Hugo's 315mm f5 Newtonian on a Split-Ring Equatorial Mounting.
The Mounting started life as a sandwich of two sheets of 12mm plywood
and one central core of 14mm MDF.
The three sheets were glued together with two part epoxy resin known
locally as "Bote Cote" due to its use in the manufacture of small
boats. As shown here, the two declination circles were cut out of the
main ring. Not shown, is the backing piece for the base which was cut
from the centre portion of the polar brace on the right. Before the
cutouts were made, a router was used to shape the ring using a circle
jig on the router. The edge of the ring was coated with fibreglass and
epoxy before giving it a final cut with the router slightly larger than
the first. Once the centre pivot is no longer needed, the centre piece
can be cut out, along with the dec circles.
The Dec circles can then be edged with a router, lined with fibreglass
and carefully shaped so that they are identical. The centre point will
be the place to attach the dec pivot bearings later.
The Polar brace can now be screwed to the Split ring at right angles
and held square with an offcut on each side. The declination bearing is
placed at the junction of the split ring and polar brace. To construct
it, I assembled it together with the dec bearing clamps as shown below
and carefully marked the centre, cut it using a hole saw and hollowed
it out with a router set to the thickness of the roller bearings. Note
that a small piece had to be chiselled out of the split ring bearing
recess edge, so that the telescope can be placed (and removed) easily.
The bearing was fitted using a bead of car body putty so that it fit
snugly and can be clamped firmly.
After this it can be disassembled, coated with epoxy or varnish as
desired then reassembled with screws.
The base (below) was thrown together using 90x35 timber with the north
bearing housed in a small recess in the back plate.
A 30mm pipe is used as a polar axis. It protrudes out of the centre of
the polar brace and fits into the bearing. I bored the hole in the
polar brace with a 30mm spade bit to a depth of about 75mm and epoxied
the pipe in place.
The base unit contains the drive system. The wheels (roller skates) are
positioned to elevate the ring to the correct angle corresponding to
your latitude, in my case, 25 degrees. I don't have any dimensions for
this - I just trimmed everything untill it was right.
Having it this way meant that the ring assembly can be easily removed
from the base. This makes transporting and set up very quick.
The drive system consists of a stepper motor salvaged from a floppy
disk drive. It is connected to a Tamiya gearbox with a rubber cylinder
salvaged from a photo print roller to reduce vibration. The gearbox
(80:1 ratio) is coupled to the drive axis (an 8mm bolt) with
brass gears from a mechanno set. Chuck in odd bits of
MDF, roller wheel bearings and you have a telescope drive!
Note that the drive wheel is free to rotate and is clamped to the shaft
using a cork lined plate with a wingnut when the motor is used. It is
shown below during preliminary setting up, before it was secured in
Here are the two parts of the mount put together ready to accept the
Originally, the mount was designed for a 250mm telescope. However the
mirror, purchased on ebay, turned out to be a shocker. So my family
bought the 12.5" f5 mirror from Discovery Optics in the U.S.A. and I
had to do a little butchery and creative design to make it all fit
The telescope is fabricated from 20mm aluminium tube. The mirror box
panels are 16mm melemine shelving. The two dec circles are attached to
the panels with internal screws and the dec bearings are held in place
with coach screws in a 30mm wooden plug as a centre support.
A trolley makes transporting it easy.
The telescope is lifted into the mount by hand. The weight is
sufficient to hold it in place, but I have two small bearing clamps to
secure it and provide a solid backing for the dec friction clamps below
the polar brace. These clamps are made from a kitchen cutting board and
a bolt that presses the board against the dec circles.
Here is a view of the dec clamps in place. The telescope mirror box has
a fan (from an old computer power supply) to assist in cooling the
mirror at night.
The secondary is made from alumunium tube supported by galvanised steel
vanes. This secondary holder is designed for two functions. One, to
support the secondary mirror in the usual fashion. Two, to support a
SAC8-2 CCD camera for digital imaging when the secondary is removed. A
single screw holds the secondary mirror assembly in place and it can be
quickly removed for imaging sessions.
Since I use this "prime focus" position for CCD imaging, the secondary
support is hollow and is accurately collimated to the optical axis
using a laser collimator. This means that the CCD camera has a clear
and unobstructed view of the primary. Since the camera is the same
diameter as the secondary mirror, the cenral obstruction for the mirror
is made no worse, and I avoid the light losses from the secondary
mirror. It happens that the camera is about the same weight as the
secondary mirror assembly. Therefore swapping the two makes for no
adjustment in tube balancing.
The only problem I have is that, with the "scraps and bits" approach to
low budget telescope construction, I can only manage up to 4 second
exposures before tracing errors appear in the CCD images. However the
SAC8-2 CCD Camnera is a very sensitive unit!
Below is an example of what 9 x 4 second exposures can produce. This is
NGC5128, the centaurus A galaxy.
SAC8 with telephoto lens
The Fine Print !
This HTML page and all contents, text,
is the sole property of Anthony M. Hugo MIEAust CPeng, copyright
1999-2006. Permission is hereby granted to copy this material for
or educational use if the teacher isn't looking. Commercial use by
entities, colourful sporting identities and all you rich folks requires
specific permission from me. So there!
Copyright symbols, logos and abbreviations
by copyright owners, logo designers and abr. ppl. Whatever happens,
happens. But only sometimes!
I started life with nothing, and still have most of it left!
I always wondered whether anybody ever
reads the fine
print at the bottom of documents like this. My advise is to use an