Tech – Telescope Control (Part 2)

As mentioned in Tech – Telescope Control the telescope control system was quite aged. It was built in the late 90ies and based on DOS(!). Not even the software was dated, also the hardware. The existing control system relies on ISA-bus cards! So using a new computer system with the dated control electronics/software was not possible. But using the old hardware and software was no option too.

So we searched for a control hard and software and found the solution at Sidereal Technology and their SiTech Servo II controller.

It fulfilled our two main requirements:

  1. to be able to reuse our 24V DC Motors
  2. to be able to reuse our Heidenhain motor encoders

And it gave us the opportunity to improve the current system with adding absolute encoders to the axes, which is also supported by the SiTech Servo II. More Information on that can be Found in our blog post Tech – Using Renishaw Absolute Encoder.

Also the first tests were quite a success as you can read in Tech – First Hardware/Software Test and Tech – Telescope Alignment & Guiding.

But what are the pros and cons (are there any?) of the system now?

The Servo Controller II during the first setup.

What can it do

It is a quite compact motor driver solution which is fully ASCOM compatible. Beside this it also supports a hand pad with basic functionality. The main operation of the controller is intended to be driven by the SiTech configuration/control toolset (Tech – Software Part 1 (SiTech Servo II)).

The controller is capable of handling up to four high speed encoders on its encoder input ports. It is also capable of handling absolute encoders via a “LBB” (Little Black Box) (Tech – Using Renishaw Absolute Encoder), you need to by extra and which plugs in to the RS232 port also present on the controller.

The SiTech Servo II also supports auto guiding with a dedicated auto guide port.

If you follow this link to the Sidereal Technology web page describing the new controller version, you’ll find some other useful features:

  • 4 extremely high speed encoder inputs
  • Above feature allows use of the new “Cascade” telescope encoder mode.
  • Industrial Terminal Blocks
  • 4A max motor current
  • Built In Autoguider
  • USB AND RS232
  • Slim and versatile enclosure
  • Overtravel Limit Switch Inputs
  • PEC Limit Switch Inputs
  • Homing Switch Inputs
  • High speed hardware to support future software features
  • LED’s to show hand pad status and Communication Status

Why it is ideal for our project

As mentioned in previous blog posts (e.g. here Tech – The Telescope or here A project begins – What we want to do) we got a very powerful telescope which we will utilize in this project. The only downside of this instrument was its dated and aged control hard- and software.

Due to its special mechanical setup, we’d like to keep as many components as possible in the drive train (e.g. motors and encoders) and just ditch the electronics. Since we use DC motors, a telescope control for stepper motors (by far the most available ones) won’t work.

And there comes SiTech into play: The SiTech Servo II controller can handle our motors AND our encoders. Due to this fact, setting up the new hardware/software to our telescope was more or less plug and play.

Another point is the possibility to extend the controller with really high end features such as absolute encoders.

The last but certainly not the least point was the software integration. SiTech works very well together with other ASCOM software; and it is fully scriptable → a feature we need for the robotic operation mode.

Were there troubles?

Honestly, no. The SiTech system works quite well for us. If I had to name a con regarding our very own setup, this would be the limitation of 4 Amps motor current per axis. If we’d like to drive faster then 3 degree/sec we exceed the 4A during ramp up and the motors went into “blinky” mode (an error state).

But beside this, the SiTech Servo II is a great telescope controller for mid to high end telescopes.


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