Mark II Bath Interferometer Tests – We Have an Interferogram
Mark, Steve and George met at the Hillestad workshop on August 21 to make the first try at getting the Bath interferometer that Steve built working. From the last post you know that one of the big challenges was not having enough space to do the test without setting the mirror or the interferometer up in Steve’s driveway. Out there it was subject to the breezes of Nature, and while pleasant, they make it hard to get an interferogram out of the apparatus.
The telescope was positioned at one wall of the workshop, dropped down to zero altitude, with the secondary spider (the temporary pyramid one that we used for star testing at prime focus) removed. At the other end of the workshop we set up the Mark II version of the Bath interferometer, as Steve began to call it that night.
This image is from the front of the instrument, with the photographer’s back to the telescope mirror. Leftmost is the aluminum lens mount containing at 9mm F/1.0 biconvex lens. The purpose of that lens is to diverge the beam of the laser that passes straight through the beam splitter, illuminating the full diameter of the 40-inch mirror. The center of the photograph shows the flat mirror, set at 45 degrees to the laser beam. It reflects the part of the laser beam that is reflected off of the beam splitter face at about 45 degrees from the original path, and bounces that undiverged beam down to the 40-inch mirror. The diverged beam from the lens is reflected from the entire surface of the big mirror back to the small flat, where it is reflected back into the beam splitter and on through it to the image plane of the interferometer. And the undiverged beam off of the flat is reflected by the center of the big mirror back through the lens where it is diverged and reflected off of the beam splitter to the image plane. The beam splitter is just behind the diverging lens in this photo.
In this photo from the side and above, you can see the layout, including the laser (a salvaged laser pointer module with battery supply). The image plane is in the upper right and that’s where we set up the camera to photograph the interferograms. Steve’s experience with this apparatus paid off pretty quickly – he was able to adjust the beam, beam splitter, lens and mirror through a step-wise process that got them all lined up, and Mark was then able to position his camera to take an image of the interferogram.
You can see in this gallery the first three interferograms we recorded. These are not yet diagnostic interferograms – they are just showing that we can get one (one step at a time!). A useful interferogram will have concentric rings, each ring indicating an additional wavelength of deviation from the central height of the mirror. Our interferograms are showing some misalignments that we can work out, including the images with the two bright red dots – those are images of the incident undiverged beams that can be eliminated by rotating the beam splitter slightly, and adjusting the other components to keep everything lined up.
Alignment with the Mark II apparatus posed some problems because of the nature of its construction from spare parts, so we decided, with this success, to build the Mark III. If you click on this link (Bath Interferometer v0.3) you will open up a PDF of the design of the Mark III using off-the-shelf optical lab parts from ThorLabs. (You’ll need to use an Adobe Reader to get the 3D features – once it opens, click on the image and then you will be able to click and drag on the image to rotate it and zoom in to see how it all fits together). The parts just arrived, and this is an image of a test assembly.
Next step is to construct the Mark III from these parts, and that will involve some low tech machining of the optical rail and a part to hold the beam splitter. Once we have it assembled, we should be able to do our first tests with the new apparatus the week of 9/10 and attempt our first diagnostic interferogram inside the darkened Hillestad lab.