Sunset behind Kitt Peak

I first started this blog nearly two years ago, partly inspired by the blog of fellow amateur astronomer Dean Ketelsen and his wife Melinda.  In what has become an annual tradition near the winter solstice, Dean and Melinda trek up to near milepost 9 on the Catalina Highway and observe the Sun setting behind Kitt Peak National Observatory (KPNO).  Each year Dean and Melinda invite folks to come along, and this year I decided to join them for the first time.  At left you can see our little group set up, passing the time until sunset, observing the Sun through Dean and Melinda's big binoculars and 5 inch SCT.

In preparation for taking pictures of the sunset, I constructed a filter using Baader photographic solar film, cardboard, and high density foam.  Mounted on the front of my Canon 70-200mm f/4 L lens, I was prepared to take white light images of the sun setting behind the various telescope domes at KPNO.  As we were setting up, Dean offered to let me borrow his 1.4x teleconverter which effectively turned the lens from 200mm to 280mm.  This proved to be a great piece of hardware, and at right you can see a test shot of the Sun about 45 minutes prior to sunset.  Note the sunspot regions on both limbs, as well as the brighter facula surrounding the spot regions.

I aligned the camera on the center of KPNO, installed the solar filter and waited for the Sun to enter the cameras field of view.  As soon as it did, I started taking images every few seconds.  I did not really time the sequence, but took 20 images during the event.  Sunset lasts two minutes, so assuming I was close, that means I snapped off one picture about every 6 seconds.  It was really a learning experience as once the Sun was about halfway set I was having to change the shutter speed in between exposures.  The reason for this is the significant decrease in sunlight with each passing second.  Below are a couple of my favorite ones, including a shot of all the domes illuminated, as well as the closest I have come to capturing the green rim phenomenon in a picture.  As always, click to enlarge the images to full size: