Star Test – Chapter 3

This will be the last star test for a while. This was the best diagnostic image we’ve taken and it was done with the Orion Starshooter Autoguider – the hand-me-down from the CCD project now that Steve Smith has replaced it with a Lodestar autoguider for better computer control and eventual remote operation.  We used Stark Labs PHD (a pre-release version) to save this 1 s integration image. The squarish shadow on the top is an 8.5″ sheet of black construction paper meant to make sure we know where on the mirror the image is oriented. That’s the “top” in the alt-az mounting, and currently the mirror is rotated so that the E zone is at the top. ‘E’ comes from the letter names for the sixteen zones we’ve marked out on the edge of the mirror.

The image we captured is from inside focus, almost 12mm inside focus. This is an image of Antares which was quite low in the sky. In addition to altitude impacts on the image, we were battling with clouds throughout the session.  You can see a very not-round part of the image shape and we think that is caused by some form of astigmatism of the mirror’s surface. These two images, inside and outside of focus, but more like 6mm on either side, and both at 1s exposures, confirm that high level diagnosis.

From these images we’ve learned enough to know that we need to make adjustments to the mirror figure before we coat it. In order to take that next step, we need to know how to change the shape of the surface, and the star test is not diagnostic in that sense – it tells us that we have astigmatism, and that it has a particular orientation on the surface, but not where the high or low spots are that we need to work.

That information may come from our next task, to get Steve Follett’s Bath interferometer working. He and Mark had tried this before but the conditions for using that measuring technique were not ideal in Steve’s shop. The interferometer must be placed at the radius of curvature (ROC) which is twice the focal length of the parabolic mirror, or 7,328mm from the mirror surface, or about 24 feet away.  In Steve’s shop that always meant the mirror was outside on the driveway and the test is sensitive to air currents. In Mark’s shop, there is room in the enclosed shop to set the full test up. The test ran into some other issues having to do with the illumination of the mirror with the laser, but we will need to try it again and start troubleshooting until we get it working.  We’ve described our situation to the Yahoo Interferometry group and have had encouraging comments that we should be able to use this method to test our mirror.

If you’d like to read up on what the Bath interferometer test consists of, you can check out this well-written description on the web:

http://starryridge.com/mediawiki-1.9.1/index.php?title=Bath_Interferometer

Steve Follett’s version is a right-angle device. The basic idea is that a laser passes through a beam splitter that divides the beam’s intensity in two and allows one half to pass straight through while the other is reflected at 90 degrees. One of those beams is diverged so that it expands and reaches the entire mirror surface, which then reflects that diverged beam back to a focus near the diverging lens where it is re-collimated by the same lens. The second beam is reflected straight back to the image plane of the interferometer where the reference beam and the test beam are allowed to interfere. The difference in path length causes fringes to appear that indicate changes in distance across the surface of the mirror compared to the test surface.  The fringes are photographed and then fed into an open source fringe analyzer to produce images of the shape of the mirror. The arrangement is straightforward, but the devil is in the details of getting the optical elements correctly lined up so that intensities match and so that we get good fringes and not too many or too few.  We have the experience of the Interferometry group to guide us, but part of what we will be doing will be a first – mainly getting this to work for an f/3.6 mirror of this diameter.

The interferometry setup sessions will begin this week. I’ll post pictures of the setup so you can compare to the StarryRidge versions in the link, and eventually we should be able to post our interferograms and the analysis that comes out of them.

And, in case you were wondering, until we get this test working and see our first analysis, we won’t know how hard it will be to get the mirror figure into the correct shape. That will determine the answer to the “when” question, as in “When will this mirror be coated?”.

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