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Phase 1: Automating A Celestron Compustar-14
The whole automation thing started with Gordon offering to house Carter Observatory's C14 which was returning from display at a convention. There had been partial success with automating it at Carter Observatory so we tried our hand by starting from scratch.

What is a Celestron Compustar-14?

The off-the-shelf product from Celestron consists of the optical tube assembly (OTA), the mount (both seen in above picture) and the telescope control computer (see below). The OTA is a C14, a 14-inch (36cm) Schmidt Cassegrain working at f/11. The mount is an equatorial fork design which is altogether too light for a 14-inch telescope; typical of a cost-conscious commercial offering. The flexure in the forks is perfectly acceptable for the visual tasks it was designed for but causes some problems when under computer control.

The control computer (seen right) is a now-obsolete design. It provides control of the stepper motors (the drivers for which are in a separate power-supply box) and generally coordinates the telescope's movement.

The C-14 installed at KPO.

The Celestron Compustar control computer 

Polar Alignment

The Compustar only works for correctly aligned telescopes as it cannot do the co-ordinate transformations required for an arbitrary (or even slightly-off) alignment. Alignment was done by successive approximation by pointing the telescope at a star in the normal and inverted telescope positions.

Cable Fouling

The Compustar is totally ignorant of the fact that there are cables of limited length running up the forks (including its own declination motor cable) and a simple software work-around was required to prevent the Compustar from tying itself in knots. A zero hour-angle position is defined as the "centre" and the cables prevent the telescope from moving more than 180° each way. When the software detects that the Compustar would slew past the 12 hour-angle point (slewing from the current place to the new place normally goes the shortest way) it simply does an interim slew to the meridian guaranteeing that a slew to anywhere cannot foul the cables.

Lens Dewing

A well-known problem with Schmidt Cassegrain telescopes is that the corrector plate at the front of the telescope can fog up. The traditional solution is to put a dew shield on - that is to effectively extend the tube well past the corrector plate. Space restrictions prevented us from doing this as the telescope could be as little as 15 centimetres from the dome in places (the pier position was intended for a German mount, not a fork mount). We tried a variety of heated and/or moving air designs which all failed to be satisfactory.
Some unsuccessful designs to combat this problem were:
  • A short (10cm) shield with a ring of resistors generating about 20W of heat. This kept the edges clear but the central annulus still fogged up! This may have worked for a smaller diameter telescope but not a 14-inch.
  • A pair of small fans to force air past the lens, the idea being that air wouldn't stay around the lens long enough to condense on it. So much for theory.
  • A resistor ring and fan pairing to distribute slightly warmed air over the lens. Partial success but no good for unattended operation.
The design we ended up with is shown above and never failed in its duties of keeping the lens clear during our care of the telescope. There is an air duct around the circumference of the lens with numerous holes through which forced air is passed. The air is ducted from an inlet on the side of the telescope using a fan from an old PC power supply. There is a heating element just in front of the fan which keeps the air warm enough to prevent it dewing on the telescope. The fan/heater unit is mounted on the side of the telescope (seen on top in the picture at the end of the hose) and piped up to the lens duct.

Originally, this is all there was to it until we noticed that the inside of the lens occasionally fogged up while the outside surface continued to be free of condensation. We combatted this problem by warming the whole front end of the telescope slightly with a heating element wrapped around the tube. The element is a car rear-window heater (!) running at about 30 watts (when suitably arranged in series). This can be just seen as a black strip (the rubber covering) behind the telescope's front rim. This is not as extreme as it sounds because of the area over which the heat is distributed. We had a previous version of this using an electric blanket element which worked almost as well but wasn't as tidy or as efficient.

You may think this would seriously effect seeing. If it did it was barely noticeable, even on planetary objects. Even if it did it wouldn't matter that much because a star can be as fuzzy as it likes for photometry so long as it stays within the bounds of the photometer's aperture.

Backlash and positioning problems

This caused us a number of headaches and most of the problem seems to stem from the worm and wheel polar axis drive mechanism. Some of the problem is that the wheel started to wear from use over its eight year life. This wasn't helped by the fact that the wheel is less than half the size that the rule-of-thumb dictates and the Compustar slews it at a speed of 12 degrees per second. This, perhaps, isn't bad in itself but it is accelerated to (and decelerated from) that speed in less than a second.

The worm gear is very difficult to set in the correct position due to a design oversight. The position adjustment screws for the worm/motor assembly are not accessible with the telescope fully assembled and requires adjustment on the bench. Unfortunately the flexure in the system means that if it set up correctly on the bench the meshing will be too tight when loaded with the full weight of the telescope. We decided to drill holes right through the base plate to make these adjustment screws accessible.

Then we found that the aluminium construction of the housing and motor support was flexing and while the worm did not move relative to its main gear the worm was moving relative to the mount! This explained why gear backlash was practically zero but we nevertheless had backlash of some kind. This would not be a problem for a uni-directional tracking system (where some backlash and pre-loading is actually recommended) but an APT needs to move back and forth without much error.

By this point we figured we had too many problems and decided at least a new worm/wheel assembly was required and probably a more substantial housing. This problem was to be addressed by retrofitting the telescope with a new larger and higher precision worm/wheel gear from Byers. The work was to be carried out at the Heretaunga Central Institute of Technology (CIT) faculty of Technology. The telescope remained at the CIT with its owner which has left us free to go down a more practical path with our own telescope. We don't know what became of the C14 after relocation at the CIT.


During all the fiddling with the hardware, the software evolved from nearly useless "flare star monitor" to the slightly less useless differential photometry program we finished with (before discarding everything).

It had access to several observing "sets" stored on disc each of which describes the observing requirements for a single variable star. It had a list of objects (variable, comp, sky etc), observing parameters and a sequence list. Some crude bolt-on facilities gave it access to power control hardware and the dome but they were never implemented properly.

A new replacement version designed to allow proper control and monitoring of everything was reasonably well developed but never actually got to the stage of doing observing. The screen shot below shows it used a DOS windowing type thing.

A Windows version was considered in the light of problems encountered in the DOS version but no code was written.

The working photometry software running under DOS.

The intended replacement multitasking and windowy DOS version which never saw the dark of night. :-)