Sunday, December 7, 2008

Planning A Field Trip!

There are no shortage of astronomical field trips in southern Arizona, but one of them has a long history with me! Back in the fall of '87 I had an opportunity to use the little-used 16" Schmidt telescope on Mount Bigelow north of Tucson (since converted to a 26" and used to search for asteroids and comets for the Catalina Sky Survey) for nearly daily observations of bright comet Bradfield. I noticed in my frequent trips up the mountain that there existed a spot where the sun would set behind the silhouette of Kitt Peak National Observatory. I made it a goal to accomplish that feat. Turns out that near winter solstice, when the sun moves little from day to day, that there is a pullout along the highway where it can easily be done. And fortunately, there are two brief windows a couple days before and after solstice that provide alternates in case of bad weather.

The pullout is on the outside of a hairpin turn just past milepost 9 on the Mount Lemmon Highway. I've set up there several times the last few years, continuing to refine the best sun position and observing position. It turns out that just a few yards on the ground makes a noticeable change in the sun's position against the mountain. The photo shows my setup along the side of the road in December of 2006.

Here is the best shot from that sequence taken on the afternoon of 24 December, 2006. I was using eyepiece projection with a William Optics 11cm F/7 refractor to get the image size correct, and unfortunately, the seeing was terrible, and likely my focus was also a bit off. In any case, it was a successful observation, and in fact, I made an animated GIF movie of the frames, taken every 4 seconds. It can be seen by clicking here(about a 2MB download).

Always looking for a better result, last year ('07) I lead a group from the astronomy club to capture it. Unfortunately, on the Saturday chosen, it was far enough south that we had to significantly change our position away from the pullout to the next one, and it missed the profile of the mountaintop for a perfect alignment, so I went the next day again, 24 December, 2007 and got good results again, but the sun is riding low on the profile, so would prefer it a little further north in the future. The photo is from that observation. Note the change in scale from above - this time I was using the William Optics scope with a 1.4X converter, with sharper, but smaller image.


It is a beautiful view of the observatory from that spot, looking over the Tucson valley, nearly 60 miles from Kitt Peak. Seen in silhouette from the left are the Vacuum and McMath-Pierce solar telescopes, the 84", 36", and 3.5 meter WIYN telescopes, the old 50" dome, the tiny blip of the Visitor Center's 20" and the 24" Schmidt telescope near the profile center, then the 72" and 36" Spacewatch telescopes, the 90" telescope, and finally the 4-meter telescope at far right.


As for helpful hints should you make the trip, practice before the trip to make sure you have correct image scale and filtration. I use an old Thousand Oaks filter with a little higher transmission for photography which helps keep exposures short since with the sun on the horizon it can be a lot fainter than when overhead. It also helps to arrive early well before sunset to align on Kitt Peak before the sun forward scattering off 60 miles of turbulent air makes it impossible (I suggest arriving 45 minutes before sunset). If you get there early enough, you can use the sun's glint from the 4-meter to help you focus! As for dates, from the north end of the pullout, you can see last year's image was taken with the sun as far south as could be and still span from Solar Vacuum to the 4-meter in one photo. There was only a single frame, taken every 4 seconds that showed that. So the minimum declination should be -23.405. This year, the sun will be too far south on the evenings of the 18th through the 23rd. So the "preferred" dates for 2008 are the evenings of December 16th and 17th as the sun is moving south towards solstice, then again on the 24th and 25th as it moves north again (ideal dates are 17th and 24th). Unfortunately, they are all midweek dates and do not suit folks with day jobs, but it is the best I can do! Those without families can do Christmas eve or Christmas day attempts... Or another alternative is to grab an observatory shot and a sun shot and use Photoshop to combine them (not recommended)! Questions or comments, e-mail me (click on my name at upper right of blog page)

1 comment:

David A. Harvey said...

Cool post Dean! I'd love to try this some year - but looks like the Master has already done it! SWEEET!