by Peter Natscher
As two other Tacos rolled in (Sean McCauliff and ? who's name I don't remember), the clouds and fog continued to build and wane around us. It was already pretty wet being in the mountain cloud. By twilight, Jamie Dillon drove in and by the end of twilight, Bob Jardine and Pete Santangeli arrived. We all tail gated waiting to see if things would get better--no one set up yet. Then, by 7pm, the weather condition around us started changing for the better: 48°F, 90 RH, no wind or star scintillation, clouds and fog dissipating to reveal a nice Milky Way over us. We started setting up!
By 8 pm I was enjoying the tack sharp seeing with my 20" Starmaster Dob, taking looks at galaxies, planetaries, and emission/reflection nebulas with as much eyepiece power as I wished. The seeing was wonderfully sharp and the transparency was fairly good, too! The only challenge was with the 95 RH which dewed up everything that wasn't covered or protected inside of my truck. I kept swapping out eyepieces on my Dob to keep them dry. Luckily, I have a lot of eyepiece to do this with.
The seeing last night was good enough to thoroughly support the 20" f/4.25 Zambuto mirror's capabilities. Since I ended up setting up the Dob in the dark (delayed by the foggy conditions at sunset) I wasn't sure how well collimated the optics were. I usually use my Cheshire eyepiece to collimate but can't do this in the dark. The laser collimator doesn't always offer the best collimation. The only true way of checking collimation is to have an observational look at a star at very high power and see how concentric the diffraction rings are around that star when defocused at the eyepiece. So, the first thing I did was to observe a mag. 2nd star at over 500X (20"'s aperture requires over 500X) and check out the defocused star pattern. Even though the laser beam collimation was right on, the view of the defocused star's diffraction rings were decentered by 20%. I then tweaked the primary mirror's three adjustment knobs slowly to get the diffraction rings on the star visually re-centered at the eyepiece. This takes a few minutes tweaking a mirror knob and the looking at the eyepiece to see the diffraction ring movement around the star and recentering the star in the FOV. With this final collimating adjustment though, the reward is the best collimation, and views were exquisite with the great seeing we had. A short f.l. Dob (f/4.25) really performs at its best when collimation is checked using high power at the eyepiece this way.
I had my first views of this year's Saturn apparition last night with my new 20" Zambuto Starmaster. My previous three times out observing in November with this new scope was in poor seeing and it was very frustrating for me to see soft stars. But last night, I was rewarded with views of Saturn that were probably the best I've ever seen. At over 300X, the rings displayed its many subdivisions. The Cassini Division was wide and velvety black all the way around and the Encke Minima was easily seen. For the first time, I was able to consistently see the very thin Encke Gap on both ends of the A-Ring. I've only seen this in photographs and images. It was a very thin line coming in and out of view riding very close to the outer edge of Saturn's A-Ring (1/6 A-ring width distance in from the outer edge). I observed it using two eyes with my Tele Vue Binoscope and Takahashi 7.5mm LE's (575X), and also with one eye using a 10mm Zeiss Abbe Ortho and Klee 2.8X Barlow (604X). At these high magnifications, Saturn was real big in the eyepiece(s) and remained tack sharp retaining good contrast and a lot of planetary belt detail. The tiny moon Mimas was also sharply visible orbiting close by at one A-Ring width distance away. Studying Saturn last night gave me a feeling that I was on a Saturn probe viewing this colorful giant planet from only a few million miles away.
Needing to be at work on Monday, I packed up by 11 pm thoroughly satisfied with a great evening of observing with five other Tacos.