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![[Image]](pict64.jpg)
![[Image]](pict47.jpg)
Above is a picture of Geoff Regan and Dave Owen with the recently completed 30 inch (TROK) at Lynn-Brenig, North Wales,UK on Suday morning May 3rd 1998. (Dave Thomson is taking the photograph).
30 INCH DOBSONIAN TELESCOPE
(TROK)
The 30 inch dobsonian telescope shown was designed and built by Dave
Thomson, Geoff Regan, David
Owen and Steve King (also named after its team
members, Dave Thomson, Geoff Regan, Dave Owen and Steve King). It has taken
us fouteen months of spare time work (mostly on Saturdays and Wednesday evenings)
to complete both the optics, tube assembly and mounting.
The 30 inch mirror was ground and figured by ourselves and is made of 1 inch
thick plate glass. The glass was bought from a local firm in St Helens as
a 30 inch nipped edge blank and then hogged out to the approximate curve
in my garage using a 22 foot long scaffold pole acting as a radius arm. One
end had an adjustable ball joint fixed at the wall at one end of the garage.
A 6 inch carborundum wheel, bearing assembly and black and decker drill was
at the business end with a constant water slurry running over the wheel for
lubrication.
It took 6 months of Saturdays to hog out the mirror to a depth of 10.5mm
in the middle. This gave us 3/4 of a bucket of ground out glass! Tool wear
was counteracted by adjustment of the ball joint at the other end of the
radius arm. Two 6 inch wheels where used to hog out the mirror. The curve
was measured to be accurate to 1/4mm over the entire surface which was deemed
acceptable before going on to the next stage.
The rest of the grinding and figuring was done mirror face up ontop of an
old oil drum in my garage. We used a sub-diameter tool to take out irregularities
from hogging out. This was done by fixing 4 layers of 3/4 inch plywood together,
covering the surface with car body filler and then pressing into the upturned
mirror. This gave the approximate curve of the mirror for the tool. It was
then covered in epoxy glue and 2 inch metal washers were arranged on the
surface. 60 to 320 grade carborundum was used to even out the surface prior
to fine grinding with 500 aluminium oxide. The mirror was polished with a
similar tool but being slightly larger and covered with optical grade pitch.
A fast acting slurry of cerium oxide and water was used to bring the mirror
to a polish. Figuring was done with a smaller 8 inch glass tool covered in
pitch. This was done by progressively deepening the centre to the required
parabolic curve. Focult, ronchi and zonal tests where used extensively to
keep a check on the figure. Figuring proved to be a difficult but possible.
Plate glass can respond dramatically to temperature and enough time has to
be left between work sessions and testing to allow the mirror to stabilise.
The reason for building such a large telescope was inspired by reading about
the spectacular views you get through these instruments. The popular American
magazine Sky and Telescope show these large instruments being used in star
parties in the States. The costs of buying such an instrument are prohibitive
so the amateur telescope maker has to turn hand to whatever he can lay his
hands on. The total cost of the telescope was £700, cheaper than some
commercial equatorial mounted 8 inch telescopes. It is not extremely difficult
to make a mirror but it is time consuming. Literature from library books
and the Internent where used and help and advice from many people have helped
to get us this far. If you are keen then give it a go, we did, this is the
first mirror we have ever made.
![[Image]](overview.jpg) |
Above shows an overview of my garage.This shows the oil drum holding
the mirror cell and mirror (face up)
Behind on the work bench, the test rig for holding the mirror during testing. |
If you would like to see more information and images of the making of the 30 inch TROK the click on any of the links detail below. I have arranged the links in order of build:-
Introduction and
concept
Purchasing the
blank and hoging out
Testing the curve
for accuracy
Making the tools
for grinding by hand
Evening out the
irregularities left by rough grinding
Polishing and
testing
Making the mount
and tube assembly
Making the secondary
flat
Aluminising the
primary mirror
First
light
Dave Owens notes
on the making of the Mirror
CREDITS:-
This project would not have been posible without the help, however large or small, from those listed below and others yet to be mentioned. Many thanks to them all. A special thanks must go to our long suffering families for being patient, supportive and also in helping out during construction (i.e. plenty of cups of tea and cakes!).
Specialised Glass (For the glass blank being cut so well)
Steve King (Helping in many ways all the way through, Structural work, grinding,
polishing etc)
Phil Leighton (For Ideas and help in figuring)
Andrew Bates (For ideas and help)
Peter Drew (For ideas, encouragement and glass for the secondary)
Yvonne West (For making the light tight sock to go over the tube)
Phil Harman (structural work)
Steve Southern (Pictures and helping with grinding work)
The Gerard Brothers (For Ideas in mirror making)
TE Heughs & son (Castors, nuts bolts, tools etc.)
Layton fasteners (Nuts Bolts etc.)
Craven Engineering (For the focus mount)
B&Q (Taking time out to cut the Tube and mounting).
Orion Optics (for advice and grinding materials)
LATEST NEWS !!
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After a few years of using the 30 inch we have gained a lot more experience
making mirrors and complete telescopes. Some of the achievements can been
seen on the
Telescope Making
Home page .
We have been very pleased with the 30 inch, but it is only usable at low power. This is not a function of the glass but of our own limitations at that time. We are taking this experience and using it to re-make the primary to hopefully get a 'Good' (Mel's rating) mirror form a 38mm blank just purchased. We hope to get started early next year. We have some materials for a new LARGE grinding machine also. Left is an image of the blank that had been hiding under someone bed for many years ! Now it will have it's time reflecting photons into our eyes !! |
LINKS TO OTHER SITES:-
My Telescope Making Home page MORE STUFF ADDED ON MAKING THE NEW 16 INCHERS !!
LASSA Liverpool Astronomical Society Sidewalk Astronomers Home page
Click here for Links to many other Interesting Astronomical sites !!
NORTH WEST ROCKETS (my high power rocket web site)
Let me know what you think about my page. Send mail by clicking here.
This page has been referenced times
since 4th of January 1999