240 mm f/4,5 travel Dobsonians: The Quintuplets – Mirror Making

The fabrication of the 5+1-lets starts

The mirror making novices an me took 18 mm thick borosilicate glass, the lightweight specialists chose 15 mm thick blanks.

Video rough grinding: 7 MB .mpg4 Datei (turn speakers on!)

Mirror Making thread in Astrotreff (German text)

To save time, I hogged out the blanks and the tools with an angle grinder and ground them with grit #80 by changing tools and blanks, to get an equal focal length for all. The tools were cut out from a 20 mm thick large porcelain tile. It proved to be a nice tool material, no scratches, or other problems. The back sides were ground up to grit #320 against each other.

Polishing on a flat table with a hard carpet underneath. Thanks to the flat back sides and frequent rotation of mirror and tool on the table, we observed almost no astigmatism.

The mirror testing was carried out with the Bath interferometer of the club (made by Tassilo) and data reduction with DFT-Fringe. We averaged 4 interferograms for 0° and 4 for 90° rotation and averaged them out, to get rid of the test stand astigmatism. Here is the final result of my mirror. All the other mirrors got very fine as well, even those of the novices.

For comparison I took Foucault images and evaluated the whole Foucault series mit Horias FXL.

Packaging of the mirror for transport. The mirrors were coated with pure aluminum and no protection layer over-coating in the vacuum chamber of a well known ATM.

To overview quintuplets
To mechanical components