Wednesday, December 23, 2015

Five Sunsets!

We've just passed the Winter Solstice, and for those that know me, it's the season for imaging the setting sun past the silhouette of Kitt Peak National Observatory. Like the ancients gathered around Stonehenge for celestial alignments, I've been drawn to this one for well over 20 years, first recording the sunset behind the Observatory in 1988! Those days was with my 12" Newtonian telescope and film (!) camera pointed out of my old van! It is much easier now with portable refractors and digital cameras. I can park and be set up in minutes these days. At left is a GIF file I put together on the "optimum" date of 17 December - click for a larger version. At right is shown my current setup - TEC 140 triplet apochromatic telescope and the venerable Canon XSi camera. Note the home-built solar filter that we made in the TAAA filter workshop about a month ago - it has gotten lots of use lately!

The GIF image at left is quite remarkable. While it shows a few circular artifacts that I didn't see in the original files, what is amazing are the layers in the atmosphere. Formed from air at different temperature profiles, you can see their effect on the image both at the suns edge, and also in the appearance and spacing of the two sunspots. It looks like watching an image at the bottom of a wavy pool!

Why the "Five Sunset" title? Well it hit me that I really didn't know how the image correlated to Kitt Peak very exactly. I knew if you were at "the spot" a few days before and after solstice, you could catch the alignment. I wanted to learn a little more about the situation. About the time I decided to do this, it was 10 days before solstice, so went up alternate days 5 times. It really was an amazing experience! Every sunset was different. Most were clear, one had some clouds during sunset, the last night it completely clouded up 30 minutes after sunset.

The end result is shown here at left.  I started on the 13th, when the sun was still moving southward more than a fifth of its diameter every 2 days. Of course, its southward motion slowed and stopped on solstice day on the 21st, taken just a few hours before the start of Winter. While the casual observer might think the sun wouldn't cover the observatory on most of those dates from this image, note the suns sweeping left-to-right motion for us here in the Northern Hemisphere. Because of the horizontal motion, I caught the full Observatory silhouette on 3 of the 5 evenings, all observations from the exact same spot. As you can see at right, after the first alignment happens about the 17th, the sun continues to move southward, appearing to lower from day to day as it sweeps past the Observatory. On solstice day, it BARELY covers the full silhouette - it helps to move northward as far as you can along the curve to help you catch more solar disk.

But the silhouette wasn't the only thing to see! Like I said above, every one was different - the first day it was very hazy - at least the inversion layer was above the observed level of Kitt Peak and it appeared very hazy. The result was that when the sun dropped below the observatory profile, the peaks projected their shadows into the hazy air, shown at left. Similarly, the next trip up on the 15th, I was shooting the disk (Meade 80mm F/6) as it dropped below the mountain and I caught a bit of blue/green flash between a crack in the mountain, and at the same time, caught more rays being cast upwards and outwards from the suns position, shown at right.

It often paid to not to hurry home. On the evenings when there were clouds, there were spectacular sunsets. At left is a nice example - a 3-frame panorama taken with a 300mm lens plus a 1.4X tele-converter. On another night, I had planned to pause at "Bad Dog" (actually Babad Do'ag) overlook near the lower slopes of the Catalinas to shoot Kitt Peak silhouetted not by the setting sun, but with late twilight glow with the city lights in the foreground. At right is a 5 frame panorama taken with the Meade 80mm F/6 (480mm focal length) and 10 second exposures. I love catching the contrast of the dark-sky observatory within view of the urban lights of Tucson. Besides the lights of Tucson's downtown skyline, the bright lights are from the UA football practice field, where the team was practicing for their upcoming bowl game.

I've got some more items of interest here, but will have to wait for a subsequent post.

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