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Frequently Asked Questions

Q: The N and S button behave in an unexpected way

A: Normal OnStep Behaviour.

If this is near the celestial pole of your hemisphere, then it is the way OnStep normally behaves. It is the classic question of what is north of North (or south of South). North and South reverse when you pass the celestial pole; so the next guide North can move in the opposite direction.  A guide North always starts by moving in the direction needed to get to the NCP.

Q: OnStep doesn't point where it should

A: There is a problem with your drive design and/or configuration and/or software setup, etc.

Drive design:

  • Motors and drive components move the mount reliably, not slipping or skipping steps.


  • Correct steps per degree settings for Axis1 and Axis2.
  • Correct stepper micro-step mode is actually set and used in the driver (hardware/connections setup.)
  • Motor direction of motion is correct.
  • See the Wiki section here for "Preliminary Firmware Configuration" instructions.

Software setup, etc:

  • Correct date/time and location in both OnStep and any control software.  This includes OnStep UTC Offset and control software time-zone settings.
  • Also be sure to set the mounts start position, etc. correctly as described in the Using OnStep section of the Wiki.

Q: The RA (Right Ascension) reported by OnStep at startup doesn't correspond to the expected HA (default Hour Angle at startup) relative to the current LST (Local Sidereal Time.)

A: Normal OnStep Behaviour.

The default reported RA/Dec are in Topocentric coordinates (which considers atmospheric refraction) not Instrument coordinates.  Refraction makes the celestial pole (NCP or SCP) appear higher in the sky hence instrument coordinates are pointing just a little below where the refracted pole is, as though our the Earth's atmosphere were not present.  Thus, the reported RA at startup lies opposite (12 hours from) the Meridian.

Q: When tracking, or slewing, OnStep pauses intermittently

A: Overheating. When the stepper driver overheats it temporarily shuts down, then restarts. This causes pauses in tracking during which time the stepper motor isn't energized.  Overheating is caused by the setting Vref being too high.  Vref should be set to give a current that matches your stepper motors within the stepper drivers' thermal and current limits.  More in this discussion as well as this message.

A: Insufficient power supply current.

Q: Tracking motion isn't smooth or "jumps" once every second or two

A: Zero-crossing "step-stuck" trouble which A4988's and DRV8825's sometimes have happen, search here in the group or google "step stuck.  Change the stepper drivers, change stepper motors, use the diode hack, or hack to switch to fast decay mode.  No guarantees that any of these will work or be acceptable (audible noise of fast decay, wasted power of diode hack, etc.)

A: Insufficient wire gauge to the stepper motors' can cause poor micro-stepping performance.

A: Vref set incorrectly (too low or too high.) Too low or high and the motor can cog to full step positions.

Q: How can I reset all the EEPROM values in OnStep?

A: You need to use the Arduino IDE and compile/upload a specific sketch. The exact sketch depends on the microcontroller you are using. For the Arduino Mega2560 you have to use the EEPROM Clear Sketch. For the STM32, you need the EEPROM Wipe sketch.

Q: How can I configure the focuser?

A: This is covered in this message.

Q: Which Micro-controller option should I choose?

A: That depends on your budget and requirements.  The micro-controller you use can dictate what performance and limitations you will have with it.  In this section, we detail the major differences between different micro-controllers supported in OnStep.  Below is a breakdown of the performance and features for some commonly used options:

Micro-controller Performance Differences:
-----Board/Processor----- -----MCU~Speed---- ---Memory--- --Flash-- ---EEPROM/NV--- ---Hardware--- Timers --Hardware-- FP Math -Fastest*- Step Rate
MKS Gen-L/Mega2560 16 MHz 8K 256K 4K 3 x 16 bit No 13kHz
STM32/Bluepill 72 MHz 20K 128K 4K (on RTC) 3 x 16 bit No 50kHz
MiniPCB/Teensy3.2 72 MHz 64K 256K 2K 4 x 32 bit No 71kHz
MiniPCB/Teensy4.0 600 MHz 1M 2M 1K 4 x 32 bit Yes 667kHz
MaxESP/ESP32 2 Core, 240 MHz 512K 4M 4K 2 x 32 bit Yes 62kHz
MaxPCB/Teensy3.5 120 MHz 192K 512K
4 x 32 bit
MaxPCB/Teensy3.6 180 MHz 256K 1M 4K 4 x 32 bit Yes 384kHz
Board/platform Features:
-----Board/Processor----- ----RTC---- --Encoder-- support --Focusers/-- Rotator --Stepper~Driver-- Mode Control
-TMC2130- Drivers Aux~I/O -Dew~Heaters~etc.**~ -Board+Processor- Approx. Cost
MKS Gen-L/Mega2560 DS3234+ + 3x On-the-fly+ Yes Many, 3x MOSFETs $20
STM32/Bluepill DS3231 + 1x On-the-fly Yes ? $20
MiniPCB/Teensy3.2,4.0 - + + On-the-fly Yes Aux0,1,2,3,4 $70
MaxESP/ESP32 DS3231 Built in 2x On-the-fly+ Yes Aux3,4,7,8 $70
MaxPCB/Teensy3.5,3.6 DS3231 Built in 2x On-the-fly Yes Aux0,3,4,7,8 $100

+ Means the feature can be added but it requires extra effort and might increase the difficulty of the build or otherwise has limitations.
* These figures assume square wave mode, pulse mode is 1.6x faster.  Where "on-the-fly" micro-step mode switching can be used the maximum goto speed is almost always limited by the motors/drive design not OnStep (even on a "slow" Mega2560.)  Teensy4.0 does not allow pulse mode.
** Aux0 is normally the Status LED.  Aux1 & Aux2 are normally MISO for the TMC2130 (or for ESP-01 firmware upload control.)  Aux3 & Aux4 are normally for Home SW support.  If not used for the default these can be PWM (dew heater or screen for flats) or SW (OTA cover,) etc.  There are rules about which pins can do PWM not covered here.

Alignment Capabilities/Accuracy:

Note: the maximum alignment points and resolution are Goto Assist (GTA) limitations within the micro-controller only.  Sky Planetarium can be used on any Eq mode OnStep to perform the n-star alignment with up to 50 stars and 1 arc-second resolution.

Arduino Mega2560

Maximum number of alignment points: 6
Minimum alignment resolution 128 arc seconds

STM32 Blue Pill/Teensy 3.1/Teensy 3.2

Maximum number of alignment points: 6
Minimum alignment resolution 64 arc seconds


Maximum number of alignment points: 9
Minimum alignment resolution 16 arc seconds