Living in an area subject to a lot of cloudy weather, I use my telescopes sporadically. The Christmas and New Years' periods additionally result in family and friends getting together. That means that some clear sky conditions are also bypassed. Because of several days being available, I had time in which I could, and did, check the operation and possible problems on my EQ type telescope mounts and my telescopes.
From my previous blogposts you'll know that I much enjoy the public astronomy nights which SFU runs on clear Friday nights. I've been using my 3" refractor most often for our RASC participation. The telescope is not heavy and can be put onto the telescope mount quickly. As well, it is easy to align with the celestial North Pole. The reason, of course, is the automatic tracking of any object being observed by moving the telescope westward at the same rate as the Earth's rotation eastward.
The EQ4 tracking mount I use for the 3" refractor has somewhat of an issue though. No matter how closely I align the telescope with North, the object in the telescope's field of view slowly drifts out of view. It's easy enough to correct this by using the control buttons on the Dual Axis Motor Drive which powers both the Right Ascension and Declination motors. However, it requires my periodic attention to make sure that our public guests will actually see the target to which I pointed the telescope.
There are many cloudy days and nights at this time of year. I set up the telescope and mount in our recroom at home to move through a 24 hour tracking run. The idea was to set the time and direction scales on the mount to a start point (I pointed the E marker on the telescope RA axis to the 0 hour mark on the time scale) and to read the scales at the end of the run, 24 hours (actually 23 hours, 56 minutes, and 4 seconds) later. At that point I would have expected to see the E marker to point again at the 0 hours mark. Lo and behold, it pointed instead to 2h and 50 min beyond the 0 mark. This means that the telescope actually indicated that it moved past the 0 hour time marker; it incorrectly showed that it had moved a total of 26 hours and 50 minutes. The tracking motion of the mount is therefore 11% faster than the daily sky motion. Since the Earth turns 15 degrees per hour this result moves the telescope to a position almost 45 degrees ahead of the position it should have after 24 hours tracking, Instead of moving through 360 degrees, it moved almost 405. I had noticed smaller overruns on other days, even on much shorter test runs. This is what actually made me run the 24 hour test.
My motor drive unit has 4 control buttons to speed up the RA and DEC stepper motors 2, 4, or 8 times by means of a separate speed switch. Each button is dedicated to one particular direction. The RA motor is controlled by an East button and a West button, the DEC motor by a North and a South button. The East button can be used to compensate for the 11% excess tracking speed, but it requires me to push and hold the button to move the telescope back to the observed target. It is a very basic and very slow slewing system - it would take many minutes to move from Vega to Deneb for example. As it exists, this whole system is not suitable for long-exposure astrophotography, in my opinion. It's ok for visual observation and possibly photography of the Sun or the Moon.
proposed astable timer (below)
Now the question: what causes this? I was thinking about the gear ratios in the RA motor, or in the mount, the clock frequency in the electronics of the Dual Axis motor drive unit, the pulse rate applied to the RA motor stepper windings, searched for an adjustment capability in the electronics board (found none), and other electronic possibilities. I also searched the internet for electronic schematics (there are dozens) but did not find the one which exactly matched the Dual Axis motor drive above.
I'm thinking of putting together an astable timer based on the 555 precision timer chip. Its output voltage could control the power provided to the motor drive, i.e. shut off power 11% of the time. That way, the movement of the sky could catch up with my faster-moving telescope. It's a crude approach, but in my working days I built many similar timers to control printers, vending machines, debit card readers, and other equipment. I found the timer shown above in one of my "junk"drawers which contain all manner of parts, most of which are decades old. The shown timer is an unused piece I started about thirty years ago, and never finished. I'll check it out and may modify it to counteract the tracking error.
Stay tuned. .
