In part one of this series, I talked about the hardware required to capture images of satellites, like the International Space Station (ISS), or the Hubble Space Telescope (HST), crossing in front of the sun, moon, or a planet. Here is a link to Part One.
In this installment I'm going to tell you how to find out when, and where one of these crossings will take place. I will also talk about how to determine which crossing will give you the best results, as well as how to optimize your chances for success.
The first key to success in this undertaking is planning. If you use the applications and techniques I talk about in this post, you will greatly increase your chances of success.
Let's look at how these crossings occur, and the easiest way to explain it is to think of a satellite crossing the sun as a solar eclipse, only the world doesn't go dark. In a solar eclipse a satellite orbiting earth, the moon, moves between the sun and the earth temporarily blocking the suns light from a portion of our planet. The shadow of the moon is cast on the earth, and if you are in that area of shadow, know as the Umbra, you experience a total solar eclipse.
If you are outside the shadow of the moon, you will experience a partial eclipse. Of course the satellites we want to photograph are much, much smaller than the moon, so to capture them crossing the sun, or other celestial object, we must be in the path of the satellite's shadow. The width of totality for the solar eclipse on August 21, 2017 will be about 70 miles, give or take a few miles; the width of the path where a satellite, like ISS, can be seen transiting the sun is on the order of three miles. As you can see, accuracy in your positioning is critical for success. How, you ask, do I find out where to position myself in order to capture a transit? It's fairly easy, with the help of a wonderful website called CalSky.com.
CalSky is a free website that will calculate transit visibility locations for any satellite, and almost any celestial body. While the website is free, you are limited to an hour of access per day. A small donation is always helpful to the good folks that maintain the site and it will get you longer access times. To calculate pass opportunities near you it will be necessary to let the app know your location, this and other useful information can be entered in the "SETUP" section, be sure to enter your time zone as well. The link above will take you to a landing page that is set up for ISS transits of the sun, but you can also configure it for the moon. Take some time to poke around the website, learn what it has to offer, which is A LOT, and become familiar the interface, it's fairly simple. There is also an introduction page that will answer a lot of your questions.
What you get from CalSky will be a map showing the center line of the transit. That transit line will have several markers along its length, click on a marker and a popup bubble will tell you everything you need to know about the transit for that spot along the track. Below is a transit map generated for 7 days of transits beginning on May 7th, that are within 500 kilometers of my location in Central Florida.
As you can see there are seven opportunities during the 7 day period, but on closer examination of the pass data I would only try for one of these, the third one from the left that goes from the panhandle of Florida, across the Gulf of Mexico and grazes the lower portion of the Florida west coast.
Why would I pick that pass rather than one that is closer to my location, like the pass that goes across Tampa and Miami? Because the more distant pass happens closer to solar noon than the others, and that means the station will be going almost directly over head. Why do I want the satellite to pass directly over head? Because it is the time that puts it as close as possible to my position, making the relative size of the object, in this case the ISS, as large as it can be. It also reduces the amount of atmosphere you have to shoot through, giving you a better image. The bad news is that a direct overflight makes for the shortest transit time.
And now a reality check, don't think you can just pull up a transit, good or bad, just any ole time. Nope, there are times when there just may not be any passes near you for days, maybe even weeks. And you can't plan ahead too far, the orbital elements, known as State Vectors, used by CalSky, or any other app that computes these passes, are only good for a short period of time. This is because the satellites orbits vary slightly over time and a current state vector is needed to get an accurate pass prediction. Because of this, I always recheck the pass on CalSky as close to the pass time as is feasible, that way I will have the most current timing, any possible shift of the center line. Remember, timing and location are very important when you are trying to capture an event like this.
My goal is to be set up at the location I have chosen, ready to shoot, at least an hour prior to the transit. It has been my experience that something will go wrong that you will have to fix, and you don't want to be rushed, that's not fun.
Since we are talking about timing anyway I'll put in a plug for a little app I use that displays the current time of your smart phone in HH:MM:SS in big, friendly, easy to read numerals, it's called ClockZ. Using this app or another similar app allows you to check your smart phone against UTC (Coordinated Universal Time). The time stamp on the above pass is in UTC converted to your local time zone (make sure you put your timezone in CalSky!). I use Time.is to check the accuracy of the time on my smart phone, it's usually accurate within about a half second, I make a note of this difference, and apply it to the displayed time. On location, with the app running on my phone, I have a big clock telling me when the transit is about to happen. Remember, if you're off by half a second, you could miss the whole thing. But don't sweat this, I'll explain how I deal with this in part three.
Another thing that I always do is set the alarm on my phone for five minutes prior to the transit, this way if I get distracted by something else, the alarm brings my attention back to reality.
A second useful app is GoISSWatch, this little program shows you exactly where the ISS is over the planet. With this running you can actually watch as the station approaches, which I find to be very cool. If you want to track other satellites you will have to upgrade to GoSatWatch ($9.99). The primary function of both these apps is to tell you when and where to look in the evening/morning sky to see the spacecraft fly over. Both apps have a "fast forward" feature that will allow you to roll time forward to the moment of closest approach to your selected transit location. This is how I find the altitude of the satellite at closest approach, which is part of my planning process.
I always have a space blanket somewhere in my road kit. It comes in handy for protecting the camera and lens from the elements while you are waiting for the shot. In the photo (right) you can see the blanket and the mylar film solar filter in place on my 800mm. Remember, cameras don't like to get hot, at best they will produce a poor image, at worst they will shut down completely.
The space blanket can also serve as a "focus cloth" when viewing the monitor in bright sunlight. Just throw it over your head and let it drape down to shade the monitor. You can do the same thing with the LCD on the back of the camera, just drape the blanket over the camera and your head, works like a champ!
Over the years of shooting space shuttle launches for multiple IMAX films I have gotten the habit of making check lists. When setting up multiple remote cameras in the days prior to a launch it's very easy to forget something, but having and following a check list helps insure you have covered you bases. I say this because I apply the same check list mentality to all complex photographic work I do. I start the check list in the planning stage by listing all the gear I will need. Nothing worse than getting up early to drive a hundred miles to a transit location only to find you have forgotten a critical piece of equipment. One of the first things on any check list I make is spare, charged, batteries. Can't do much photography these days without batteries.
I suggest setting everything up, at home, the day before the transit, call it a dry run. If you are going to be shooting a solar transit, this will be a good time to shoot exposure tests with your solar filter. I would even try to shoot the test at the same time of day you plan on shooting the transit. This dry run will get you familiar with what you have to do on the day, this way there won't be as many surprises on location.