Star Test Images


Images through a 10" Newtonian telescope, at focus as well as in and out of focus, were taken at F/15 using a barlow lens into a ST-7 SBIG camera with color filter wheel. Altair was near the meridian, and the seeing was pretty good. A green filter was used to reduce any possible chromatic aberration from the barlow lens. A fan was used to help stabilize the air turbulence, but since the temperature dropped only a few degrees in the few hours before this test, it didn't matter much. The secondary obstruction diameter is 1.9", mounted on a 4-wave spider. Exposures ranged from about 0.1 seconds to several seconds (on the images well out of focus), and are plotted on the same scale.

The first star tests are shown here. The numbers refer to the travel in-focus or out-of-focus, in relative numbers. Clearly, this mirror was not very good.

I returned it to the vendor for refiguring, and then retested the mirror to get the patterns in the next figure. Unfortunately, the test results were nearly the same. This indicated that the mirror maker used a faulty technique - his results were reproducibe, but always bad!

10" star test old mirror

A new mirror was tested from a second vendor, with much better results shown below. The sequence starts at the top right, progresses toward focus, and then continues to the right in the bottom row. A movie of the entire series (about 300kB avi file) is available by clicking anywhere on the image below. The movie starts outside of focus, goes through focus to inside focus, then backs out to the same starting point. If you set your movie player to "continuous repeat", it will look like you are moving the eyepiece in and out without jumps. Please let me know what you think of the movie! (e-mail to

new mirror star tests

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