Whilst browsing www.transit-finder.com, I noticed a huge swath of England and Wales will be able to see the International Space Station (ISS) transit across the moon. For the first time a couple of weeks back I was able to capture my first ISS transit, and as many people will be able to see the transit in the UK, I’d like to share my tips to getting a great shot.

My shot was captured in the Peak District under clear, dark skies. Luckily, you don’t need dark skies to capture the ISS, only clear ones! The ISS usually reflects the sun down to earth and can become as bright as -4.0 magnitude, easily visible from the brightest cities.

The first step to capturing the lunar transit is to find out where the next transit will occur on www.transit-finder.com. Here, you can input your location for anywhere in the world, set a date range, and the site will calculate where and when a transit will take place for you. Transits have a specific area where they can be seen from, ranging from a few miles across to hundreds of miles, the darker blue line represents where the ISS will transit in the middle of the moon.

Sidenote: Some great apps to tell you when the ISS will cross over your location are ISS Detector and Heavens Above. Stellarium can show you exactly where the ISS will be in the sky.

I’ve selected this transit as it covers a huge amount of central England and southern Wales, very unusual as most transits are only a few miles across. The most important figures that you need to know are these:

Date.

ISS angular size.

Azimuth.

Altitude.

Centre line distance.

Visibility path width.

Transit Duration.

The “How to use this website?” section on the transit finder site gives in-depth explanations on what each of the figures means, but I’ll try to simplify it here. From top to bottom: Date is pretty self-explanatory, just don’t turn up on the wrong day! Don’t become confused between days, anything after 23:59:59 is the next day, so if there is a transit at 00:01:00 on Wednesday, most people would call that Tuesday night, even if that is technically wrong!

ISS angular size is how large the ISS will appear in front of the moon, measured in arc seconds. For context, the moon or sun are generally 1800-1900 arc seconds across, Venus can be up to 60 arc seconds across. For this transit, the ISS will be around 16 arc seconds across, quite small for a transit.

Azimuth defines where in the sky the moon will be, it is measured clockwise from North, so North is 0°, east is 90°, south is 180°, and west is 270°. You can use Altitude in tandem with Azimuth to find exactly where the moon will be, in this case, the altitude is roughly 6.1° above the horizon, quite low for an ISS transit. Think of a degree as your little finger held at arm's length, the width of your finger is roughly 1°.

Once you’ve found where the transit will happen, you’ll most likely want to get the station passing directly in the centre of the moon, Centreline distance will tell you how far away the centreline of the transit is from your location. The dark blue line on the map will also show you where the centreline is. It is worth checking this regularly as the centreline can change quite drastically as predictions get better closer to the transit.

Visibility path width tells you how wide the path of the transit is from top to bottom, in this case, if you are within 70km north or south of the centreline you will be able to see the transit, the total distance north to south is 141km. The ISS will appear either above or below the centreline depending on where you are.

Finally, and one of the most important numbers is transit duration. ISS transits are usually brief encounters, this is one of the longer ones at a staggering 2.24 seconds at the centreline! The further away from the centre line you are, the less time the ISS will transit the moon as it will have less distance to cover.

The last thing to mention is the difference between illuminated and in shadow. an “illuminated” ISS means that it is reflecting sunlight towards earth, in the evening you should be able to see a bright white dot moving across the sky before it crosses the moon. If a transit is “in shadow”, that means it is not reflecting any light from the sun to earth, so the only time you’ll see it is when it passes over the moon, in silhouette.

For this example, the ISS will technically be “illuminated”, however as it occurs at 4pm, it won’t be able to compete with the sun, so the station should appear as a silhouette against the moon.

Part 2: Camera Setup

So you know when and where your next transit is, you’ll now need to know how to photograph it!

Let's talk focal length first. At a minimum 300mm might get you somewhere, however, you’ll need a lot of cropping which will degrade the image quality drastically unless you’re shooting at ultra-high resolution. I use a Sigma 50-500mm lens for my moon shots, and I still needed to crop, so a teleconverter would be a great addition to your kit bag to cheaply increase the range of your lenses.

An essential piece of gear is an intervalometer or shutter release cable. This allows you to trigger your camera without touching the body, this helps reduce camera shake and makes stacking the images in post a much easier task. You can get official ones from your camera vendor, or cheap £10-£15 ones can be had from Amazon or eBay.

Contrary to most forms of astrophotography, you’ll need a fast shutter speed, a low ISO and a mid-range aperture. Let’s go through these one by one.

Shutter speed: You’ll want at least a 1/500th shutter, whilst the moon will stay pretty stationary, the ISS is bombing it across the sky at over 17,500km/h. If you can go higher do so, my shot was at 1/500th and there is still some motion blur on the ISS. Around 1/800-1/1000 at 500mm would be perfect to freeze the ISS, if you have a longer lens or a teleconverter, consider increasing that even higher.

ISO: To keep sharpness and limit noise in the black areas of your shot, use the lowest ISO you can get away with. For my shot, I had to use ISO400 to get a correct exposure at 1/500th and F/5.6. If you have to use a high ISO don’t be afraid of doing so, it’s more important to use high shutter speed in this scenario.

Aperture: In general, the more you stop down your lens, the sharper the image will be, however, this will restrict the amount of light that can reach the sensor (I.E: going from F/4 to F/8 will darken your shot by one stop of light and generally increase sharpness). The most important variable is shutter speed for this shot, so if you need to shoot wide open to avoid a high ISO then do that.

RAW or JPG: If you happen to own an EOS R5 that can do 20fps RAW until your card fills up then shoot RAW! For everyone else you’re better off with JPG for this shot, the main reason being that you can shoot way more images in JPG in a row then you can with RAW, and there’s not a huge amount of post-processing where you’d want the flexibility of a RAW file.

FPS and buffer size: Let’s get into the technical stuff now. A camera that can capture more images per second will yield more frames of the ISS for you to pick from and merge into a composite. I used a Canon 7D Mark II for my shot, which has a 10FPS continuous shooting mode, This allows for 10 images to be captured every second.

You might have noticed that there are some gaps after the ISS has passed by the moon in my shot, that is caused by me not realising how fast the buffer in the 7D can fill when shooting. As mentioned above, JPG will allow you to shoot more images in a row then RAW, in the case of my 7D, there is a buffer of 31 RAW images, which gives you a 3.1 second window before the camera starts trying to offload the images onto the SD or CF card. In contrast, I’ve never hit the buffer whilst shooting JPG. You may need to shoot for 10-15 seconds continuously for a full transit, so to avoid hitting the buffer limit, shoot JPG.

That’s all for this post, the next instalment will be all about post-processing your transit images, don’t miss it!

The Ultimate ISS Transit Guide Part 1: Planning and Setup.

Part 2: Camera Setup