Telescope making 1997 - 2000

TM logo Telescope Making
Web Ring
Next | Previous | Random | List Sites
Next 5 | Previous 5 | Join


In the beginning...

It was a fine sunny day in the summer of 97, and I was sitting in a beer garden with my friend Barmy.

Since we're incredibly sad people, we were of course talking about physics, and were wondering if there was a quick and easy way to produce a mirror for a newtonian telescope. We came up with a few ideas, such as spin-casting epoxy resin, or vacuum forming mylar film, and decided to look on the net (since somebody must have already thought of all this in their own beer garden somewhere else in the world). It was there that I found the ATM (amateur telescope making) mailing list.

I posted a tentative email, asking if our ideas had any merit, and found that the list was full of friendly intelligent people, who were more than eager to help. I received various replies telling me how mylar, despite being shiny, wasn't truly smooth enough for a mirror of quality sufficient for astronomical observing, and that the epoxy idea had been tried by several people, and only scientific labs had come close to finding the magic formula of an epoxy that set uniformly with no thermal effects, and left a smooth (or polishable) surface.

Fortunately they didn't stop there. My eyes were opened to the world of glass pushing, the one and only (well, almost) true way of making mirrors for telescopes. And to my amazement, I was told that it was possible for normal people to create mirrors with parabolic curves that were accurate to within a fraction of a micron. I bought a book (recommended by the ATM list), and picked up two glass blanks and some grinding grit from Beacon Hill Telescopes, and started on my first ever telescope.

 

 

The mirror.

The glass blank I had was almost 10 inches in diameter, and the f6 newtonian that the books recommended doing as your first scope seemed like a good start (I wasn't much of an astronomer then, so I really didn't know whether I wanted a deep sky telescope, or a planetary, or an rft scope - at this stage I was more interested in the optics rather than using the thing. Maybe that sounds odd, but to put it into context at that time I was physicist working with optical experiments - lots of lasers and mirrors. I was still in awe of creating such an optically perfect surface).

Most of the pictures here are towards the end of the process, the mirror is almost fully polished (you can also see the pitch lap in some of the pictures, where squares of pitch are laid on the tool that was used to form the curve in the mirror.

The pitch is soft enough to conform to the mirror exactly, while holding the final polishing agent, CeO). Looking carefully at the pictures you might be able to make out that the mirror is still a bit grey at the centre, so there was still a little more polishing to go at that stage. Once that was done it was time to figure the mirror - basically the process of polishing had produced a spherical surface, but for a telescope I need a surface that was paraboloid. To test the shape of the mirror during figuring I had to build a foucault tester. This was the part that really appealed to my physicist side. The test was so simple it was hard to believe that it would work, and yet it gave the impressive precision required to create that perfect surface. The foucault test used a knife edge which passed through the focal point of the mirror. Any aberrations of the mirror caused light to be focused away from the ideal point of focus, and the while the knife edge blocked all the light coming from the 'good' areas of the mirror, you can still clearly see the 'bad' areas lit up (well, there's a bit more to it than that, but that's good enough for a starter).

 

The rest.

I suppose since I was mainly interested in the mirror, it wasn't surprising that the rest of the scope was not exactly a mechanical triumph. Of course, I'm aware of the exacting requirements that a telescope

needs to function properly, and I'm sure my telescope falls far below that. However, another reason for throwing together the rest of the scope out of any bits of junk I could find was that I become extremely interested in astronomy, and couldn't really wait to have a telescope. The way I see it, every improvement I make now will be a noticeable one, rather than a little tweak. I also didn't have access to any work shop, or have many materials to hand.

 

Present situation.

At the moment, my telescope still hasn't seen first light (sigh). Three years is a long time to be making a telescope (a really dedicated person could knock one out in a few weeks), but I've moved house five times since starting, and run out of materials a few times, and generally had several month long gaps.

Never mind, almost there now. The mirror isn't figured yet, although it is just about polished enough to start figuring. All I've got to do is put together a permanent Foucault setup, and I haven't really the room at the moment. The main mount and housing is pretty much done (apart from the spider and the focusser, but I might buy those). It moves fairly smoothly, but could be better. not far to go now.


The latest news is that Tim Walters has sent me a 16inch plate glass blank. It's on the thin side (only 1 inch thick), but according to the atm list, that's not impossible to make into a mirror. This'll be somewhat into the future, but one day...


TM logo Telescope Making
Web Ring
Next | Previous | Random | List Sites
Next 5 | Previous 5 | Join