Building+version+R01+of+the+Bath+Interferometer+(Original+layout)

The images below show the layout and initial images from version R01 of our Bath Interferometer. This was re-built, tested and fringes seen in one night. We now plan to upgrade some of the parts now we have seen fringes and proved to ourselves that it is worth developing, and should (reading what others have done) provide better quantative data on the mirrors we are making. Hopefully this will enable us to make better short focal length mirrors.

Image below shows me with version R01 of the Bath Interferometer and holding the web cam in place for taking images. A bracket will be made to hold the web cam square to the image plane and have the ability to focus the image too. The diverging lens is 12mm FL and taken from an old eyepiece. This is too long a focal length for this set up and only just illuminates all of the mirror. A shorter focal length is required which should be 200% the size of the mirror. The beam splitter is Ian's and was obtained originally from Surplus Shed for $7. We will order a 10mm one from Stanwax Laser in the UK to see if we can get the beams closer together. The X/Y/Z stage set up was limited in use. The Z axis was shimmed into place. Only one decent linear stage was used in the set up. We intend to have all 3 axis to have fine adjustment for alignment.



A close up of the set up early on. We were not getting good images initially. This was due to bad alignment. You can see that the laser is on the edge of the 45 degree falt and not fully on the surface. A good source of information of how to set up the the alignment of the Bath interferometer is : []

Image below is the mirror under test. This was my old David Hind's 8.75 inch Pyrex mirror of known good quaility. The laser is not aligned is this image. The mirror is in it's original cell and fixed to a back plate for fine adjustment of the returning beam to the interferometer.

Image quality was not great. There appeared to be circular rings on the expanded test beam. Inspection of the laser dot at distance showed that this may be due to the Laser itself? A replacement Laser has been ordered. This is a Green 532nm Laser. Hopfully this will provide a better expanded test beam. If not a spatial filter may be required.

The image below shows one of the first images we took using the Phillips Toucam web cam. Not ideal, but it does work. Our alignment was poor and there is dirt on the optics too. Time to refine the set up! The circular effects on the fainter expanded test beam can be seen in the image also. This is due to the laser lens quaility we think.

To complete the loop, we put one of the best images that night (not a great image!) into FringeXP. This was done to see how 'easy' it was to test a mirror. FringeXP is really easy to use with a simple help file that works. The resulting data seemed to confirm the surface quaility fo the mirror which was a quaility Hind's 8.75 inch Pyrex mirror from around 1979. The image quaility not being great would not allow for teh fringes to be auto traced. We had to do this manually. Hopefully image quaility will improve as the design improves to allow for quicker auto tracing.

To complete the loop, we put one of the best images that night (not a great image!) into FringeXP. This was done to see how 'easy' it was to test a mirror. FringeXP is really easy to use with a simple help file that works. The resulting data seemed to confirm the surface quaility fo the mirror which was a quaility Hind's 8.75 inch Pyrex mirror from around 1979. The image quaility not being great would not allow for teh fringes to be auto traced. We had to do this manually. Hopefully image quaility will improve as the design improves to allow for quicker auto tracing.

The iamge below shows that the mirror was 1/3 wave rms on the wavefront. This is obviously wrong as the mirror is near perfect. The poor alignment, poor image quaility and circular fringes were probably to blame for the poor data. This mirror is very good quaility with no astigmatism and good inside/outside focus pupil plates on sky. We intend to get a good reference sphere to test and do lots of tetsing to understand the setup issues.

Removing some of the terms improved the quaility of the mirror to a Strehl of near 0.6. That shows you that you need to have good data, and must not cheat with a software fix! In the end the stars will tell you if the mirror is good or not. That is the advantage of the ATM, we tend to use the optics on sky as a whole system in the telescope to see if the star images are good or not.