10 inch Polishing Progress

After fine grinding was completed the working surface of the mirror felt silky smooth and would provide a dim reflection of bright objects if viewed at a shallow angle.

The next step (after a thorough clean up) was to construct a pitch lap to polish the glass.

I first built a new plaster base for the lap, then pained it with exterior varnish to seal it. Then I slowly melted the pitch, which took 2 hours on my small electric hot plate. The mirror was prepared by warming it in water and then covering it with a separating solution (a mixture of polishing agent and washing up liquid).

The moment to pour my first pitch lap was rapidly approaching, to say I was nervous would be an understatement. After a deep breath I poured the hot pitch and applied the mirror.

Unfortunately, a combination of just too little pitch and a very cold December morning meant that the pitch didn’t spread to the very edge of the tool. No amount of warming and pressing would make it stretch, besides I didn’t want to spread the pitch too thinly. After much deliberation I finally decided not to scrap the lap and give it a try. It was too small, and had several ‘craters’ where I’d accidentally pressed it with my thumbs, but it was my first pitch lap, and I was determined to use it.

I decided to use a soldering iron to cut the lap surface into facets. This was a slow and smelly process, that ultimately lead to me ruining my iron (now that it’s encrusted with pitch it’s useless for soldering anything).

I found using the soldering iron quite difficult, but the best method I found was to use the thin edge of the iron first, moving it point first instead of dragging it through the pitch. Once the first cut was complete then other passes could be made using the thick side of the iron. Often rivulets of molten pitch would pour down the faces of existing facets and the edges of others would rise up. Although this was alarming at first these features quickly disappeared when the mirror and tool were warm pressed together.

I also made the mistake of scoring the surface of the facets with a knife, but using too much pressure. This caused several facets to fracture and chip.


December 2003
 During December of 2003 I did 9½ hours of polishing, most of it was completed sat in my front room catching up on watching my film collection.

The polishing progressed remarkably quickly, very soon the fine ground surface became a mirror surface.

Once the polishing was complete, I decided it was high time to build a tester.

January to March 2004
 I built a slitless tester to gauge the accuracy of the mirror surface. The image below was my first attempt, it was an utter joy to see the Ronchi patters appear across the surface of the mirror.

My first Ronch test!

This Ronchi picture was take just inside the centre of curvature of the mirror. Once I’d got over the joy of actually seeing this pattern my eye became more critical. I think I saw a slight zone and edge defect, which matched with the problems I’d had when creating the lap.

I decided to make a few modifications to the tester and to rebuild my lap.


June 2004
 Contrary to some advice I’ve read (and being pressed for time) I decided to pour the new pitch lap directly onto the old one, forming it in exactly the same way as my first, except under warmer conditions and with a little more pitch.
I was concerned about reusing the old lap, but it was a risk I was prepared to take.

Again I used my soldering iron to cut the facets (since this is just about all its good for these days). I used more pitch this time, and had to trim the excess from the edge, which is why it has a slightly irregular appearance.

Once I was happy with the initial press (without any extra thumb marks this time) I warm pressed the lap and mirror with a net made from old fruit bag stretched in between.
Unfortunately when I separated the mirror and tool to remove the net thousands of tiny pitch flakes that had been stuck to it flew everywhere.
If you’re trying this method, it would be better to do your separating outside instead of in the front room like I did!

After a cold press I did 2 hours polishing with the new lap. It worked well. The Ronchi pictures below were taken afterward. They show the Ronchi patterns both about a centimetre inside and outside the radius of curviture.

To my inexperience eye they seem to show that the smoothness of the mirror surface has improved.

About 1cm inside Radius of Curviture About 1cm outside Radius of Curviture

July 2004
 After receiving several assessments from the ATM community I believe that my mirror is an oblate spheroid (too flat) and that it has a turned edge.
To remedy this I've decided to now polish the mirror with the tool on top, hopefully this will deepen the curvature.

After two hours polishing, tool on top, I obtained the following results.

About 1cm inside Radius of Curviture About 1cm outside Radius of Curviture

August 2004
 After a further hour polishing, tool on top with short strokes and light pressure, I obtained the following results. To my inexperienced eye the figure has hardly budged, although the ronchi lines appear smoother.

About 1cm inside Radius of Curviture About 1cm outside Radius of Curviture

September 2004
 I was surprised at how slowly the figure was changing, so I decided to try heavy pressure to see what difference that would make. One and a quarter hours polishing with a thicker mixture, tool on top, with heavy pressure (a 6kg weight) The figure looks closer to spherical.

About 1cm inside Radius of Curviture About 1cm outside Radius of Curviture

October 2004
 Using a 6kg (12 lbs) weight on the back of the tool I polished for two hours (short stroke, < 1 inch overhang Centre over Centre). During this time I trimmed the channels on the lap using a razor blade, I also started using some new 'white' Cerium Oxide.
After this polishing session I think the mirror has moved significantly towards being spherical, although I think the heavy pressure has roughed up the surface a bit.
Also its still showing a narrow turned edge.

About 1cm inside Radius of Curviture About 1cm outside Radius of Curviture


November 2004 Comparison
 I thought it might be interesting to compare the polishing progress made so far. All these images were taken approximately 1 cm inside the Radius of Curviture (they are infact minature versions of the images shown above).
The first image was taken after the mirror was fully polished, this had been done with the mirror on top and had therefore produced a mirror that was too flat. All the polishing since then has been done with the tool on top and a centre over centre stroke. As the polishing has progressed I have used shorter strokes and heavier pressure.
The goal at this point is to achieve a good sphere, if the mirror's surface was sphereical then the ronchi bands would be straight lines.

Start of figuring 2 hours 4 hours 5 hours 6.25 hours 8.25 hours
It seems to me that the figure of the mirror is improving, the lines are getting straighter. Although the use of heavy pressure seems to be making the mirror surface rougher (the ronchi lines are beginning to look a little uneven). I will continue with the heavy pressure until the mirror is fully spherical and then I'll do some further more gentle polishing to reduce the surface roughness.

Last updated: 1st November 2004