Capturing an ISS Transit
Astrophotography can be a slow hobby. Everything above us works in a predictable ballet and since a lot of stellar objects are captured with long exposure there is little need to rush. About the only time I feel the need to hurry is during deep winter since Astronomical darkness closely follows the end of my work day. Observation and photography of planetary phenomena (eg, bodies within our solar system which typically use sub-second exposure times) can be an exception. Events like solar eclipses or the movement of Jupiter’s moons are just as predictable as the rise and set times of any distant nebula (in fact, the movement of Jupiter’s Moons have been used to set clocks before), but timing is much more critical in a successful capture. One of the best examples of this is capturing the International Space Station crossing in front of the Sun or Moon.
The ISS, which has been in orbit in varying forms for over 20 years now, orbits the Earth every 90 minutes at an altitude of 400 km / 250 mi. To the naked eye this football field-size station appears like a bright, moving star - if you see it at the right time. The ISS is only visible using reflected sunlight which limits its visibility to morning or evening twilight. Too early and it will be exceedingly hard to spot against the daytime background sky, though it can sometimes be seen through a telescope. On the other end, if the station passes overhead more than about 60-90 minutes after sunset it will be within Earth’s shadow. These limitations are mainly for naked-eye observing though - as the goal for this article is to teach you how to capture the ISS transiting in front of another body, we can rely on these background objects to silhouette the ISS regardless of whether it is reflecting sunlight.
The catch is that the transit times are fast. The ISS is moving around the planet at 7.6km/s and your average transit is only 1-1.5 seconds. Hours of planning, transportation, and equipment setup are all in pursuit of a window which lasts about as long as it took you to check the time in the corner of your screen just now.
Planning for Capture
First and foremost you need to know when and where the transit will happen, and a few websites can help with this task. Transit-finder is perhaps the most well-known. This website displays all upcoming transits (within 240km & 30 days) through use of a color overlay on Google Maps. I find this service the easiest to use; the interface is simple and to the point and so far its predictions have been accurate.
A closeup of a typical transit window. Transit-Finder displays the time, altitude, and duration of the transit and the red overlay shows its area of visibility
Selecting a Transit
The first property of these transit windows you may notice is that they are visible for several hundred kilometers - yet only a few kilometers off the center-line. Your placement along the length of this line does not matter much, but you should plan to set up your camera(s) as close to the the center-line as possible since that point will give you the longest transit window (and as bonus will make the resulting photo more symmetrical). Even a few hundred feet on either side of this line will cause the ISS to be noticeably off the center-line of the Sun or Moon.
A typical transit line is hundreds of km / mi long but only a few wide
Second, not all transits are of equal quality. While CalSky is able to limit predictions to a certain altitude, Transit-Finder will show you every visible transfer, including those where the Sun or Moon are barely a few degrees above the horizon. Low-altitude transits offer a trade-off: the transit time will often be much longer than the typical 1.5 second window and may reach as high as 3-6 seconds. The price for this longer transit time is lower image quality as photographing night sky objects near the horizon involves shooting through the thickest part of our atmosphere. Another possible downside is that the ISS will not have its iconic “Tie Fighter” shape. The station takes active measures to always point the same side towards the Earth (it makes antenna alignment easier among other reasons) so during a low-altitude transit you will be photographing the station from a side angle. In general I avoid shooting anything under 20-25 degrees whether it is a transit or regular night of long exposure.
A Solar Transit near zenith from February 2020 (Left) vs a low-altitude Lunar capture from September 2018 (Right). In the left image, the ISS has the iconic “TIE-fighter” shape while the lower-altitude transit on the right shows the station tilted with solar panels viewed from the edge.
Backlighting
Even though the Sun and Moon are our only two useful backdrops the main difference in capturing a transit is really whether or not the ISS is visible outside the solar or lunar disk. For solar transits the answer is always no since 1: you will be using a solar filter and 2: lacking a filter, the camera will be unable to successfully capture any contrast on the ISS while it is caught in the glare of our home star (also the camera will be on fire). In the case of the Moon, which is typically much safer to observe than the Sun, late night transits of the ISS will be practically similar to their solar counterparts in that the ISS will not be visible on either side of the Moon (such as the example in the photo above). If the transit occurs during twilight hours when the ISS is still sunlit (or even daytime Lunar transits, provided they are a safe angular distance from the Sun), then it will have lower contrast on the Moon but will be visible on either side of the Moon as well.
The practical difference between these two is the focal length in use. For those using deeper focal lengths of 1000mm+ the Sun & Moon will fill a decent portion of your camera’s Field of View. For most of my transits I have used a 2000mm Celestron C8; combined with a focal reducer this telescope has 1366mm of focal length but with my T3i the effective focal length is closer to 2050mm (remember that 1.5x modifier for crop sensors). If you only have a small refractor or zoom lens to work with (300-600mm or so) then I recommend limiting your captures to twilight transits. In these wider focal lengths the Moon will be small but the sunlit station will be visible on either side of it, such as in the photo below.
A twilight Lunar Transit from January 2018 captured with a reduced C8 and a T3i. Since the ISS was sunlit it was visible on either side of the Moon
Why am I trying to photograph the station multiple times? I just want one photo!
One photo is certainly fine, but spamming the capture button (or using video capture) will give you the best chance of capturing a sharp image. Your focus may be slightly off or the atmosphere may be particularly turbulent that day so most of your individual captures will vary in quality or sharpness. In addition to this, many people who capture these transits like to combine all the photos together so the ISS appears in a line corresponding to its rapid movement.
Mobility & Location
The transit is almost certainly not visible from your backyard so be prepared to travel. If you shoot using a tripod, DSLR, and a zoom lens then this is hardly an obstacle. If you use a heavy equatorial mount with an 11” SCT then your setup and tear-down time may be considerable. Issues of mobility are not unique to shooting this kind of photo and if you routinely transport your equipment to dark sites then it is of little concern.
“Time to go mobile” - the astronomer Bane prepares to travel to an ISS transit window in The Dark Site Rises (2012)
Your setup location will be unique for each transit. I use the Google Maps overlay on Transit-Finder to find open areas and most of my transit have been captured from parking lots on or near the center line. Those living in more rural areas may find success in simply pulling over to the side of a country road. Unlike a typical astronomy outing to a campground or dark site, your possible locations for setup are heavily constrained by both the center line of the transit as well as the time of night (or day).
Typically good locations may include public parks or parking lots near schools, churches, or shopping malls (with considerations for the time and day - you won’t find much parking at a school during a weekday morning and security might not want you there anyway), but I try to set up away from crowds or sources of high traffic. My most recent transit capture was visible over a high school but the transit also passed over a shopping area intersected by two major roads. As the transit was taking place on the same Sunday afternoon as the Superbowl the school parking area was predictably empty but the shopping area was extremely busy with people out buying food & drink for the upcoming game. The last thing I need when setting up nearly 100kg in optics and tracking equipment is attention from passerby or possibly even police, even if the transit occurs during midday (I don’t own a Dobsonian telescope, but they resemble large cannons and SWAT teams have been called on unsuspecting astronomers before). Night-time transits can actually make setup easier since businesses are closed and traffic is minimal - just be aware of your surroundings.
Countdown to Capture
1-3 Weeks Out
You found an upcoming transit in your area! Examine the transit line and look for good locations for setup. If this is your first attempt at a transit capture, do a backyard test run with your equipment to see about how long you will need for setup, tear-down, and any recurring issues like trouble finding good focus or bad alignment with your tracking mount.
1-2 Days Out
Double check the transit window. Placing the ISS in Low-Earth Orbit (LEO) makes it easier to reach but those solar panels cause a lot of drag so it periodically boosts itself back into a higher orbit. This is great for the continued survival of the astronauts on board but it may also move the transit line. Transit-finder is quick to update based on these new orbital parameters, but if you find the line has suddenly moved one day now you know why. This actually happened about 18 hours prior to my most recent transit capture which forced me to look for a different place to set up my equipment.
12-24 Hours To Go
Set up your equipment completely and make sure everything is photo-ready, then disassemble it straight into your car - this ensures you have everything you need. “Blind” packing is a great way to forget a critical piece of equipment. This advice goes for any astronomical outing and I do this every time I am packing for a trip to a dark site. Double check the transit prediction again as well.
T-45 Minutes
You should be arriving on-site for equipment setup right about now. For more equipment, give yourself more time, but err on the side of caution. Aside from equipment setup, use this time to allow the telescopes to reach ambient temperature and fine-tune the focus using test shots. If you are shooting during the day bring a heavy towel or blanket of dark color to drape over your head so you can see your camera or laptop screen. This can be uncomfortable during warmer summer months but the discomfort of finding your photo was out of focus lasts much longer in my experience. As I write this the Sun is currently in Solar Minimum and finding proper focus on the Sun lacking any sunspots can be a challenge.
A Solar ISS Transit from October 2017 (taken from the Cosmosphere parking lot). My focus was poor since the Sun was “blank”
Accurate setup of an equatorial mount during the day is a challenge in itself, but can be mostly mitigated by ensuring the tripod is level and using a compass to roughly align to North (assuming you know your local declination). Arriving well ahead of time and watching how the Sun drifts in your camera’a field of view may also allow you to make adjustments to your polar alignment during the daytime. If the transit is at night then simply use your typical polar alignment routine.
If using a dedicated Hα solar telescope, take this time to tune for the best surface detail as well
T-10 Minutes
Verify focus and exposure with more test shots. Remember that the ISS is moving fast so excessive exposure times may cause it to blur in your images. The exact limit depends on your focal length but the deeper focal lengths require shorter exposures. At the effective focal length of ~2000mm I don’t shoot slower than 1/1600” but wider focal lengths will be more forgiving. For most Solar or Lunar passes this may not be an issue since both the Sun and Moon are fairly bright and even at low ISO values the exposure times will be short.
T-2 minutes
There is no time for significant adjustments now. Make sure the Sun or Moon is centered in your Field of View. The Transit-Finder prediction will tell you the exact time to expect the ISS, but don’t trust your watch or even cell network for an accurate time. Instead, have time.gov open as this clock has always been accurate to the transit window. If capturing a twilight Lunar transit you may even be able to see the ISS approaching with the naked eye which will give you a reasonable estimation on how much time you have before the transit.
The Shoot
The exact time you start to take your photos will depend on how you plan to capture this event. For Solar and late night Lunar transits start your exposure several seconds before the expected time. For sunlit Lunar transits this depends more on your field of view. Transit-finder tells you the duration of the transit across the Moon or Sun, but if you use a wider focal length then you may have to eyeball roughly how many Lunar diameters fit into your Field of View and multiple the transit time based on that.
As for capture methods, many new DSLRs are capable of capturing 4k video at a decent FPS and if you have one available this is probably one of the best methods of capture. For older DSLR models like my T3i, video mode is actually just a series of 1080p JPG images (plus an audio channel), so I like shooting with individual shots in high-resolution JPG or RAW. This is because even at 2000mm of focal length the ISS is small, so if I get away with less total shots of it in frame it will still probably have better detail. I would prefer to shoot entirely in 16-bit RAW but my T3i buffer fills up after 6-7 shots whereas JPG can fire continuously for minutes at a time.
For those using Dedicated Astronomy Cameras, assuming your laptop has good USB throughput and harddrive speeds you can start a video capture in your program of choice (I use Firecapture) whenever you desire. My most recent transit capture involved two telescopes & cameras so I started a video capture with my dedicated astronomy camera about 15 seconds before the transfer so I could focus on locking down the remote shutter on my DSLR.
T+10 seconds
Take a breath & check your photos. If you have a Fitness Watch, check your heart-rate monitor because it probably spiked in the last 5 minutes. Next, take another stack of photos as you would for any regular photo - I’ll explain why in the next section.
Stacking & Edits
Post-processing a successful transit is similar to any solar or lunar imagery - just with the bonus of having an interloping space station in the way. For Lunar imaging I recommend you check my article Shoot for the Moon as it goes into a lot of detail for recommended steps in processing. Processing solar images is actually quite similar, though. A Solar image (filtered, obviously) and a Moon photo both have comparable properties regarding luminosity and detail. The major difference in stacking a transit image is that you may need to do so twice for best results:
First take any images which have captured the ISS and blend them all together either using darken or lighten blending (depending if the ISS was sunlit or not) - this will be your “Transit Stack”
Next, take your second capture (taken right after the actual transit) and stack them together as you would for any regular Lunar or Solar image (you can find a list of the software I use for this one my article Astrophotography on a Budget). This will be your “Regular Stack”
Finally, layer the Transit Stack onto the Regular Stack and mask in each iteration of the ISS. Combining these two images will enable the best overall image quality; the ISS will be shown exactly as you captured it and you also gain the benefit of a smoother image of the stacked Sun or Moon behind it
You don’t need to perform steps 2 and 3 though; if this was your first transit capture, you may be satisfied only using step 1. If you do take a second capture for a stacked image then, before, during, and after Step 3 should run sharpening or noise reduction processes on your photo as detailed in the Shoot for the Moon article linked above. Be aware that small but high-contrast details like the ISS may easily form rings or halos from being oversharpened so remember to closely examine your photo between each step in editing and use appropriate masking.
Good Luck!
Capturing a photo like this is one of the best challenges for an astrophotographer. A successful capture require precise timing, the ability to operate your equipment under pressure, and the cooperation of local weather. With adequate preparation, the right equipment, and a little luck, you may be able to capture a transit like this yourself.
The ISS transiting the sun in false-color Hα from February 2, 2020
Coronado SolarMax III 70 Double Stack, ZWO ASI1600MM-P
The ISS transiting the sun in false-color White Light from February 2, 2020
Celestron C8, Canon T3i
Don’t Care About Photos? Make it an Experiment!
These transits aren’t just opportunities for a cool photo. If you are able to compare the time of your capture to that of a friend’s capture along the same transit line you can calculate the altitude of the ISS. World-renown Kerbal Engineer Scott Manley details the how-to in this video. The typical limitation encountered by an astronomer in this scenario will be the part about having friends