Photographing the ISS and Iridium flares
Equipment and Getting Started
There are a couple of essential tools for satellite photography. A tripod is an obvious necessity for any night-time photography, and satellites are no exception. A cable release or means of keeping the shutter tripped on BULB mode is essential for exposures longer than the cameras default maximum length (usually 30 seconds). Although not necessary, a compass helps in finding the exact direction you should be looking in order to catch those Iridium flares.
Before we get started I should mention that in all cases it is best to put you camera on manual focus and focus the lens to infinity. Especially in dark skies it will be nearly impossible for your camera to autofocus in the dark. As well, in all cases I am working in full manual mode on my camera. Your camera DOES NOT know what you want you are trying to do for this kind of photography and you need to play around with all three controls. There are two main types of satellite images you can make: images where the stars rotate in the background (star-trails) and those where the stars do not move (pin-point stars).
Superimposed on star-trails
Superimposing your satellite image on a background of star trails is the easiest method since you don't have to worry about keeping your exposure times short and precisely timed. In this case you are taking a long exposure with your satellite event happening sometime during that long exposure. In my experience, ~ 5 minute exposures are long enough to see the stars coherently rotating, include a ISS pass or Iridium flare without worrying about timing, and short enough that you can do a couple of test shots before hand if you want. The first thing you want to do is frame the image the way that you want it to be. Personally I like to try and include some foreground interest so that the image is not just sky. In order to verify that your image is framed the way your want it, take a test shot just long enough to get the stars exposed (and your foreground). Since this is a test image only, I use a high ISO and an open (low number) aperture. Once the image is set up properly, bring down the ISO to a lower level (100 or 200 will probably work) and close your aperture a bit (make the number bigger). This way you can set up your long exposure, without overexposing your sky. Take a couple of test shots with the same settings you intend to use for the satellite before it comes. Once you are happy with how the image is going to look, time your next exposure accordingly. For an ISS pass, you can either start your exposure before the Space Station is supposed to be visible (so you don't miss anything), when you see the ISS appear, or part of the way through your frame. Similarly, you can stop the exposure after the ISS pass is done, when you know it is out of the frame, or even part of the way through the frame. For an Iridium flare, plan the flare to be somewhere in the middle of your exposure to be sure that you include it. For example, for a 5 minute exposure, start at least 1 minute before it is scheduled. In both cases, make sure you use the exposure times you planned for. Resist the temptation to stop the exposure early to see how your image turned out. You made those test exposure times for a reason!
Getting an ISS pass or an Iridium flare on a background of pin-point stars has all the same principles as for the star trails image but with two main challenges: exposure time, and timing. Getting the an appropriate exposure time depends on the lens that you are using. The wider the lens, the longer exposure you can expose before star trails become apparent. I have found (on a cropped sensor) that at ~17mm you can use an exposure time of ~30 seconds without significant star movement. Once you have this exposure time you need to adjust your ISO and aperture accordingly so that your sky is properly exposed. With respect to timing for an Iridium flare, you can either wait until you see the beginning of the flare (and risk missing the start of the flare) before starting the exposure or try to start it a few seconds before the flare is scheduled to be seen. If you use your exact coordinates in heavens-above and a cell phone or computer clock, it is surprisingly accurate.