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 And to make things not too easy the focuser motor is also a an outdated hollow shaft stepper.
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:
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.
Soon after our First Hardware/Software Test we again went to finish what we could not finish before, namely the 16-point alignment to increase pointing accuracy, reading out PEC-Data and enabling auto guiding.
First we started with the alignment. Having read the corresponding section in the Sitech Controller manual we did the alignment by memory and not by the book, probably the first mistake we made that night. Getting 16 stars across the sky centered and saved took a while and the sky was already nicely dark when we finished, so we were already getting anxious to make some pictures. Calculating the pointing model however took no time, so we instantly applied it and thus made our second mistake that night.
We should really have tested the pointing accuracy because as it turned out later that night we seriously worsened the accuracy of the telescope with the 16-point model in comparison to a 1-star alignment.
Not knowing this, we went on to installing the guide scope, a Skywatcher 80/400 Refractor. We planed to initially use an old ASI 120MM camera which we had lying around, and already good experience with. However, we could not get it to work with the PhD2 guiding we use on our telescope control server. After some At-Home investigation, we assume this has to do with a combination of the USB-Port handling of our virtualization and the known issue that the ASI 120MM sometimes does not like newer USB ports. Don’t get me wrong, this is the first time the camera does not want to work and our solution for this is using the ASI 120MM-S which is newer, has USB3.0 and simply works after plugging it in.
Beside the technical stuff we also wanted to make some pretty pictures. For this we chose NGC7635, the bubble nebula. We took 5 exposures for 30sec, 60sec and for 120sec exposure time for each LRGB filter. For the H-α (12nm) filter we took 20 exposures with 30 sec, 10 with 60sec and 5 with 120sec exposure time. And to be honest, due to the night being a bit cloudy and our technical problems the raw material did not look very promising.
During the testing we observed another problem: the RGB filters have a different focal plane then the H-α filter. Therefore we always have to refocus when changing between RGB and H-α. And this was quite challenging for the auto focus since we needed up to 3sec exposure times for each focus image. But the focus handled the situation quite well.
One problem you can see in the picture is that the camera does not have a anti-bloom gate. Additionally we didn’t make bias or dark frames (jet), but will make some in teh future. Also the sky was a bit dusty, but the biggest problem was already mentioned above, the guiding did not work and the 16 point alignment worsened the accuracy, so we had to turn it off an re-shoot the 120sec exposure (because we did not check the alignment immediately).
So the next steps are to install an test the ASI 120MM-S as a guiding camera, but more importantly redo the 16-point alignment step by step according to the manual. After this is done, we can try out the automated alignment using plate solving (see here page 78).
Between 2005 and 2016 the telescope was not really used. In 2016, the LAG started a project to revitalize the observatory and the telescope itself. The project was called LAGST540 (LAG Starlight Team 540). Shortly after starting this project we got the opportunity to join the Star Park Hohe Dirn project and we decided to change the project goals: 2018 the KRO project was born.
First of all, to be on the same page, we refer with tube to the white part of the telescope. The blue part is the fork mount and was discussed here. The tube holds the main mirror, the corrector and the camera.
The mount and the tube were designed together by Rudolf Pressberger, so they fit together perfectly and therefore the telescope can’t be used with another mount and also the mount can’t be used with another telescope.
The tube was built from light weight steel panel. A clever internal design makes sure, that the tube is also very stiff. The spider ring (holding the Keller corrector and the camera) is also built from light weight steel panels and is held in place by hollow steel tubes following the principle of a Serrurier truss. Only this design in combination with the very stiff spider ring, prohibits the tilt of the optical axis during a position change of the tube.
On the side faced towards the mirror cell, a similar construction as the Serrurier truss facing the spider ring, was used. This made it possible to bring the mirror cell as close as possible to the load-bearing part of the tube and thus to the DEC axis.
Due to this yet stiff but light weight construction of the tube, it only weighs a little bit over 100kg – of course without the mirror cell (the round assembly at the bottom of the tube, see picture above) and mirror.
Mirror and mirror cell
We are using a mirror cell which supports the mirror in axial direction with a 9-point whiffletree design invented by an Irish optician called Thomas Grubb. The radial bearings for the mirror are three temperature compensated ball joints which support the mirror on six sliding surfaces on the outer rim of the mirror. Due to this construction, the mirror can be centered and fixed to the optical axis. The ball joints provide free axial movement.
The studs holding the ball joints are made of zinc to compensate the temperature expansion of the mirror cell. This special construction prohibits the deformation of the mirror but locks the mirror into position, so that it can’t slide out of place.
The telescope lacks of a secondary mirror, instead there is a corrector and the camera is directly mounted to the corrector. So the camera is in prime focus which (in combination with the corrector) results in the quite fast aperture ratio of f/3.3.
Another advantage of the prime focus design is, that the mirror do not need a hole in the center to be able to output the light path at the back of the telescope.
The disadvantage of this design is, that the spider ring needs to be able to take a lot of a load (Camera + quite large corrector). Luckily the spider ring design by Pressberger is really stiff and can handle this load with ease. And also the rest of the telescope design has to be stiff to minimize tube bending.
Since the telescope is a standard Cassegrain type, we need some sort of correction due to the fact that a Cassegrain telescope tends to introduce distortions to the image. This is due to the fact, that the mirror is spherical not parabolic. Another important point for a telescope used only as astrophotography tool: the image area should be flat.
In our case this is achieved with a corrector mounted in the spider ring. It was designed by Philipp Keller, a physicist specialized on optical components. The corrector eliminates the coma of the optical system and also flattens the image (this is very important for cameras with a big sensor area).
Our corrector also incorporates the focus. This is done via a slide in/out lens at the camera faced side of the corrector. This focus is motorized and will be described in a future post.
A view days ago we achieved an important milestone, as we could complete the facing of the building.
The observatory was designed and planned by our LAG member Ing. Johann Bachlmayr who is a professional master-builder.
The next in completing the building, is the installation of the electrical stuff: wiring, server rack, lighting (read and white), network/internet access, etc. we will focus on in one of our next posts.
In the following Gallery, you’ll find some impressions of our observing site Star Park Hohe Dirn.
View from the path to the Anton Schosser hut down to the Observatory in east direction. (Photo by Günther Truhlar, 13.07.2018)
View in south direction over the national park towards the alps. (Photo by Günther Truhlar, 17.08.2018)
Below you’ll finde some Photos taken by our colleague from the “Sternfreunde Steyr” Bernhard Mayr during mid of September.
View from east to west over the domes of the “Star Park Hohe Dirn” observatory. In front there are the two domes of the Sternfreunde Steyer and in the back the dome of the KRO. (Photo by Bernhard Mayr)
The path to the Star Park Hohe Dirn and the Anton Schosser hut. View direction west. (Photo by Bernhard Mayr)
View from west to east over the KRO dome. (Photo by Bernhard Mayr)
The dusk in the Star Park. (Photo by Bernhard Mayr)
The dusk in the Star Park. (Photo by Bernhard Mayr)
The two domes of the Sternfreunde Steyr and already some fog in the valley below. (Photo by Bernhard Mayr)
Milky Way over Star Park Hohe Dirn
The following photo of our own galaxy, the Milky Way, over the observatory was taken by another colleague from the “Sternfreunde Steyr”, Rudi Dobesberger.
As mentioned in previous posts, the telescope for the KRO already exists. It is located in a observatory north of Linz, quite easily accessible for us. So we can test most of the modernized functions already there.
What was the goal of the first test:
controlling the telescope with the SiTech Servo II
testing the myFocuserPro2 (a DIY motor focuser)vand the autofocus mode
testing if all works with Astroart (image capturing, filter wheel control, auto focus control and telescope control for fine adjustments
using Stellarium (and StellariumScope) to find and GoTo to the desired Object (you can find a German document pointing out the usage of Stellarium with a telescope here, by Johannes Stübler
Remote control (remote connection to the telescope server)
First of all, the test went really well, all components worked together and we ended up needing just two programs to do the job of finding objects and take images of them. The underlying ASCOM, SiTech system doesn’t concern the user and therefore the user do not have to deal with the complex lower layers. Users only have to deal with Stellarium for finding the objects and pointing to them and of course the image capture software Astroart.
The software is preconfigured and ready to use. So the first thing is to locate and GoTo the desired object. For this the we only have to open Stellarium and open the telescope dialog.
Once the we hit “GoTo” (in the image above “Schwenken”) the telescope will point to the desired object and we now can switch to the image capturing tool.
The first step there is to focus the camera. For this Astroart supports an auto focus feature which is compatible with our ASCOM motor focus:
After a successful focus procedure, we only have to select the desired filter and trigger an image acquisition (either single mode or as predefined sequence or as predefined script).
We demonstrated that the basic user interface and telescope controls are working quite well together and it is already now possible to control the scope fully by remote connection.
We plan a second test session, discussed in a future post, where we focus on the 16-point alignment to increase pointing accuracy, reading out PEC-Data (to be able to compare before/after absolute encoders) and enabling auto guiding via PHD2. Perhaps we can shoot the last pretty picture on the old telescope location.
Once the tests are completed, we will disassemble the telescope on its old observing site, rework it, add the additional absolute encoders and rebuild it at its new observing site on the “Hohe Dirn”.