Bath+Interferometer

__Introduction__
Why build a Bath Interferometer? Well, we have been building telescopes now since the mid 90's and we have been using Focult, Ronchi, Couder mask and Star test methods for testing our optics. This was fine for longer focal length telescopes. We have made F4 telescopes, but these have never really perfomed that well, and have been very difficult to test with the methods stated. In particular, testing the outer edge of the mirror. We began to re-investigate null methods, and in particular the Ross Null test. This is well documented and explained here:- []

But, we had some issues with set up and seeing the whole of the mirror initially. At present (December 2012), we are still persuing this method and getting Ross Null lens made of sufficient size and of the correct FL to do large fast mirrors.

We then came across the Bath Interferometer by chance surfing the web on optical testing methods. Interferometry in the Amateur world has always been seen as too hard, and too expensive to do. Well, not any more. In fact, ATM's have been using the Bath and other types of Interferometer for years now. By far the easiest and cheapest type of interferometer to make is the Bath. So we decided to make one. Between us we had all the bits anyway! We decided to make the right angled set up first, but we did not have a lot of success getting fringes.

Due to the size of the beam splitter immediately available (30mm x 30mm) we decided to rebuild and opt for the original layout. This allowed for the beam separations to be closer together. The laser was changed also as the laser was an old model that did not have a good beam and spot. The laser is supplied from a remote battery, via switch and potentiometer to reduce the beam strength. This is mounted in a small box. This set up version R01 gave us fringes and gave us hope in development of a better system.

Further development included a loan of s smaller beamsplitter cube 10mmx 10mm. This together with a new green 532nM laser and finer X/Y/Z adjustment resulted in R03 of the Interferometer (completed December 2012). This has given us the best interferograms so far.

Latest arrangement (Version R04) is below (Summer 2014) which has a high resolution webcam mounted to the vertical stage. This makes alignment much easier. The web cam lenses are changed to suit the size and focal length of the optic under test so that the image of the fringes fill the monitor. This gives much better fringe resolution for OpenFringe to analyse. Lenses are obtained for not much on Ebay. These are standard M12 thread web cam lenses.



__Laser Calibration__
Calibration of the interferometer is done on a regular basis. The wavelength of the Laser used is checked using Rspec and a Neon light source. This method has been checked against a calibrated Ocean Optics spectrometer I use in work and the error in the test is +/- 1nm.

[]

A webcam with a zoom lens is used to image the source in a dark room a few feet away. A Star Analyser is then mounted over the end of the webcam lens and rotated to orientate the spectra. The web cam is a 2 mega pixel camera and used at high resolution.

[]

__Checking the radius of curvature__
The radius of curvature is checked with the Foucault tester at the vertex with a laser ranger that is checked for calibration on a regular basis. This gets the ROC to within +/-2mm or better. A piece of masking tape is used over the centre to ensure that the rear of the mirror is not being measured. The ranger is inserted into the tester and is repeatable. The tester has an offset of 11.3mm from the moving knife edge and source. This is added to the reading taken. The staging for the tester is par-focal with the Bath Interferometer used for surface measurements. This makes testing easy and quicker to set up.





= = =Previouse development versions prior to version R04=

Building version R03 of the Bath Interferometer (Right angled layout)
= =

=__Some of the mirrors made so far using the Bath Interferometer:-__=

20 inch F4 Dobsonian
=__Useful links__=

tech group:- []

Articles on Interferometry:- []

Software:- [] []

Web sites on designing and building Bath type Interferometers:- [] [] [] [] []

Other Web sites on designing and building Interferometers:- []