by Robert Vanderbei

I find it amazing how accurate the Clear Sky website is at predicting in advance the quality of an observing session. I am signed up for their email alerts. A few days ago, I started getting alerts that last night would be a good night. In fact, the seeing was predicted to be “excellent” (5/5). In my experience, nights of excellent seeing only come a few times a year usually in the hot months of July, August, or September, and they are not to be missed. Excellent seeing is especially critical for viewing/imaging planets and star clusters. Since no planets were in good viewing position late last night, I decided to do globular
cluster M15, which culminated about 3:00 a.m. The seeing was indeed excellent. I took 20 second unguided images.

In the best 60 out of 90 luminance frames, the FWHM varied from 1.06 arc seconds to 1.41 arc seconds. That’s pretty amazing given that the diffraction-limited resolution (defined as 1.28 Lambda/D) for my 10″ RC is 0.51 arc seconds (in green light). Here’s the best picture that I took…

More are on my website. The stars are amazingly sharp. But, my new image doesn’t go as deep as some of my earlier images of M15. This is consistent with the fact that the transparency wasn’t that great. There was a thin layer of fog/haze. Of course, transparency is often inversely correlated with seeing.