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ATM: 90 mm (3.25 inch) f/7.6 Project

This mirror grinding project is based on a request of another amateur astronomer. He needed a replacement for one of those common 75 mm, f = 700 mm Newtonians. Below are some pictures:

2001-12-12: The Duran blank was slightly too large in diameter and I had to cut it down to 91 mm. This was done by setting the blank into dental stone within a PVC cup and then trepaning it with a commercially available tool, running backwards, using 80 grit and water. This was the first time I used this technique, surprisingly it was quick and clean.

After several hours of grinding on a one-piece tile tool the desired radius of curvature (RoC=1380 mm) was almost achieved. For the test the mirror was polished with a HCF tool and Cerium Oxide.

Polishing with the pitch tool showed some residual pits from grinding, so the mirror got another three wets with the finest grit I had. Here also the too short focus was corrected to the planned 690 mm. Polishing with an (almost) full size micro-facetted lap and figuring took a while, but the mirror came out fine. On the last day 8 'wets' were done with testing in between, slowly working towards the parabola. My limited test equipment shows an acceptable result.
 

The blank, slightly oversized at 94 mm, center marked for trepaning 
 

The blank set in dental stone (oops, pink!) 

The cutter at work, the greyish colour comes from the grit slurry 

The mirror partly polished for testing.

The HCF-Tool used for polishing, attached to the tile tool. Later on a pitch-tool was used.
The pitch tool on dental plaster, mounted on a wooden disk

The ready polished mirror, error <1/8 lambda on the wavefront, Strehl >0,96.
Below is a picture I took with the following setup:
Illumination by a point source, reflection photographed with a hand-held camera through a grating. I have no idea if this setup gives a valid picture, but it looks fine to me. The contrast isn't too good, I am happy that the fully automated camera took a picture anyway. The strongly curved lines at the edge are (diffraction) artifacts, I hope.