AO-7 Mirror Fix Announcement

MODEL AO-7
ADAPTIVE OPTICS SYSTEM
MIRROR FIX
January 27, 2000

We have received periodic reports from some AO-7 users that they are unable to obtain round star images with the AO-7 unit attached. This problem was aggravated by two mechanical issues that we have already solved. Copper shims used in the mirror assembly have been replaced with thinner plastic shims and a spring loaded plunger that tolerates greater temperature extremes is used as the pivot point for the mirror. Even with these mechanical updates, there are still some users reporting star images that are not round. As a result we took our entire last shipment of mirrors for zygo testing by an independent lab. We found that some of the mirrors we were receiving from our original supplier were of unacceptable surface figure. This alone could account for the out of round appearance of the star images being reported. As a result of our tests we have changed to a different mirror supplier. Our first shipment of new mirrors has arrived and we are satisfied that the new mirrors are of the quality we require for our AO-7 product. We apologize for not catching this problem sooner.

We cannot say from our test samples what percentage of AO-7 units were affected since they were first released. Even a few is too many. In this case it is probably on the order of 20%. From the numerous user images we have received (and used in our ads!) it is clear that many AO-7 units are working fine and do not need to be returned. But if you have an AO-7 unit and you are bothered by star images that do not appear to be round, we will replace your mirror at no charge. You can easily test the mirror by taking an image through the AO-7 on a night of good seeing (e.g., at 9 microns star images of 2 - 3 pixels). Attach the AO-7 but do not turn it on. Use the AO-7 as a passive diagonal mirror only and take a short (e.g., 1 - 3 second) high resolution image of a star field high in the sky and examine the stars in the center of the image. If the stars are not round appearing, try the same test without the AO-7 attached. If the star images are OK without the AO-7 but elongated with the AO-7, send it back to us and we will replace the mirror. Be aware that star images taken near the horizon may show an elongation due to atmospheric refraction and that images taken through a telescope that has not sufficiently cooled or is out of collimation can show elongated stars. This is why the test should be done on the same star field near zenith with and without the AO-7 attached to the telescope.

If you are in doubt about whether you should send your AO-7 in for a new mirror you can send sample images to SBIG for analysis.