Click on each image to get a larger one.
Description:
The base:
This part of the telescope includes the RA axle, th motor drive and
of course the three feet which are 16mm threaded rods. For polar
alignment,
the south rod can move to adjust the latitude. There is no adjustment
in
azimuth and I need to push the whole telescope. This can be done when
the
feet are on a wooden board but it is not easy and it's difficult to be
precise. I will need to improve this part.
The axle is the rear axle of a 2 CV (a popular car here in France which
is not build any more). It is a hollow steel tube with has 65mm outside
diameter and 5mm walls. The old bearings (from the car) were replaced
with
new ones.
On this view we can see the RA drive but I will give more details at
the bottom of this page.
Under the polar axis there is some place where I will be able to put
a small battery.
This part weights about 15 kg.
The fork:
The fork fits on the RA drive and can be tightened with a bolt. The
black disk on the side is a counterweight. On the picture we can see
the
openings where the declination axis comes and the brake on the side.
The
fork is made out of 15 mm plywood (used in double thickness in some
places).
Cutting the wood has been difficult because everything needs to fit
perfectly.
The final assembly required a lot of hand work with a file. The boards
are screwed and glued together.
The fork weights about 12 kg.
The OTA:
The walls are 10mm thick. A door at the bottom of the tube gives
access to the mirror. The inside diameter is 270mm. The helical focuser
can accept 2" or 1"1/4 eyepieces as well as a camera at prime focus.
There
are a lot of small holes in the OTA to attach piggybacked cameras,
counterweigths
guide scope, finder, ... Since I didn't know exactly where I wanted
them
to be, I drilled a lot of holes nearly everywhere on the tube. The
finder
scope has a 44mm diameter. It is made of PVC pipe and an old achromat
(probably
from binoculars).
There are 2 ball bearings in declination (50mm outside diameter).
At this time, the movement in declination has no motor (but I will
add one later). When the telescope is in use the brake is not tightened
strongly. When th OTA is well balanced the brake adds only some
friction
to the movement and the telsecope is moved like a dobsonian. You push
the
tube where you want and it doesn't move any more when you stop pushing.
There is no need to use the brake knob in the whole night. The manual
control
is used when guiding astrophotos or for recentering at high
magnification.
This is made possible because the brake disk turns around the
declination
axis (there is some teflon to alllow a smooth movement).
The OTA weights about 12kg.
The RA drive
A stepper motor is used to make a stainless steel threaded rod turn.
A nut which cannot turn translates on the rod and tracts a ribbon which
is attached to a sector of a circle (fixed on the RA axis except when
doing
some manual moves for finding objects).
Every 2 hours, I need to make the nut come back to its starting
position.
This is done with the motor running at full speed in the reverse
direction.
This takes some time and I need to improve this.
The stepper has 200steps/revolution. It is used with a 25:1 reducer.
The threaded rod (14mm in diameter) rests on 2 bearings. The nut (2
nuts
in fact) cannot turn. The arm has a ball bearing at its end which rolls
on a flat piece of aluminium. The weight prevents the nut turning in
the
other direction.
The nut tracts a steel ribbon which is fixed to the wooden sector of
a circle. On the back of the sector there is spring which applies a
force
opposite to the traction of the ribbon.
The sector can be tightened to the RA axis. It is used in the same
way
as the declination brake. It is possible to move the telescope manualy
and when I stop pushing the telescope there is enough friction between
the sector and the axis for the motor to make the telescope move at
sidereal
speed. All in all for visual use, the telescope is used in the same way
as a dobsonian with the added advantage of the motor drive. For
photography
however, the brakes are tightened so that there is no accidental move
during
the exposures.
Finally, this telescope is stable and easy to use. The only drawback
is its weight (more than 40 kg). It is already satisfying for visual
use
but I need to make some improvements for long exposure astrophotgraphy.