Manufacture
of a Stevick-Paul telescope.
I have always wanted to build a scope for lunar observation and got
really excited when I came across the “weird Telescope” pages and saw
some of the long focal length beasties featured there.
I was at the same time
playing with “winspot” software and was amazed to see the theoretical
optical performance possibilities of the stevick paul design, and all
without complex toroidal mirrors or flexing yokes (scary things)
I should point out that
although I am practical and have a mechanical / engineering background,
I have never ground a lens or mirror before and my only experience with
telescope manufacture is a 125mm newtonian built with optics purchased
from IPC in India (fantastic company, service and prices)
There isn’t a web site
to do with ATM that I haven’t poured over in the last 18 months, and I
have read “advice” on mirror grinding that ranged from “its easy, just
knock one up overnight with nothing but a glass blank and an old tile”
to “it takes years to get one right and it isn’t even worth trying”. I
decided that the world had room for one more sad git with nothing to
fill his evenings and decided to start ordering glass.
Having looked at the
maths on Mr. Stevick’s web page I decided to make a 10 inch aperture f15
scope. This I figured would still be just moveable on a decent mount but
was unlikely to instantly make me wish I'd built a bigger one. (go on
then, what criteria do you use?) I did the maths and came up with a
requirement for a 10 inch and 2X6 inch blanks. I was instantly lucky as
a nice man at vacuum coatings in London had the blanks that I needed,
and sold me the 10 inch with two six inch “tools” saving me some money
straight away…
…which I spent on a
Rigel finder that same afternoon (awfully good aren’t they?)
The glass arrived and a
few days later so did the grits and pitch kit bought from Beacon Hill. I
laid it all out, checked it over, checked my maths, cleaned the corner
of the garage, got scared, and put it all in a box for three months
while I tried to convince myself it had all been some bad dream.
It took me a long time
to talk myself into actually starting, and it was that which prompted me
to write this. You see, with all that “good advice” from the web
spinning in my head, I could not see a clear path to achieving a
finished mirror, let alone a working scope.
When I got the kit for
the earlier newtonian I had built, it had included a booklet from Edmund
Scientific Co. which I had largely ignored as my optics came completed.
It was while reading it later that I realised it had all the friendly,
picture filled advice that I could possibly want to get started.
The very next day I
attacked the web anew, this time with specific questions regarding tool
construction, and came away with the information required to build a
tile tool.
I decided to attack (?)
the 10 inch mirror first as it was going to be main mirror and if I
couldn’t do that I could at least sell the other blanks.
I used repair cement
from local builders merchant as it was strong, cheap, quick setting and
very heavy (less manual effort required for grinding). I made a wrap for
the mirror blank from cardboard and cling film and poured the mixture
straight onto the surface of the mirror smoothed off at about an inch
and a half thick and left to set.
Once the tool was
completely set and dry (about three days) I separated it from the mould
and blank and turned it up the other way for tiling.
I had some small tiles
lying around which I cut into approx. inch square pieces, then I coated
the surface of the tool with a waterproof tile adhesive and stuck all
the tiles on upside down (no glaze to scratch) to the surface. I placed
the mirror blank back on top with a heavy weight and left to dry.
Isn’t it funny how wrong
good advice can be?
Nobody had ever said, on
any web page I had visited, how it is important to check the shape of
the surface of your blank before you use it to mould a tool!
When I started out with
the 80 grit all excited at full of vim and vigour, I was doing so
unaware that I had a convex blank and therefore a concave tool
See if you can guess how
long it takes to correct that by grinding? No you’re wrong, you’re not
even close.
I re-read all the web
pages about tool manufacture and not one of them mentioned checking the
blank, except I found a couple that talked about pre-formed curves. The
penny dropped. Everybody (else) buys blanks with pre generated curves so
they don’t need to worry when it comes to making a tile tool.
My tile edges wore out
at the same time as I achieved a flat blank so I ground as best I could
until I got a close to sagitta (I was aiming for 63 thou). I then
cleaned the tool thoroughly and used epoxy to seal the tool and stick
steel washers all over the surface. Sound crazy I know, but several web
sites talked about it and I figured that I was only on the rough grind
so what’s the worst that can happen?
Nothing bad happened at
all!
The blank ground out
fairly fast and definitely evenly until I had frosted glass right out to
the edge of the chamfer.
By this time the sagitta
was a little too deep at 70 thou but fine grinding will take care of
that (?)
Tool No. 2 is required
as No. 1 tool has washers stuck all over it ( incidentally, don’t forget
to clean, grease and clearly label old tools with radius of curvature,
before packing them away for that future project, or the mirror you make
to replace this one that you dropped?)
I used the same process
to make this tool as I used for the first except I used plaster instead
of cement as I was sure I wouldn’t need as much effort to drive this
stage.
I used the mirror to
make the mould for the tool just as before, but this time of course the
curve had been generated in the mirror surface already. That’s got to
make things quicker right?
I was right, and with a
good collection of cd’s to listen to it only took two weeks of evenings
to go from rough ground to nice and smooth with the right sagitta,
progressing by one grit size each night.
I then made a third tool
from plaster and learned the joys of hot pitch.
Tool No. 3 is made in
the cleanest room you can find, on top of a cloth, sheet or board with
clean hands and when you are not going to be interrupted by anything.
Kitchen table is perfect but prepare to explain funny smells to anyone
who asks.
Once the plaster has
hardened paint it with a paint that has as strong a bond as possible,
PVA is a good start and then the idea is to prepare the tool for a short
but hard life immersed in water and being warmed and cooled continuously
without letting any of its surface chip crack or flake off (it will
bring tears to your eyes if it does)
Once you have achieved
that you lay the tool face up on the table with a strip of paper or card
taped all around the edge so as to make a tray with a depth of around
10mm. Warm the pitch in a container that doesn’t matter anymore, (I use
a bean tin with a file handle nailed to the rim) and pour it evenly and
carefully over the tool.
Use a blade or spatula
if you like to even out the surface as it cools and once it no longer
moves around easily, you are ready to place the mirror on top.
There are many different
approaches to putting the grooves into the surface of the tool, from
specially designed plastic matting, to a sharp knife. I decided that I
would simply press the grooves into the tool surface by hand using a
warm steel bar. I did this first before I placed the tool on top for its
first press and then again later to help open up the grooves after
subsequent presses.
I made a waffle pattern
with the grooves about 30mm apart, the important things to remember are
not to have junction at the centre of the mirror, and that you will have
to chase out the grooves several times before your mirror is finished.
Under no circumstances;
