Monday night seeing

by Robin Casady

Did anyone else get unusually good seeing Monday night? I'd like to get an indication whether Monday was an exceptional night over a wide area, or specific to our location.

Arthur Babcock and I were doing some site testing on a cattle ranch. We got rather amazing results. We were taking CCD images to evaluate star sizes (full width at half maximum) and looked at Mars for a visual reference (and fun). We were using a new Astro-Physics 130 f/8.3 refractor.

This was the third night out for this refractor. The first was a quick view from home under poor conditions. The second was a MIRA star party at Chews Ridge. The view of Mars through this scope was the best I've ever had up to that time. Rod spent quite a while viewing it while babysitting my scope so I could look through the 36".

Monday, Arthur and I packed up his SUV and headed up a dirt road to a grassy knoll where we hope to put an amateur observatory. It was a good night. The cow pies were dry and he seeing was great. The view of Mars was as good as it had been at Chews Ridge.

We spent most of the evening shooting stars with a Starlight Xpress HX516 CCD camera through an Astro-Physics 2X barlow. I was concerned that we were underexposing the stars because the disks were so small. So, I kept encouraging Arthur to increase the exposures. After processing the images today, here is what he reported.

"EVERY SINGLE ONE of those images is over-exposed: even on the tiniest of those star images, peak value is 65535 (i.e., the maximum). Which means that the program can't calculate FWHM, since it doesn't know what M is...

FORTUNATELY, displaying the images in MIRA reveals another, fainter star, totally hidden in the Starlight Xpress software. This one doesn't peak, so I calculated FWHM as...

1.55 arc-seconds!!!! Yes!!!!

Based on an effective focal length of 2160 (1080 x 2) and pixel sizes of 7.4 microns. This gives 0.7 arc-sec per pixel, meaning that FWHM of 1.55 is practically at the Nyquist limit!!!!! Meaning that according to sampling theory, the best FWHM we could get with that setup, no matter how good the seeing is, is 1.4!!"

A FWHM of 2 is considered quite good. SBIG reports that most amateur images they receive from locations lower than 5000 ft. are 3 to 4.