OnStep Conversion Guide: Orion SkyView Pro (SVP) Mount

This guide documents my personal conversion of an Orion SkyView Pro (SVP) Equatorial Mount into a computerized OnStep GoTo system.

I encourage you to review the steps and parts I used, and refer to this guide if you find this method useful for your own project!

Parts Required

To complete this conversion, you will need the following components:

Category	Item Description	Quantity
Timing Belts	128mm Close Loop Timing Belt, (Compatible with GT2)	2 Pcs
Pulleys (Motor)	2GT Aluminum Timing Pulley, 16 Teeth, 5mm Bore	2 Pcs
Pulleys (Gear)	2GT Aluminum Timing Pulley, 40 Teeth, 6.35mm Bore	2 Pcs
Motors	High Accuracy 0.9° Step Angle Nema 17 Stepper Motor, 1.7 A, 0.52 Nm (74 oz-in)	2 Pcs
Connectors (Motor Side)	Aviation Plug GX16 Connector, 4 Pin Socket with 20CM Wire	2 Pcs
Connectors (Cable)	GX16 Double Female Electrical Plug (Female-to-Female), Circular Aviation Connector with 1M Cable (4 Pin)	2 Pcs
Small Connectors	JST PH 2.0mm Connector Kit (6Pin Housing, Female/Male Adapter Cable Connector Plug)	2

Tools Required

Precision Crimping Tool: 

ENGINEER Precision Crimping Pliers Best Made Precision Crimping Tool for AWG32-AWG20 Wires across 80 Different Pins & D-Sub Connectors Contacts, Oil-Resistant Grip (PA-09)  

3D Printed Parts

 The required mounting brackets must be 3D printed using the files available here:

https://www.thingiverse.com/thing:5902846

Important Note: There is a slight alignment issue with the 3D-printed RA bracket. This may need to be corrected or adjusted before final assembly to ensure proper meshing of the gears.

Orion SkyView Pro Mount Specifications

Once all parts are gathered and assembled, the following technical details for the Orion SVP mount are necessary for the OnStep configuration:

Parameter	Value	Notes for OnStep
RA Worm Gear Teeth	144	This is the crucial information needed for the GR2 Ratio field in the OnStep Configuration Calculator spreadsheet.
RA Gear Ratio	704:1
Microsteps per RA Revolution	4,505,600
Microsteps per Worm Revolution	31,288.89
Resolution	0.287642 arcseconds
Microsteps	32

Source of information: https://eq-mod.sourceforge.net/prerequisites.html

Configuration Steps

1. Define the System Parameters: The next step requires using the OnStep Configuration Calculator. Inputting the mount's core specifications (like the 144 RA Worm Gear Teeth) and your motor specifics allows the spreadsheet to compute the values for the OnStep firmware.

2. 

2. Access Controller: Once the mechanical conversion is complete and the OnStep controller is powered, log in from your laptop by entering the default IP address in your web browser: http://192.168.0.1/index.htm

After calculating the values using the Configuration Calculator spreadsheet, input them into the OnStep controller's web interface as shown below for Axis1 and Axis2. These values define how the controller converts motor steps into angular motion for your specific SVP mount.

My reference notes 

OnStep Configuration Calculator notes

GR1 = gear/pulley reduction from the motor's output shaft to the worm's shaft. (GR1 ratio: 2.5)

GR2 is the reduction provided by the worm gears. (GR2 Ratio: 144:1)

On the SVP and the vast majority of similarly sized mounts this reduction is 144:1 meaning the worm has to rotate 144 times to turn the RA or Dec axis 360°. Each worm rotation advances the worm wheel 2.5°. The planet's rotation rate is 1° every 4 minutes, 15°/hour or 1440 minutes in 24 hours. Hence, we need to rotate our SVP's worm once every 10 minutes. The  calculation is (400*64*2.5*144)/360° The variables are Micro steps and GR1 (in RED). Everything else is fixed.

Hold Current

Hold current is the amount of current supplied to the stepper motor when it is stationary (not moving). It keeps the motor locked in position to maintain its holding torque, preventing the telescope from drifting or losing alignment.

Why is it important?

• If the hold current is too low, the mount might lose position accuracy or drift.
• If the hold current is too high, the motor can overheat and waste power.

Recommended Hold Current:

For most telescope setups:

• Set the hold current to 30-50% of the tracking current.
• This is sufficient to maintain position without overheating the motors.

Assume the tracking current is 1100 mA, the suggested range would be:

• 30% of 1100 mA = 330 mA
• 50% of 1100 mA = 550 mA

Suggested Setting:

• Start with 400 mA for hold current.
• If the mount drifts or loses position, increase to 500 mA.
• If the motors get warm when idle, decrease to 350 mA.

Tracking Current

For a stepper motor rated at 1.5A (1500 mA), the ideal tracking current is typically set to 70-80% of the rated value to balance performance and heat management.

Recommended Tracking Current:

• 70% of 1500 mA = 1050 mA
• 80% of 1500 mA = 1200 mA

Suggested Setting:

Start with 1100 mA for tracking current. This is a good balance between torque and keeping the motor cool. If needed, you can adjust slightly up or down:

• If the motor feels underpowered or skips steps, increase to 1200 mA.

If the motor runs hot, reduce to 1000 mA.

Slewing Current

The Slew Current is usually set higher than the tracking current to provide extra torque during fast movements (like slewing to a new target).

Recommended Slew Current:

Typically, the slew current is set to 10-20% higher than the tracking current. Since your tracking current is set to 1100 mA, the suggested range would be:

• 10% higher:  1210 mA
• 20% higher:  1320 mA

Suggested Setting:

• Start with 1300 mA for slew current.
• If the motors skip or struggle during slewing, you can increase it slightly.
• If the motors get too hot, decrease it a bit.