(Simplified) How To Photograph an ISS Transit of the Sun or Moon

by Jeff Bondono

One-Time Preparation

• Note the latitude and longitude of your house. If you don't know this off the top of your head, go to google maps, click the Layers box at lower-left to switch to a satellite view, right-click on your house, and write down the GPS Coordinates (latitude and longitude of the spot you clicked) from the first entry on the menu which appears. For the next step, you need those two 7-digit coordinates rather than more accurate ones, so write them down.
• Now go to Transit-Finder.com. In the first location box, enter the coordinate you wrote down before the comma, probably something like 42.75377. In the second location box, enter the coordinate you wrote down after the comma, probably something like -83.05376. Then click the "Calculate" button.
• You should now see a list of ISS and Tiangong (the much smaller and more difficult to photograph Chinese space station) transits of the sun and moon during the next few weeks which are close to your location. The arrow over the Sun or Moon on each thumbnail shows how centered or how far away from the Sun or Moon the space station will pass. You'd strongly prefer to photograph a transit crossing the center of the Sun or Moon. The closer to the centerline, the longer your transit will take, and the more photos you'll be able to fire off while the space station is crossing over the Sun or Moon. The Center Line Distance tells you how far you'd need to drive to have the space station cross right across the center of the Sun or Moon. The Transit Duration tells you how long it will take for the space station to cross the Sun or Moon. It's usually between 1 and 2 seconds, so a very brief event. If you find a transit that's of interest to you, click the "Show On Map" button to see where you'd have to go to get right on the centerline, or at least whether you're within the colored region (the area that will have some transit, albeit maybe short-duration). You can zoom in on the map, and drag it around, and click a spot to get the statistics for the transit from that location. If you don't find a desirable transit, come back in 2 weeks and check again, but be sure to read on for the other things you need to prepare for your transit.
• If you intend to photograph a transit over the Sun, you MUST HAVE A SOLAR FILTER OVER YOUR CAMERA LENS OR ELSE YOU WILL BURN UP YOUR SENSOR. For transits over the Moon this is not necessary or advisable. If you need a solar filter, search Amazon for "solar filter for camera lens" and find/buy one appropriate for the lens you intend to use.
• You also need an app on your phone which displays the US Naval Observatory's Atomic Clock (just search your app store for "Atomic Clock" because you'll need to know the precise moment to press and hold your shutter button to capture a sequence of photos of the ISS traveling between you and the Sun or Moon. There's nothing visual to clue you in, you just need to trust the time and "fly blind" using super-accurate time to know when to start taking your exposures.

Preparation before a transit

• Check Transit-Finder.com every 2 weeks to see whether there's a transit near your home or that you're willing to drive to.
• When you get lucky and have one, check Transit-Finder.com again the day before (or even the morning of) the transit to see whether conditions have changed. The ISS sometimes needs to fire its thrusters to avoid collision with an object, which changes its orbit and moves the centerline or time by various amounts.
• Check the weather at the time of the transit, perhaps at Weather Underground. Be sure to change the location to the one you're shooting from. If total clouds are predicted, forget about this transit and wait for a better chance at clear skies.
• Once you're happy with a location and time, write that time, minus one second, in big numbers on a piece of paper you'll take with you. Be sure you're writing down the time for the location you're going to photograph from.
• Review that you understand how to do the things listed in the "On the Day of the Transit" section below.

On the Day of the Transit

• Take Camera, Tripod, Remote Shutter Release, Paper with the time (minus one second), phone with Atomic Clock app, and Solar Filter (if you're shooting a transit of the Sun)
• Plan to get there 15 minutes early (maybe 30 minutes early for your first attempt)
• Setup your camera on your tripod with the Remote Shutter Release, and your Solar Filter mounted to your lens (if you're shooting a transit of the Sun).
• Point the camera at the Sun or Moon
• Turn off Image Stabilization since you're on a tripod.
• Set your camera to Manual Focus and manually focus the shot, using Live-View if necessary, and reviewing focus in test shots. Keep working until the edges of the Sun or Moon and the Sunspots or Craters are super-sharply in focus in a magnified view of the test images you take. Take your time to do this correctly; it's difficult to get right but critically important.
• Set your camera into Manual Exposure mode and find the proper exposure of the Sun or Moon. Start by setting your ISO to the lowest value (usually ISO 100), set your aperture to f/5.6 or f/8, and set your shutter speed such that the exposure you obtain is not over- or under-exposed (no blinkies when you review the shot). You'd like this to be a pretty quick exposure, like 1/250 or faster, to freeze the motion of the space station. When photographing a transit of the moon, you might need to open your lens aperture wider and/or increase ISO in order to get such a fast exposure. Whether you're shooting the Sun or the Moon, set your exposure correctly for that object. The space station will be a silhouette in front of the Sun or Moon.
• Set your camera to take continuous photos at it's highest speed.
• Open your Atomic Clock app on your cell phone and wait until the time that you wrote down arrives.
• You'll probably need to re-center the Sun or Moon while you're waiting due to its motion across the sky. Be careful not to change your focus.
• When the Atomic Clock reaches the time you wrote down, press and hold your remote shutter to force your camera to take rapid-fire images of your target. Keep holding your remote shutter button down until your camera can no longer keep up. Then pack up and go home to see whether you succeeded.

Post-Processing (using Adobe Lightroom Classic and Photoshop on a desktop)

• Import images into Lightroom
• If needed, select all images and apply identical edits to each to obtain great exposures.
• We don't expect the first or last images to contain the space station because we started shooting 1 second early and shot a second or two too long, just in case the timing changed at the last minute.
• Pick which image is your best of the overall Sun or Moon. Sharpest focus, no motion blur, best contrast, etc.
• While all are selected, right click and pick "Edit In --> Open as Layers in Photoshop"
• Move your best photograph so it becomes the bottom-most layer.
• Add a layer mask for each layer above that bottom-most layer, and use the Brush tool to add each instance of the ISS onto your base layer, doing so one layer at a time. You should use a soft round brush set to opacity and flow of 100% and color black. Make it small enough that the central part of the brush (the part before it begins to fade) is just barely bigger than the space station. Then the outer part which fades will blend the edges into the base image. Just position your brush over the space station and click once. That will make the space station become invisible, press Ctrl-I to invert the mask and make everything except the space station disappear. Repeat for each layer except the bottom-most layer.
• Save your image (if it says the image is too large, go ahead and let it save as a large document format .PSB file).
• Once it's back in Lightroom, make final developing adjustments (like white balance, since your filter probably has a color cast, and sharpening).

If your first attempt does not get the results you want, try, try, try again. You might need to begin shooting 1/2 second early instead of a full second if your buffer filled up too early, or you might need to reduce your continuous shooting speed if that's possible on your camera. This is not your only chance. The ISS will continue to drift across the Sun and Moon every once in a while, perhaps with even better circumstances, as you take advantage of the lessons learned during this first attempt. You might want to review my more detailed instructions to get a different explanation of the process.