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This is the motor unit showing the rubber drive wheel and the two
idler wheels that will keep the rubber wheel in firm contact with the
track. One idler wheel is on a hinged arm with a
quick release mechanism to allow dome rotation to be done by
hand.
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4th July
2011
Aluminium bracket now made and fitted
to the interior of the dome. The bracket holds the 12V battery and supports the
motor. |
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Quick release latch in open
position |
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Quick release latch in closed
position |
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One complication is that at each join in the fibreglass segments
there is a raised section that can interfere with the motion of the motor
assembly. These will have to be smoothed off. |
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Picture below shows the offending bump removed. I first tried
using a plane. Not easy on fibreglass. Then a Dremel with a metal cutting bit.
Hard to control and masses of dust. Then I tried a hacksaw. Not enough
distance available to get a to and fro action. I then tried the Dremel with
a miniature silicon carbide cutting disc. Perfect! It cuts smoothly and quickly
and there is less dust. I used a vacuum cleaner
held near the cutting disc and the dust problem was gone. The other three
joins are much smoother than the first one.
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7th July The rubber
wheel does not provide enough grip to rotate the dome efficiently. It works,
but there are times when the wheel slips, so this method is now
abandoned. This
company supplies timing belts in lengths up to 50 metres, so
I have ordered 8 metres, and a suitable pulley. This will give a much stronger grip, and should be able to rotate
the dome smoothly.
12th July
At the 4
joins the fibreglass is thicker, something like 6 - 7mm. Other places are
closer to 4mm so each join has to be thinned down a bit.
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Filled in with car body repair epoxy and smoothed. |
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The inner surface of the fibreglass was rough and dimpled - to
ensure smooth running I filled and sanded the whole track. |
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... and with a couple of coats of paint. |
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8th August 2011 The rubber
wheel now replaced by a timing belt pulley. Rob Januszewski at
Epsilon
Telescopes made the central locking shaft to
fix the wheel to the wiper motor. |
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The
timing belt pulley runs on the inside of the track. This is because it
will be much easier to glue the belt on the inside. At present there is only one idler wheel, running on the outside
of the track. The motor assembly pivots around the idler wheel, and there
will be two tension springs in the positions shown to keep the pulley wheel
firmly against the belt. Trial runs using elastic bands show that this will
work.
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12th August 2011 Close
up of the belt and pulley wheel. The belt was glued on with superglue which
sets alarmingly quickly. There is almost no time to reposition the
belt. |
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This video
shows
the first trial run. An elastic band does the work of the tension
spring. |
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13th August
2011 Tension springs at the left. The
aluminium strip folded over the plywood can easily be slipped off to allow
rapid slews by hand. (I had intended a second
spring at the right hand side but it may not be necessary). |
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This video
shows
the dome rotating well.
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28th August
2011 A small job to make life easier. The shutter opening rope often
slipped off the pulley. This aluminium attachment keeps it neatly in
place.
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Note the extra bolt. Only one is
supplied and it is impossible to tighten it up enough to avoid
movement.
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4th September
2011 This is the first try at wireless
remote control. The receiver is from a toy car, operating at 40 MHz.
When it gets a signal from a joystick control, or the IR sensor, it will
activate the motor. The device does what it is supposed to do, but the range is
limited, and even a metre or two distance between transmitter and receiver
causes intermittent performance. I will move to a
433MHz system with a controller normally used to open garage doors and
the like.
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In the
meantime, I have cobbled together a simple timer that will rotate the dome a
small amount every 10 minutes or so. The box to the left of the battery
contains the electronics. The toggle switch sets the direction of rotation, and
the dial allows the speed to be varied, according to where the telescope is
pointing. Experiments are now under way to
determine how often the dome should be moved, and how far each step should
be. |
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How
fast should the dome rotate? A first try at finding out involved cutting a slit
in a paper sheet to represent the dome aperture, and timing how long a star in
the planetarium software Stellarium
would take to cross the slit. This was not done in real time but by observing
the time reading on the screen, and the star motion could be
accelarated. Readings obtained were accurate
enough, but to cover the whole sky would require many hours work. Places where
the star crosses the slit at a steep angle were also difficult to
evaluate.
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The
next attempt also used Stellarium, which allows both an equatorial grid
and azimuthal grid to be displayed at the same time. In the picture below the brown grid is Azimuthal and the blue grid
is equatorial. The star is positioned at 220° azimuth and
25° altitude. The precise declination of the star is
displayed.
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A
screen grab pasted into Photoshop allows angles to be measured. In this
example, for my latitude, a star at Az 220° and Alt 25°
is seen to have a declination of -5° 01' 42" and is moving at
23.1° to the horizontal. The horizontal
speed = maximum sidereal speed x COS(-5° 01' 42") x COS 23.1°=13.74
arcseconds per second. This method is far more
accurate than the first, but is still very time consuming.
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The
ideal solution would be to generate all the angles
mathematically. Fortunately the spherical
trigonometry involved is not too difficult. In the
diagram below the blue lines demarcate a spherical triangle which obeys a
simple cosine law:
cos(PX) = cos(PZ)*cos(ZX) + sin(PZ)*sin(ZX)*cos(azimuth)
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An
Excel spreadsheet allows all the angles to be speedily calculated. It is then
quite straightforward to calculate a conversion factor for how to set the dial
on the motor control box. When the star motion is almost parallel to the dome
slit, or very slow near the Pole, a value of 1.0 is given. In these
positions the dome motor could be switched off. The Eastern hemisphere has identical values. First trials worked very well and the dome has kept pace with the
telescope for over 3 hours on one motor setting.
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Although it is most unlikely that anyone would aim an astronomical
telescope at the horizon, it is interesting to note that in that position the
horizontal motion of the stars remains constant for the whole 360 degrees. In
the above example for altitude = 0 the motor setting is 8.5 for all positions
of azimuth.
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17th September
The
433MHz transmitter and receiver arrived (£8.90 from
Singapore).
The
right hand pin is Vcc, 3rd from the right is ground. When a
button on the transmitter is pressed whatever is on the 2nd pin is transferred
to one of the output pins.
Connecting the 2nd pin to Vcc via a resistor is an easy way to get
a logic 1 transferred to the outputs, which is easy to interface to the Picaxe
microcontroller.
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See this
video
to
see the dome operating remotely.
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19th September The
completed installation. Wiring tidied up, and an ON/OFF switch provided
at the right hand edge. No more danger of hooking
up the crocodile clips the wrong way round... |
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28th September The latest
addition to the hardware is an Infra-red security camera. This is wireless and
runs at 2.4GHz. The telescope and dome aperture can easily be seen from
indoors, and the 433MHz link enables the dome to be rotated when necessary
from the comfort of my study.
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4th October There is a problem
with using an infra red camera to monitor the dome position - the infra red is
picked up by the imaging camera and produces some nasty gradients. This can be
seen at the right hand edge of this image of the veil
nebula.
I have tried blanking off some of the
IR LEDS, but if there is so little IR that no gradients are produced, then
there is not enough light to be able to see the telescope and dome. The
answer may be to have a system that can switch the IR camera off while imaging
and turn it on at intervals to monitor the dome. This will require additional
remote capabilities and may not be worth the effort!
Test image. Most of the light
causing the smeared gradients is getting into the telescope through the vent at
the rear. This image is 10 minutes exposure with the dome shutter
closed.
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