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Interference testing optical flats

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About this observation
Observer
Mike Harlow
Time of observation
06/09/2020 - 21:00
Observing location
Ipswich, UK
Equipment
15cm optical flat
26cm objective prism (under construction!)
Astrodon OIII filter (5nm)
Canon 550D DSLR
Exposure
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Something for the telescope makers out there:

Although I don't make mirrors any more I have one last optical project before I hang up my lab coat: A 26cm diameter, 3 degree objective prism.  It is specifically for low resolution spectroscopy of faint objects, down to near mag. 15.  I ground and polished it several years ago but the two faces have yet to be figured accurately flat.

Optical flats are usually tested by putting two surfaces in contact and illuminating them with monochromatic light.  The resulting interference pattern gives a contour map of the surface under test.  In the past I have used a Helium-Neon laser (632.8nm) for testing an elliptical flat mirror and a green laser pointer (532nm) for testing a Schmidt corrector plate.

Now I am experimenting with using a white light source, an LED light bulb(!), but with a monochromtic filter on the camera imaging the interference pattern.  Early results look promising.  The image above shows a 15cm optical flat on top of the 26cm prism imaged through an Astrodon OIII filter at 500nm+/-2.5nm.  Fringes are clearly seen and good enough to reveal the surface accuracy of the prism.

A little "optimising" will be done to give clearer fringes before figuring is started.

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