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
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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.
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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.
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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. |
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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.
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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.
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Software
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. |
