As we found out, the Sidereal Technology SiTech Servo II supports absolute encoder we decided to go for it. To be able to connect the 26bit Biss-C compatible Renishaw Resolute read heads to the SiTech Servo II (which speaks RS232 and not Biss-C) we needed to obtain a small interface box provided by Sidereal Technology: the so called LBB (Little black box).
Essentially it is a prtocol converter which converts the Biss-C protocol electrically and logically to the RS232 interface of the SiTech controller.
Therefore you find some level converters and a small microcontroller inside this box.
So what’s next?
We now have to order the correct sized encoder rings and read heads (in our case the 413mm diameter ring for the RA and the 200mm diameter ring for the DEC axis) and once we got them adapt the mount to be able to mount the rings to it. The main problem here will be to find a suitable and easily adjustable read head mount (the tolerances here are quite narrow). But let us discuss this in another post.
The stock dome itself is quite well made, but we identified some points to need some improvement – mostly related to the harsh conditions we face in the winter (high snow, fast winds and low temperatures).
The first issue we will solve is related to the shutter mechanism. This was identified through a small incident: we had some very strong winds a few weeks ago, and the wind managed to lift the shutter a little bit so that the drive gear jumped out of the gear rack.
We are currently investigating on how to improve this situation. The Problem is, that the shutter has too much play, and therefore it is possible that the gear jumps out of the gear rack.
We think that in this case the wind entered the dome through the several gaps at the shutter and at the gear rim.
To prevent that the wind pull through the dome in the future and to prevent drift snow to enter the dome, we installed a rubber lip at the shutter.
We also plan to install similar lips inside and outside the gear rim, to prevent that insects have easy play when entering the dome.
Another improvement dedicated to the gear rim, was to exchange all screws with stainless steel screws. We also reworked all gear rim mounting holes to make sure, the rim is 100% even and therefore the dome can run very smooth and the driving gear doesn’t get detached from the gear rim.
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:
to be able to reuse our 24V DC Motors
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.
But what are the pros and cons (are there any?) of the system now?
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
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.
As also mentioned in Tech – Telescope Control, we are planning to use the SiTech (Sidereal Technology) Servo II controller. In this document we found a reference to using absolute encoders with 26 bit encoder resolution. However this reference was more or less a sub clause in the document. It also mentioned a “LBB”, a little black box which is needed to interface with the encoders.
After some research we didn’t find out what this LBB is and if absolute encoders really work with the SiTech Servo II. We had our doubts, since there are no dedicated connection ports for absolute encoders and only this document mentioned them. You even do not find the Box in the SiTech online shop.
Our last hope then was the registration in the SiTech support forum and asking our questions there. Btw., we should have done this earlier, since the SiTech guys are really responsive and the forum in general is really accommodating.
So after a short chat there, we got the info that they do have this ominous LBB and they also sell it.
This box is designed to work with absolute encoders supporting the BiSS-C protocol. However SiTech recommends encoders from the British company Renishaw. On the SiTech controller side it will plug into the RS232 interface of the controller. The LBB is also prepared to directly work with the plugs used by Renishaw on their encoder systems.
The Renishaw encoder system
Now that we knew SiTech indeed supports absolute encoders and we also now knew the encoders to go with, we now started the evaluation on the encoder system we really need.
The main problem there was the fact, that our mount does not have “real” axis as described in the post Tech – The Telescope Mount. So the first important point was found: if we would like to use absolute encoders, we have to go with a system which has ring encoders — other designs won’t work on our mount.
The second important point was also figured out quite fast: the minimum inner diameter of the ring encoders. On the DEC axis we have to use one with minimum of 14cm of inner diameter. The problem is the RA axis, since the ring encoder has to fit around the quite thick neck of the fork part of the mount. Here we, at least need an inner diameter of 38cm. Another important point was the supported temperature range of the encoder rings and read heads.
It is compatible to the LBB (natively → plug and play)
It supports the required 26bit encoder resolution
It will support our need of an extended temperature range (-40°C to 80°C, the important value is the -40°C since we can reach temperatures well below -30° at our observation site) with their ETR read heads
The scale rings are available in sizes we need
The scale rings and read heads are small enough to fit with our mount.
RA and DEC Mounting
The last issue we had to solve when we use absolute encoders, is the mounting of the encoder on the axes.
On RA it was quite easy, apart from using a really big scale ring, since there is enough space. The other thing is, due to its bad shape, we have to replace the RA friction wheel at all. This makes it now possible to lathe the encoder mounting directly to the RA wheel during remake.
For DEC it is a little bit more difficult, since the free space is quite narrow. But also in DEC we plan to redo the friction wheel and the bearing cover plate. The bearing cover plate is then an ideal point to mount the encoder scale ring. For this purpose, the cover has to be remade including the mounting ring.
We continued with our Hard and Software test regarding the telescope controller and the control PC. Since last test was quite successful (just the small problem with the guide cam and the pointing model) we didn’t expect much troubles ahead. — Oh man were we wrong!
We tested the guide cam, a ZWO ASI120MM-S, previously and it worked as intended. It worked also with the actual control PC and its virtualized Windows.
As we tried to fire the thing up on the test site in Davidschlag, it suddenly didn’t work. The cam managed to acquire some images, but it randomly lost connection to the PC.
This was so often, that a guiding via PHD2 was simply not possible. PHD2 lost its guide star constantly.
We are now a little bit concerned if the problem is related to the telescope control server. It seems that it s USB hubs are faulty and quit working when a lot of USB devices connected to them.
As already mentioned in Tech – The Telescope Mount, the friction wheels are quite corroded, which results in a quite bumpy ride for the telescope.
In our first tests this was annoying and caused position errors, but we were able to move the telescope to the positions we want.
But in this night, the ride was even bumpier, and constantly caused the SiTech Controller to go in “Blinky-Mode” — a failsafe mode which causes the telescope to immediately stop the movement. So this time we were completely unable to point the telescope to any position at all.
We were quite puzzled what happened, but after some thinking we identified the Problem: Temperature.
Our last tests took place in quite warm weather conditions, this one however was during a quite cold night (in comparison to the last test nights). This causes the metal of the mount to contract and therefore we ended up with different pressures on the friction wheels. This seems to interfere with the bad friction wheels a lot more than in warm weather conditions.
We now decided to stop testing and therefore disassemble the telescope and start with the refit and repair tasks. This means to update the telescope with new friction wheels, additional absolute encoders and motorized mirror/corrector shutter.
However, we obtained some nice images during the last tests, which we plan to present here in this blog and on our other social media channels during the telescope downtime.
This post should be the prelude to several posts dedicated to the software we are using. The first software we want to discuss is the command and control software of the SiTech Servo II telescope controller.
The software is split into two more or less independent parts:
This tool is dedicated to configure the controller with all sorts of parameters needed. E.g. motor speeds, encoder ticks, backlash settings and so on.
The program is a little bit colorful, but it nicely sums up all relevant information in the main screen. You can easily see the status of both axes (motor status, encoder status) and you can also see other controller settings below.
You also have a little virtual hand pad, which is quite handy when one is fiddling around with parameters and want to test them.
On the left side you’ll find a menu where you can access the settings. The most important menu item is “Edit Parameters” which takes you the parameter setup screen:
All in all the basic setup is/was done quit fast and straight forward. The most difficult part was to determine the actual values for the parameters to set. Some were found in the config files of the old DOS based control software and some were found by trial and error – but with the help of this tool and its documentation we were able to do the whole setup in one day. Seeing the motors moved by the new controller was quite satisfying.
For further in depth information check the documents provided by Sidereal Technology:
The basic handling of the scope, e.g. unparking the scope and other stuff can be handled directly from the main screen.
For pointing to object, SiTech.exe also have a full planetarium subprogram included. At this you can easily find objects and point the telescope to them, however we will not really use this part of the program since it is a little bit clunky to handle and pointing to objects through Stellarium is far more comfortable – so we decided to use Stellarium instead. The good thing is, due to full ASCOM support of SiTech.exe, this is quite easy to do.
So we basically use SiTech.exe as interface to the controller itself. We also use it to configure the pointing models and park/unpark the scope. Perhaps we also use it as main script controller for automation tasks.
SiTech.exe also provides powerful ASCOM scripting extensions. A documentation of them can be found at this web page (SiTechExe ASCOM Extensions Help as of Version 0.91Xe).
In upcoming blog entries regarding software, we will discuss Stellarium, which we use to point at objects and AstroArt which we use for acquiring images.
As mentioned in an earlier blog entry, one major part of the project is the upgrade of the telescope controller.
As it can be expected, the PC and the software never got an upgrade. Never touch a running system! So currently we have a custom built software running on MS DOS on a vastly outdated PC where getting spare parts is a science of its own. Luckily, we have some well aged connections of our own!
So while parts kept dying we could find replacements in a reasonable time. However it is not feasible to upgrade the current system for Remote Control.
And then we said: “Cool at least we can keep everything from the Motor Driver onward.”
Sadly the answer was a clear “No!” You see, not only do we have a custom built software, but also is this software is communicating with custom built ISA cards. Said ISA Card incorporate all the necessary hardware and the DC drivers to move the telescope, just updating the software part would be asking for hardware trouble later on.
Thus finding a controller that fulfills our needs was not so easy. Especially problematic is, that with our telescope size we are in a bad spot (we discussed the telescope itself in the post Tech – The Telescope). To big for the cheap stuff (also we have DC motors and not steppers) but to small to realistically afford the stuff the big telescopes use. And we really wanted to keep the motors and especially the encoders.
We already had a go at it and played around with the controller and one motor of the telescope. Looking at the 50+some page Manual and the 32 page config guide we were a bit scared about the whole process. But in the end we managed to get the controller working and the motor running quite nicely.