Tech – Using Renishaw Absolute Encoder

The search for a LBB

As already mentioned in Tech – First Hardware/Software Test and Tech – The Telescope Mount we plan to upgrade the telescope mount with absolute encoders to increase pointing precision and to get rid of aligning the telescope every now and then, and after every shutdown.

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.

We found, that the Renishaw Resolute measuring system, in combination with their RESA scale rings, will fulfill all of our needs:

  • 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.
The Renishaw absolute encoder system showing two RESA scale rings and the corresponding Resolute read head. (Image by Renishaw)

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.

Here you see the current RA friction wheel with the neck towards the fork. The RA wheel will be replaced by a new one including the mounting ring for the encoder scale ring. (Upper blue part = fork, lower gray part = pole block, marked section = RA friction wheel + neck) (Photo by Erich Meyer, 1999)

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.

The silver disk is the current bearing cover which will be remade a little bit bigger and with the encoder mounting. (Photo by Erich Meyer, 1999)




Tech – Alignment & Guiding (Take 3) – Fail

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!

First Fail

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.

Second Fail

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.

If you like it like it 🙂 …

Of course we will keep you up do date on the repair tasks.

Tech – Software Part 1 (SiTech Servo II)

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:

  • Sidereal Technology Servo Configuration Utility
  • telescope control (sitech.exe)

Servo Config – Sidereal Technology Servo Configuration Utility

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.

Servo Config tool main dialog. (Screenshot taken from the SiTech Servo Config manual)

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:

The first tab of the “Edit Parameters” screen, where you can change motor parameters. (Screenshot taken from the SiTech Servo Config manual)


SiTech.exe  main window.

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:

SiTech.exe – Telescope control

The more complex part of the SiTech software bundle is the actual command and control software, called SiTech.exe

First of all, you can configure the pointing model there, which as you can read in Tech – Telescope Alignment & Guiding, was not so easy to handle.

The SkyView and InitPoint dialog from SiTech.exe are sufficient for their Task but for Day to Day operation something more widespread will be used.

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.

Connecting Stellarium to the SiTech.exe (Via StellariumScope & ASCOM) makes pointing the Telescope quire easy, at least on the Software side.

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.



Project presentation in Linzer Wissensturm

On Monday (22.10.2018) we presented the current project status to LAG club members and the interested public. The event took place as part of LAGs monthly public talks in the Wissensturm¹ (Wissensturm web page) in Linz.


Markus Hoflehner and Günther Truhlar from the KRO team. (Photo by Robert Mayrhofer, 22.10.2018)
(Photo by Günther Truhlar, 22.10.2018)

You can download the slide set presented here: Monatsvortrag_Wissensturm_22.10.2018 (German)

Also in this monthly talk, our LAG colleague Harald Schmidt, presented his aurora borealis pictures, taken this year in March on a cruise in Norwegian waters.

Harald Schmidt from the LAG. (Photo by Peter Lagler, 22.10.2018)

For additional information about our monthly talks please visit our LAG main website.

The upcoming talk will take place again in the Wissensturm and will have the topic “250 Jahre Meteoritenfall von Mauerkirchen” presented by Herbert Raab. (Date: 19.11.2018, 19:30)

¹ rough translation of Wissensturm: Tower of Knowledge

Tech – Telescope Control

As mentioned in an earlier blog entry, one major part of the project is the upgrade of the telescope controller.

The outdated Telescope Control which is running on MS DOS needs to be replaced by a newer version. (Photo by Johannes Stübler)

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.

What we found was the SiTech Servo Controller II.

The ASCOM capable servo controller from SiTech is our controller of choice.

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.

We have now an equipment page

Perhaps you have already noticed, there is a new menu link called “Equipment”. It links to a static page where we listed the technical equipment we use at the KRO. E.g. telescopes, cameras,…

This site will be updated if something changes, if we add a new equipment or if we ditch one.

KRO in dusk, waiting for the night to come. (Photo by Bernhard Mayr, October 2018)

Tech – DIY Motor Focuser

After rudimentary setting up the Sitech Servo II Controller and the telescope, we went out to test it and make some pretty pictures. Sadly before the fist exposure the motor focuser died, after years of standstill the lubricant got really sticky and the load killed  our USB_Focus v3. After a failed repair attempt it was clear we needed something new, something that is ASCOM compatible and can be easily repaired or at least cheaply replaced. And to make things not too easy the focuser motor is also a an outdated hollow shaft stepper.

Here you can see the sticky mess, also it should be pointed out that the focuser sits on the keller corrector and the little motor has to lift the SBIG Camera. (Photo by Markus Hoflehner)

Looking at the Cost of a ready made focuser an the possible adaptions, we still had to do for the stepper, we decided to make a quick google search. An this was the Result!

The myFocuserPro2 Project, was exactly what we were looking for. Yes at a first glance the project is quite huge and intimidating but it is surprisingly well organized and works very neatly even if you do not have a lot of experience in programming. Dont get me wrong, there are quite a lot of options and extras and we suggest you have a good look a the documentation to see what is already available before deciding what to build, we sure did the same. But when you have decided it, it is very straight forward.
Many Thanks to Robert Brown for providing that Project!

As we were primarily interested in the ASCOM compatibility the first thing we built on a breadboard was an ASCOM focuser. No buttons, no display, just a Arduino Nano and the DRV8825. And it simply worked, so we added an LCD, a rotary encoder and a temperatrue sensor (DS18B20). The final version which went for a test with the telescope looks like this:

The preliminary build of our myFocuserPro2 which we will take for testing to the telescope. (Photo by Günther Truhlar)

Yes it is a mess, but after a successful hardware test we will clean that up. More details on what we exactly did and the cleanup in our next post.