TC400 Telescope Drive
What is isn't
The TC410 is not a complete "goto" system that you might get from
commercial telescope makers like Meade and Celestron. It has no
knowledge of geographical position or siderial time, it doesn't
understand equatorial coordinates and it doesn't have a database of
objects that it can "go to".
Despite this is can be used stand-alone as a pure "dumb" equatorial
drive system since the tracking and 2-axis joystick control are all
that's needed for this job. Of course it could be made into a complete
integrated "goto" system by just throwing more software into it and,
should the need ever arise, then that will undoubtedly happen. The
thing is that we have no need for this functionality.
It is not a retail product and as not been through a certification
process to determine whether it passes EMI and safety standards.
What it is
In a nutshell, it's a dual-axis motor controller. In fact it knows so
little about astronomy it could probably be used to drive things other
than telescopes. This was designed from the outset to be the drive
component for a larger automation system. As such there is always a
computer running the show so why should the embedded software do what
is easier to do on a desktop computer?
The TC410 is an evolutionary version of the TC400 with a better integrated design and revised embedded software.
The motor coils are driven by switch-mode
current drivers. The maximum phase current is software-configurable
between 1.0A and 7.0A in steps of 0.1A. Motors are microstepped at
1/10th of a full step for the Gecko (on the secondary axis) and either
1/16th or 1/1256th of a step for the IMS (on the primary axis).
Maximum step rate is 20,000 microsteps/second which, on a a typical
200-step motor is 10 revs/second (600 rpm). 1 arc-second steps require
a drive ratio of 1:648 which permits an equatorial tracking rate of 15
microsteps/second and a maximum slew rate of 5.5 degress/second.
Tracking smoothness and maximum slew rate can be traded off by
appropriately designing the drive train. Velocity is controllable to a
resolution of about 1/1000 microstep/second allowing theoretical
maximum tracking drift of about 1 arcminute/day.
Maximum motor velocity is soft-configurable (per axis) because maximum
theoretical speed is not necessarily attainable with a constant-power
motor. Similarly, acceleration and deceleration are independently
configurable for each axis to allow suitable values to be chosen for
the intertia and responsiveness. Slow acceleration is generally
preferable but is annoying when driving manually with the joystick.
Velocity control is prioritized as follows:
Slew-to-target is used by software running on the main computer to slew
to a position. The computer calculates the axis positions appropriate
for the object and the TC410 automatically accelerates, coasts (if
necessary) and decelerates to the target position.
- Slew to target (goto mode)
- Remote speed set
- Joystick input
- Autoguider input
- Default track rate
Joystick input is normally logarithmic allowing guiding speeds with
small joystick deflections and slew speeds with large deflections.
The autoguider supports analogue inputs (similar to the joystick input) or digital.
A default track rate can be set-and-forget if the telescope is mounted
equatorially, otherwise the computer can periodically update the
tracking velocity for non-linear two-axis tracking.
The TC410 only keeps position in units of motor steps about each axis.
All conversion to other coordinate system is done externally.
Internally, the (preferred) position is keep to 10^-8 of a microstep to
allow low cummulative tracking errors but, of course, physical position
resolution is 1 microstep and this is what is reported if the position
Encoder inputs support up to 16-bit Gray code. Relative/incremental
encoders are not supported. The main use of encoders is so the system
knows where it is on power-up and to re-establish position in the event
of a drive anomaly (eg motor stall, clutch slip). This includes
allowing the telescope to be pushed (by hand) without losing its
bearings. The encoders are not processed in any way by the TC410 (it's
basically just a unified interface for them) so their integration into
the feedback loop is entirely done with external software. Typically if
the motor position differs from the encoder position by some threshold
and the telescope is stationery (or just tracking) then the motor
position is reset according to the encoders.
Currently, we only have (GUI) software for RISC OS. We have a
stand-alone RISC OS driver which allows the drive to be configured,
monitored and controlled as well as providing a two-way communication
channel to ROCchart (our
RISC OS star chart software). This allows the telescope's position to
be shown in real-time on the chart and for the telescope to be driven
to an object by clicking on it.
Technical manual (revision 2, 638K pdf)
RISC OS stand-alone driver (194K zip)
Partial source code for RISC OS driver (71K zip)
ROCchart (RISC OS startchart supporting TC410)