Interferometer for Testing Astronomical Telescope Mirrors

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While a Foucault tester can show symmetrical erros only (spherical aberration, zones, turned edge), an interferometer sees the complete 2D surface profile and thus asymmetrical errors can be evaluated as well (astigmatism, trefoil...)

My Bath- interferometer (common path interferometer) in 90° geometry is made with parts, that were selected from the ingenious Alois Ordner, to provide best perfomance:

- Well collimated 5 mW Laser (Roithner DPSS Laser module CW532-005F, green with a wavelength of 532 nm) with a focusable 4 mm lens, powered with a stabilised 3V power supply. The non focusable version CW532-005 should work equally well.
- Multicoated 50/50% beam splitter cube with 20 mm side length, surface flatness Lambda/10.
- Multicoated biconvex lens with 5 mm diameter, 5 mm focal length, accuracy Lambda/4. The lens holder can be adjusted lateral and in height.
- 3 D adjustable surface coated optical flat mirror.
- Bath beam separation approx. 7 mm.

The testing device is mounted on a 4.3 kg heavy duty XYZ- stage with a platform size of 125x125 mm. The stage stays on 3 feet with 100, 50 and 30 mm segments for rough height adjustment. Thanks to the high total weight of 6.5 kg it stays rock solid and can be fine adjusted in all 3 axes tremor free and without any backlash.

Video all-round view (7,5 MB)

Video "first light" (12,5 MB)

Introduction at Astrotreff (German forum)

Top view

Side view

Front view with the milled down lens holder edge to minimize the beam seperation

The camera stays at a right angle to the laser beam with the lens opening close to the beam exit of the instrument. I use a 100 mm f/2.8 fix focus low distortion lens + an APSC format camera, which can capture test mirrors of up to f/3.8 complete and without internal beam vignetting. For faster mirrors like f/3.3 a full format camera is needed... or an APSC in combination with a smaller focal length lens.
Sample Interferogram from a not yet finished 24 inch f/4,5 parabolic mirror. The device delivers clean high contrast interferograms with only vew "unwrap errors".

Interferogram analysis software DFT Fringe and a tutorial
General info on Bath interferometers
Optimize Bath interferometers at Astrotreff (German) with valuable tips from Alois Ortner and others.

In comparison a point diffraction interferometer (PDI) is easier to make at lower cost, but is more limited in terms of mirror size and focal ratio. Further info and supply of PDI plates and lasers see Astro Electronic, Michael Koch.

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