At 6:35 AM this morning, the International Space Station made a somewhat rare lunar transit which was observable here in Golden, Colorado, passing in front of the moon just before sunrise at a distance of about 730 km. Images were captured using a tripod-mounted Canon EOS 7D, with EF70-200/2.8L IS lens + 1.4x TCII. Planning for this event took quite a bit of time and effort trying to minimize as many possible confounding factors as possible. I’ve adding details to the bottom of this post if you are interested in background and planning information, including some of the various apps I used.
Some Background on Planning and Methods:
I started planning for this shoot about a week ago, when I was alerted via CalSky.com that the ISS would make a lunar transit, with Golden placed right near the line of intersection. I didn’t get too serious until about three days ago, since long-term weather forecasts tend to be rather marginal beyond about 5 days, and conditions for optimal astronomical ‘seeing’ are far more specific and complex than just a nice, clear night.
As the date neared, I watched CalSky’s refined predictions of the flightpath of the ISS in comparison to the location of the moon; the path was, at its nearest, 5 km from our house, so it wouldn’t be logistically difficult to plans for changes at the last minute. The path I needed to locate myself on was approximately from Hwy 93/64th extending southeastward to the I-70/Hwy 58 junction.
The only bad part about this particular transit was the time of day, a bit too close to sunrise for optimal twilight conditions. I had planned on using two cameras, one for the moon, the other a wide-view time-lapse of 30-sec exposures that might have been either composited into a single image, or compiled into a time-lapse. Due to the sky being too bright from the rapidly rising sun, the images aren’t usable for either endeavor, although you can see the ISS trail in the sky. Oh well, I knew it wasn’t likely to work out perfectly.
The close-up images of the transit, however, I assumed to be a bit more straightforward to plan for, since the ISS would be illuminated by the sun against a relatively dark sky. I went back and forth, debating whether I should rent a 500/4 lens, but I considered the $100 ‘gamble’ to be unsavory, since I can’t control the weather, despite how hard I try. So, I decided to ask my neighbor John if I could borrow his Canon 7D body, since the 1.6 crop factor puts more pixels on the subject, even though my camera is higher resolution. He agreed from London, and I owe him one!
Even though I am a devoted raw file shooter, I decided to shoot JPEG, since the buffer depth at 8 frames-per-second is greater than the 9 seconds of continuous shooting time that I wanted to capture. Shooting raw, most cameras need a break to process after about 3 to 4 seconds, not enough to capture the track of the ISS across the entire frame of the image. Precise focus and tripod stability were the main criteria for this aspect of the shoot, along with forgetting about the ramifications of the fact that I only have one remote shutter release, which meant I’d need to ‘manually’ trip one of the camera’s shutters. I chose to go manual on the telephoto setup, since I’d only need to shoot continuously for about 9 seconds, after which I could forget about that camera. It did cause some post-processing headaches, since I then had to register (align) the images for the time-lapse sequences (even with a tripod, just lightly touching a camera using a 448mm-equivalent lens causes significant movement).
The last decision to make was in regard to exposure, and three aspects could prove critical to image quality: exposure, lens aperture, and ISO setting. Not having done this before, I was guessing at how fast the shutter speed would need to be to freeze an object traveling at 17,000 mph, even if it was 460 miles away. I settled on 1/1000th of a second, which worked well enough. Lens aperture vs. resolution is pretty well-established among experienced photographers; any lens stopped down approximately 2 to 3 stops from wide open will be near its sharpest. However, there is a tradeoff here in that stopping down for increased resolution will require either a slower shutter speed to offset the change in exposure, or increasing the ISO to do the same, both of which could decrease sharpness in the image. Since I’d ‘randomly’ decided on 1/1000th for shutter speed I decided to stick with it, so I decided to test only for ISO vs. f/stop settings. Having owned a 7D, I knew quite intimately its tendency toward–we’ll say ‘enhanced’– luminance noise characteristics, so I knew my goal would be to keep ISO as low as possible, given the circumstances. After tens of thousands of sports images, I like and trust my Canon EF70-200/2.8L IS lens, even with a 1.4x converter at 200mm, so decided to roll the dice and shoot wide open. The result: 1/1000th at f/4.0, ISO 400; trusting some serious gut here, new terrain that it is.
A 40-pound backpack and five hours of sleep later, I found myself hiking under a nearly full moon to a hilltop where I thought the best composition for the wide-angle shots would be. CalSky’s final prediction last night moved that location from 500 meters north of the centerline to only about 200 meters, fortunate luck given that predicting paths of satellites in comparison to planetary bodies at altitudes other than sea level gets rather complex.
Once set up, I stoked up the ‘Satellites’ app on the iPad and started the wide camera shooting, and watched the WNW horizon for the first sign of the ISS which was due to break the horizon at 6:30. The ISS came into the frame of the telephoto rig at 6:34:59, and crossed nearest the center of the lunar disc at 6:35:04.37, close enough to the prediction of 6:35:04.09 to be precisely accurate. Being only 200 meters off of centerline , I’m not sure why the transit didn’t get closer to the center of the lunar disc, but I assume it is related to CalSky not taking into account elevation at the observing point (not sure about this though, have read differing opinions).
About all you ever wanted to know and more about threading a needle in Earth’s orbit, eh?
Enjoy the images!
PS: While testing the lens/body setup last night, I had a random encounter with something as the moon was rising; I’ll post a tidbit on that later, when I’m not so tired… :)