms guiding might be sub-arc seconds. The rms value does not take into account the peak to peak excursions of the guide star, which can be 5 times the rms value. It's a lot like sticking your head in the oven and your feet in the freezer. Your average rms body temp might indeed be very comfortable.

You all may not realize this but most of the inexpensive mounts being sold today with microsteppers and belt drives have these fast moving p-p excursions. That's because they don't have any kind of encoder feedback, either on the motor shafts or on the mount axes. It's a well known effect of microstepping where the individual step sizes are not constant but vary by 5% in a typical stepper motor. These are fast moving errors that occur between 1 to 3 times per second depending on pulley ratio. Without encoder feedback you simply won't get smooth tracking. These show up as fast oscillations in the RA direction when used with longer focal length scopes such as SCTs. They may not show up when using short wide field instruments and they certainly won't show up when guiding at 3 second frame rates.

That's why DC and AC servo motor drives with spur gear reduction have smooth tracking rates because they must use encoders on the motor shafts simply because these motors won't work without them. Microsteppers do work without encoders and have been the cheap solution for low end mount manufacturers, but the downside is tracking wiggles.

Rolando



-----Original Message-----
From: badgerz49@... [ap-gto]
To: ap-gto
Sent: Sun, Sep 15, 2019 3:11 pm
Subject: [ap-gto] Re: Tracking and guiding with and without encoders - Part2



Roland, 

Without encoders your mounts already guide up to the limit imposed by seeing.  How much will encoders improve the results of guided imaging?  No one is questioning whether encoder mounts track better or are superior for unguided imaging.

A test like this would be helpful:

- Have encoders on, calibrate PHD2, use default PHD2 settings, set a 3 second guide interval, run 200-400 samples .... check the total RMS error

- Turn encoders off, have PEM disabled, recalibrate PHD2, default PHD2 settings, 3 second guide interval, run 200-400 samples .... check the total RMS error

Repeat the test a few times and compare.  It would be best to use optimal PHD2 settings with both encoders on and off.  The above test would be simple and give at least some idea of the difference though.

Chad






Join main@ap-gto.groups.io to automatically receive all group messages.