Observation by Mike Harlow: Prism Making: Part 2

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Mike Harlow

Observer

Mike Harlow

Observed

2026 Jul 20 - 14:08

Uploaded

2026 Jul 20 - 14:09

Objects

Equipment

Title

Prism Making: Part 2

About this image

From Part 1:

Once the correct angle and thickness have been achieved fine grinding is done using 180, 280, 400, 600 and 1000 grade abrasives exactly like mirror making. Regular checks with a spherometer are made to maintain surface flatness.

Image A Polishing is carried out with the prism clamped to the grinding stand with the top surface horizontal. A sub-diameter lap made from hard pitch is used on top and the strokes adjusted to maintain a smooth flat surface.

Image B Testing is done in contact with a master optical flat illuminated with monochromatic light from a green laser pointer at 532nm. The interference pattern shows the surface profile. Straight lines show that the faces are within ¼-wavelength of flat.

Image C The finished prism mounted ready for fitting on the telescope.

Total time for the project was about 60 hours spread over 4 months. The final weight of the prism is 2.0Kg

A more detailed write-up of prism making can be found in the Orwell AS Newsletter for March 2021.

Files associated with this observation

prism_making_2.jpg (564.0 kB)

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James Weightman, Paul Whitmarsh, David Swan, Des Middlemass

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