Capturing something as small and fast as the ISS has to be planned using patience, precision and a bit of luck concerning the weather (as always in this hobby…). You will need a good mount (ideally with tracking), a DSLR/mirrorless camera or dedicated Astro-camera and the longest focal length telescope/lens you have, to get the most possible details in the shot.

There are a few reasons why this plan is so difficult:

• The ISS is just 73 meters wide and orbiting at around 400 kilometers. So obviously it will appear very small (from about 10” [arcseconds] to about 1’ [arcminute], depending if it’s passing low on the horizon or directly above the observer). For comparison, the Full Moon viewed from earth is around 30’ wide!

• The average distance to the Moon is 384 400 km. Considering the ISS is only 400 km away, the path where the ISS can be seen in front of the Moon is very narrow at around 8 km. So, in order to be able to see the transit, you may have to travel quite the distance to get into the visibility path.

• Spontaneous change of route. The ISS is affected by atmospheric drag and has to boost regularly to stay in orbit, which can change the path both pretty drastically and instantly. 

• And last but not least, even if you did everything right, the Universe might put some clouds right in your face…

The basic equipment you need includes the following:

• DSLR/Dedicated Astro-camera
• High focal length telescope/lens
• (Tracking) Mount

 
My equipment for this image:

• DSLR:           Sony a6400
• Telescope:   RC8
• Mount:          SkyWatcher EQ6-R Pro 

Camera settings:

Setting up your camera for an ISS-Transit is different from "standard" astrophotography.
You either want to capture a video of the pass or use the burst mode of a DSLR.
I would recommend the video method, as you won't have to worry about timing that much as in burst mode, and also won't run into trouble with the saving speed of your memory card.

Software:

• PIPP
• Autostakkert! 3
• Registax 6
• Photoshop (or similar)

Here is the planning involved in getting this difficult and beautiful shot:

• Use https://transit-finder.com to find out when the next transit is happening at or near your location. Simply put your location and elevation details into the fields and check the results. 

• Use Stellarium to check the framing of your camera.

• Check your equipment: You should be using the biggest telescope/lens you have in order to achieve the highest possible resolution. If you are imaging from a location without power, consider using a power box to enable tracking the Moon and powering your camera and additional equipment you may be using to make your life easier. Make sure to test the whole setup before the event to make sure everything is working as intended.

• Look up the weather forecast and the path of Transit regularly.

• Set up your equipment early to avoid missing the moment due to some annoying problems in the setup. I would recommend having everything ready to image at least 30 minutes before the pass, so you can test everything for a last time.

• Start the video recording quite a bit before the calculated time of the Transit. 1-2 minutes should be enough to make sure you won’t miss it.

So hopefully everything worked out by now and you got an impressive video of the ISS passing in front of the Moon. Now you can process the video in any way you like. I myself stacked the background with the Moon using PIPP and Autostakkert! to get a sharper and better image, and then aligned the frames of the ISS manually and blended them in using Photoshop. For the next attempt, you can look at some more details, like the apparent size of the ISS during the pass (which varies quite a bit depending on where and at which distance it is passing) and wether it is illuminated by the sun or not (it was illuminated in my image, so the contrast to the Moon is quite low). 

Don’t expect everything to work out perfectly the first time, as always in this hobby. But after so much planning and working to get this done, looking at a final result like this is just so satisfying. 

Good luck and Clear Skies.

Johannes

Full image:

Written by Johannes Möslein, 24. August 2024