Finishing up the polishing of my telescope mirror

On Friday I continued working on my telescope project. Unfortunately, I didn’t have as much time to work on it as I did in previous weeks, because I had an after-work event to attend. But I did reach another milestone: I finished polishing my glass. I know I finished polishing because I couldn’t see any pits in the surface of the glass at 100X magnification. And now, some pictures.

Telescope 96

This is what my pitch lap looked like after cleaning out the grooves between the pitch segments using a razor blade.

Telescope 99

The surface of the glass after I finished up polishing. The marking and apparent gritty surface are all on the underside of the glass. The light is reflecting off the top surface of the glass. Note that no surface features can be seen on the top surface of the glass in this photograph (or indeed, even at 100X magnification).

Telescope 106

A Ronchi test on my glass. Note that the lines bow out at the middle, indicating that the shape of my glass is too oblate. In a paraboloid, the lines are straight in the middle and then bow out at the top and bottom.

Telescope 110

Another Ronchi test after a few minutes of a longer stroke on the pitch lap. Note that the lines are already much closer to vertical.

That’s as far as I got on Friday night, but of course, I’ll return next Friday to continue to do work. At this point, the figuring of the mirror is very precise and involves very short polishing runs. Basically, it consists of performing a Ronchi test, identifying how the shape of the glass deviates from a paraboloid, doing the appropriate stroke for a few minutes, and then repeating. Once I get really close we’ll switch over to running Foucault tests, which are more precise but take longer to do.

After I’m done figuring, all I have left to do is aluminize the mirror and then build the housing and mount. At this rate I should be up and ready just in time for the upcoming Mars opposition.

One Response to “Finishing up the polishing of my telescope mirror”

  1. Hans Says:

    Hi Cyde, H__ here. Nice to follow your progress here. I hope your project results in a fine scope.