ASTROTRACKER
A
Simple Equatorial Mount for Astrophotography
Ulhas
A Deshpande, Nagpur
A properly mounted camera
with capability of tracking the object being photographed is a basic
requirement of Astrophotography. However, such mountings are not easily
available in India and very expensive when available. While some mounts, which
can be constructed at home, such as the Scotch, or the Barn Door mount have
been described, these have inherent limitations. A different type of a mount
capable of carrying lightweight cameras with a telephoto lens is described
below. Most of the material required can be easily found in a bicycle parts
shop and a hardware store. Nothing more than simple skills with a hammer and
saw plus a drill machine are required to construct the astrotracker.
Material required: 1) A Bicycle Hub
with axle and its fixing nuts to make
RA axis
2) Wooden strips (Beading) 50 mm X 1 meter, 3
to 5 mm thick. These would be available
at a plywood supplier.
3) A Wooden Piece 50 mm wide 10mm thick 30
cms long
4) A Wooden Post 50 mmX50mm of length as
required for the support stand
5) A metal strip 3mm thick 25mm wide X 30 cm
long for the camera bracket
6) A GI strip 50mm
wide 15 cm long
7) A Metal Rod about
3mm dia. 20 cm long (use a coat hanger)
8) Assorted screws, nails
etc.
Tools Required: Hammer, Wood Saw, Round
File, Electric Drilling machine with Drill bits, No. 10 Spanner for axle nuts,
Cone Spanner (available at cycle shop) to adjust hub bearings, Pliers
Construction: (For details see figs 5 &6)
RA Axis: The bicycle axis, which will act as the RA axis, is fixed to the
support post by the GI strip. It may be necessary to cut a V shaped slot in the
end of the support post for proper fit. Secure the bicycle hub by the GI strip
screwed to the support post. Firmness of this fixture is essential
FIG 1
GI Strip around the Bicycle Hub, screw to support Post Bicycle Hub Support Post
Drive Arm: The drive arm is made up of the wooden
beading strips and is shown in fig.6.
Drill a hole of dia. 10-mm to fit the bicycle hub axis as shown at one
end of the strip. If a 10-mm drill is not available a small hole can be drilled
and filed to requirement. Make the guide slot at the other end as shown by
sticking pieces of beading together with Fevicol. The support arm, which
carries the drive mechanism, is made of wooden piece 10 mm thick. However the
end pieces should be pasted and screwed to the support arm for strength. Note
that the wooden beading strips are very hard and will crack if you try to use
nails. Drill holes and use screws.
FIG
2
The
drive handle can be made out of a metal hanger by straightening it and bending
to shape. The hanger rods are usually 3 mm thick The drive string is wound
around the handle, clockwise at one end and anti clockwise at the other. This
ensures that the string is released from one end when it is wound at the other.
The drive string is looped around a nail on the drive arm so it pulls or pushes
the drive arm when the drive handle is turned. The distance to this nail is
dependent upon the no turns per second of the drive handle and the diameter of
the drive handle rod. A distance of 22
CMS to the nail from the center of the axle and a drive handle rod diameter of
3 MM. will require a turning rate of one rotation per 10 seconds to track a
celestial object.
Camera Bracket: Rigidity of the camera
bracket is of paramount importance for proper tracking. The accuracy of the
bending is also important if correct tracking is to be ensured. The vertical
and horizontal arms must be at exact right angle It is therefore suggested that
the camera bracket be made out of a metal strip at least 3 mm thick, although
this will require a visit to a orkshop. Find a workshop which makes grills for
doors and windows, they should be able to help.
Counterweight: Since the mounting
requires the camera to on one side of the axis, it is necessary that a
counterweight be fixed to the other arm of the camera bracket to balance the
system. The counter weight is fixed by means of a long --200 mm--bolt so that
its weight can be reduced. The counterweight is made out of a Tin with a lid
(try a Panparag or Baba Jarda Tin or a jam tin).A hole is made in the bottom
and the lid of the tin. The tin is filled with small stones to make a weight of
about 300-400 gms. The tin is fixed to the bolt by nuts on the top and bottom.
This arrangement allows the weighted tin to be moved closer or further from the
camera bracket.
Assembly: Having prepared all parts proceed to
assemble the tracker as follows
1. Fix the drive arm support
to the support stand at 25 CMS. from the Bicycle Hub axle firmly by screws.
2. Insert the drive handle
through the end plates and secure at both ends by twisting a wire around it
just outside the end plates
3. Fix the drive arm to the
Hub axle. Ensure the handle is engaged in the slot at the bottom end of the
drive arm.
4. Wind the string on the
handle and tightly loop around the nail in the drive arm.
5. Fix the camera bracket to
the Hub axle as shown in fig 4
6. Fix the counterweight
bolt and attach the counterweight container.
Setting Up: To ensure correct tracking it is necessary that the axle of the
bicycle hub should point to the celestial north. However for most purposes
except very long duration photos, it will be adequate if the axis is aligned to
the Pole Star. The support stand will therefore have to be inclined by an angle
equal to the latitude of the observation site and the axle pointed to the pole
star. Some arrangement to fix the stand in this position will have to be made.
One such simple arrangement is shown below
FIG 3
Having set up the
astrotracker to the correct orientation fix the camera to the camera bracket.
The position of the counterweight is next adjusted to balance the system.
Loosen the nut holding the drive arm to the axle so that camera and weight can
rotate free. Adjust the position of the counter weight till the camera and the counterweight
comes to rest in a horizontal position. The system is now balanced. Tighten the
nut again.
Rotating the drive handle
now should rotate the camera. Select a bright object such as a planet or a
first magnitude star and point the camera at it by loosening the drive arm axle
nut and the camera holding screw. Center the selected object in the viewfinder
and tighten. If the camera is now left stationary the object will drift in the
viewfinder. Rotate the drive handle to bring the object back in the center.
Note the direction of rotation. Rotating the handle in opposite direction
slowly should now keep the object centered. The correct speed at which the
handle is required to be rotated has to
be determined by trial and error but should be close to once in ten seconds for
which the drive is designed. Once the correct rotation rate has been determined
you are now ready to load a film and take great astrophotographs.
Wish you clear skies and
marvelous astrophotographs