Enhanced OnStep Calculations Spreadsheet

OnStep is a full featured telescope controller firmware that was created and maintained by Howard Dutton.

As of late 2021, it runs on several types of boards with a variety of microcontrollers, including the Teensy, ST's STM32F103, STM32F303, STM32F446 and Esspressif's ESP32.

In order to convert a mount to OnStep, one needs to fill out a spreadsheet created by Howard that accounts for various parameters, such as one or two stages of gear reduction, microstepping, an motor shaft RPM.

The results from these calculations impact the mechanics of the mount (e.g. pulley sizes), speed of slewing, tracking resolution and accuracy.

Due to recurring confusion on the OnStep group with interpretation of the spreadsheet, some enhancements were added to better flag when unworkable or borderline situations arise with configuration settings.

The spreadsheet linked to below is an enhanced version that uses the exact same calculation as Howard's, but provides the user feedback when certain parameters are out of range.

Specifically, these enhancements are:

  • Spells out a lower as well as an upper range of steps per degree (12,800 to 62,100). These are mainly for imaging, which is most OnStep users. Lower values can be used for visual observing). OnStep has been tested with up to 102,000 steps per degree, but Howard still recommends staying under 62,100.
  • Lists the three recommended microstepping values (1/16, 1/32, and 1/64). Some users conflate physical gear reduction with microstepping. Too much microstepping (e.g. 1/128, 1/256) will negatively impact stepping accuracy and torque. Too little microstepping (full step, 1/2, and 1/4 microsteps) will increase the chances of resonance and stalling. The recommended three microstepping values offer a reasonable compromise and work for the vast majority of mounts.
  • Warns if the motor shaft is too fast for most stepper motors when slewing. What this does not take into account is the use of MICROSTEPS_GOTO, which allows faster slewing speeds without increasing the RPM of the motors. Hopefully, a future update will add this.

By catching these common mistakes as early in the conversion process as possible, we are avoiding a common situation where users go to the OnStep group for help, only to realize too late that they miscalculated something. For example, they need different sized pulleys, and they buy them from eBay with many weeks of shipping.

So we hope that this results in less user frustration, and reduced volume of support questions on the group.

Download the OnStep Enhanced Spreadsheet, do the what-if analysis on it, then head over to the Online Configuration Generator to easily create a Config.h file for your setup.

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