bojan
18-10-2016, 02:00 PM
After seeing a photo of Auckland Observatory Zeiss refractor on the net, I decided to tell the story about Zeiss telescope RA drive mechanism and my involvement with it on Zagreb’s Astronomical Observatory.
It must be I am getting old, recently digging out memories from my younger days.
This particular story is dear to me because then I managed to solve a riddle that was bugging our astronomical society for years.
The Zagreb’s Astronomical Observatory bought this telescope in 1964, after international annual fair, where it was on display by Carl Zeiss from Jena.
Earlier that year I actually sort of officially “discovered” astronomy as a discipline I wanted to be involved with, and it became and still remains to be my obsession even today.
The telescope in question replaced the old Austrian made 160mm equatorial (installed 1903) driven by fully mechanical clock mechanism (with weight and Watt centrifugal regulator). It had 130mm diameter achromatic lens, 1954mm FL, mounted on very solid and heavy equatorial mount. The tracking system used 24V DC motor, with appropriate reduction mechanism which also accepted external synchronization pulses (we were told so but there was no any documentation or user manual as far as anybody was aware at that time… and I was only a 14 years old kid then).
The potential to synchronise the drive mechanism with accurate clock was an issue for quite a long time… but we lived without it somehow. Yes, the tracking speed was not accurate, but it was OK for visual, and there were those adjustment knobs we used to bring the observing target back into the centre of FOV when it drifted too close to the edge. Annoying thing about them knobs was the mechanical limit – once reached, the knobs position was needed to be turned all the way back until the other end limit was hit. And, apart from rumours about the need for external sync, nobody knew how this thing actually worked, and what was really needed to be complete.
After couple of years (when I finished high school and uni (electronics engineering) and got my first job and enough confidence, and when on my annual leave for the first time) I finally decided enough was enough. The sync with external, accurate electronic clock must be implemented.
Firstly, I needed to understand the principle of synchronization with external clock. So one weekend I opened the control box on the wall and gearbox case on the pier and had a look inside.
The 24V DC motor was directly driving a worm gear, moving a puzzling electromechanical assembly. There were two additional gears, two high power adjustable wire-wound resistors/potentiometers (in series with stator of the DC motor), a wiper on one of them, additional spur gear (frictionally coupled to wiper), … solenoid with cog which interfered with rheostat wiper preventing it to rotate with the rest of the assembly … it didn’t make much sense on the first glance.
There was not a lot of documentation (only one almost unreadable photo copy of the electrical schematic of control panel), but from couple of experiments and labelled terminals inside the cupboard on the wall it was possible to figure out what was what. Inside the box there were also mains transformer, couple of selenium rectifiers and rather funny looking reed relay with a large drop of mercury as one of the contacts, with its glass tube inserted inside the large solenoid. It seemed the sync pulses should be applied to that relay. And when the reed relay was activated, it energised the solenoid with mechanical cog inside the gear box on the telescope pier.
I started to have a feeling I am onto something…
There was a toggle switch on the gearbox with two positions: “unregulated” and “regulated”.
Up until then we were always using the telescope with switch in “unregulated” position, because otherwise the speed of the motor was gradually increasing, until reaching the maximum, then it would suddenly drop to minimum, from where it was increasing again.
The toggle switch in “unregulated” position resulted in steady (but not accurate) tracking speed.
And then in one moment all that collection of details fell into place.
It was soo simple actually…
Inside the gear box there were two ~80mm diameter, independent gears on the same shaft, one with 50 (? Let’s say 50, I don’t remember now), another one with 50+1 teeth. They were coupled with smaller spur gear at their rim.
The gear with 50+1 teeth was directly driven by worm gear, completing the rotation every 2 seconds. This gear also had the rheostat mounted on it.
The gear with 50 teeth was hence rotating 2% faster, and was driving the rheostat wiper via frictional (not very tight) clutch.
This was the reason for steadily increase of the rotating speed of the DC motor.
However, when the solenoid inside the gear box was activated by external timing pulse (duration was ~0.3sec, adjustable with resistor in parallel of the coil around reed relay, the cog stopped the rotation of the wiper, so it was moved back a bit and the motor moved slower.
When sync pulse voltage went back to zero, the wiper was again free from cog and it was moved by spur gear, increasing the motor speed a bit... until the next synchro pulse. And so on… Mystery that was riddling us for so long was finally solved.
Interesting thing was that one of my colleagues was visiting Ondrejov Observatory (near Prague) at the same time and while there, he got the same idea (they also had larger Zeiss telescope with the similar but fully functional clock drive, and he wanted to see how the darn thing worked)… So when he came back he told me what he discovered… and confirmed my way of thinking 100%.
Anyway, 1-second pulse generator solved the long-lasting problem in the following couple of days.
This was done with CMOS oscillator-divider chip (CD4060). However, for this frequency to be accurate for sidereal clock display (it was not possible to adjust the 10ppm crystal that much), I designed the circuit which inserted additional pulse after every 356 pulses in the divided pulse train.
The resulting ~50kHz output (6.4MHz/128) was divided further down to 50Hz by 3xCD4029 decade counters, that was needed as input to LED clock module to display the (sideral) time.
1 sec sync pulses for telescope clock drive were derived from colon LEDs.
I attached the image of the telescope discussed in this post, along with some others from other observatories on the same/similar mount.
It must be I am getting old, recently digging out memories from my younger days.
This particular story is dear to me because then I managed to solve a riddle that was bugging our astronomical society for years.
The Zagreb’s Astronomical Observatory bought this telescope in 1964, after international annual fair, where it was on display by Carl Zeiss from Jena.
Earlier that year I actually sort of officially “discovered” astronomy as a discipline I wanted to be involved with, and it became and still remains to be my obsession even today.
The telescope in question replaced the old Austrian made 160mm equatorial (installed 1903) driven by fully mechanical clock mechanism (with weight and Watt centrifugal regulator). It had 130mm diameter achromatic lens, 1954mm FL, mounted on very solid and heavy equatorial mount. The tracking system used 24V DC motor, with appropriate reduction mechanism which also accepted external synchronization pulses (we were told so but there was no any documentation or user manual as far as anybody was aware at that time… and I was only a 14 years old kid then).
The potential to synchronise the drive mechanism with accurate clock was an issue for quite a long time… but we lived without it somehow. Yes, the tracking speed was not accurate, but it was OK for visual, and there were those adjustment knobs we used to bring the observing target back into the centre of FOV when it drifted too close to the edge. Annoying thing about them knobs was the mechanical limit – once reached, the knobs position was needed to be turned all the way back until the other end limit was hit. And, apart from rumours about the need for external sync, nobody knew how this thing actually worked, and what was really needed to be complete.
After couple of years (when I finished high school and uni (electronics engineering) and got my first job and enough confidence, and when on my annual leave for the first time) I finally decided enough was enough. The sync with external, accurate electronic clock must be implemented.
Firstly, I needed to understand the principle of synchronization with external clock. So one weekend I opened the control box on the wall and gearbox case on the pier and had a look inside.
The 24V DC motor was directly driving a worm gear, moving a puzzling electromechanical assembly. There were two additional gears, two high power adjustable wire-wound resistors/potentiometers (in series with stator of the DC motor), a wiper on one of them, additional spur gear (frictionally coupled to wiper), … solenoid with cog which interfered with rheostat wiper preventing it to rotate with the rest of the assembly … it didn’t make much sense on the first glance.
There was not a lot of documentation (only one almost unreadable photo copy of the electrical schematic of control panel), but from couple of experiments and labelled terminals inside the cupboard on the wall it was possible to figure out what was what. Inside the box there were also mains transformer, couple of selenium rectifiers and rather funny looking reed relay with a large drop of mercury as one of the contacts, with its glass tube inserted inside the large solenoid. It seemed the sync pulses should be applied to that relay. And when the reed relay was activated, it energised the solenoid with mechanical cog inside the gear box on the telescope pier.
I started to have a feeling I am onto something…
There was a toggle switch on the gearbox with two positions: “unregulated” and “regulated”.
Up until then we were always using the telescope with switch in “unregulated” position, because otherwise the speed of the motor was gradually increasing, until reaching the maximum, then it would suddenly drop to minimum, from where it was increasing again.
The toggle switch in “unregulated” position resulted in steady (but not accurate) tracking speed.
And then in one moment all that collection of details fell into place.
It was soo simple actually…
Inside the gear box there were two ~80mm diameter, independent gears on the same shaft, one with 50 (? Let’s say 50, I don’t remember now), another one with 50+1 teeth. They were coupled with smaller spur gear at their rim.
The gear with 50+1 teeth was directly driven by worm gear, completing the rotation every 2 seconds. This gear also had the rheostat mounted on it.
The gear with 50 teeth was hence rotating 2% faster, and was driving the rheostat wiper via frictional (not very tight) clutch.
This was the reason for steadily increase of the rotating speed of the DC motor.
However, when the solenoid inside the gear box was activated by external timing pulse (duration was ~0.3sec, adjustable with resistor in parallel of the coil around reed relay, the cog stopped the rotation of the wiper, so it was moved back a bit and the motor moved slower.
When sync pulse voltage went back to zero, the wiper was again free from cog and it was moved by spur gear, increasing the motor speed a bit... until the next synchro pulse. And so on… Mystery that was riddling us for so long was finally solved.
Interesting thing was that one of my colleagues was visiting Ondrejov Observatory (near Prague) at the same time and while there, he got the same idea (they also had larger Zeiss telescope with the similar but fully functional clock drive, and he wanted to see how the darn thing worked)… So when he came back he told me what he discovered… and confirmed my way of thinking 100%.
Anyway, 1-second pulse generator solved the long-lasting problem in the following couple of days.
This was done with CMOS oscillator-divider chip (CD4060). However, for this frequency to be accurate for sidereal clock display (it was not possible to adjust the 10ppm crystal that much), I designed the circuit which inserted additional pulse after every 356 pulses in the divided pulse train.
The resulting ~50kHz output (6.4MHz/128) was divided further down to 50Hz by 3xCD4029 decade counters, that was needed as input to LED clock module to display the (sideral) time.
1 sec sync pulses for telescope clock drive were derived from colon LEDs.
I attached the image of the telescope discussed in this post, along with some others from other observatories on the same/similar mount.