Witnessing the ISS Transit the Sun: A First-Timer's Tale

Have you ever wondered if you could catch a glimpse of the International Space Station (ISS) silhouetted against the fiery disk of our Sun? Well, I did, and after seeing a stunning image of such an event, I was inspired to try it myself. What follows is the story of my first attempt, the meticulous planning, and the thrilling capture of this rare celestial event.

The Quest Begins: Finding the Opportunity

My journey started with a simple Google search, leading me to . This incredible tool allows you to pinpoint future ISS transits across the Sun or Moon from your specific location. As luck would have it, an opportunity arose right in my backyard!

Setting the Stage: Equipment and Preparation

The day arrived, and with a cloudless sky (though a bit turbulent due to the heatwave), I set up my equipment. Here's what I used:

• Telescope: Celestron NexStar Evolution 8” EdgeHD with Mount
• Solar Filter: Baader Digital Solar Filter OD 3.8
• Camera: Canon EOS R5 Mark II

With my telescope pointed precisely at the Sun and focused, I initiated my camera's continuous shooting mode just one second before the predicted transit time. It's truly mind-boggling how accurate these predictions are!

The Moment of Truth: The Capture

And then it happened! The ISS, a surprisingly small speck against the vastness of the Sun, made its swift passage. The entire transit lasted only about 1.36 seconds, moving at an angular velocity of 23.2 '/s.

While I didn't see it live on my camera's screen due to the rapid nature of the event, the immense relief and excitement when scrolling through the captured frames and spotting the ISS were unparalleled. Despite the atmospheric turbulence, I'm absolutely thrilled with the result, especially considering it was my very first attempt.

The Technical Details: Data and Processing

Here's a breakdown of the technical data from the event:

Sun & ISS Data:

• Date: 12.08.2025
• Time: 07:01:40 UTC
• ISS angular size: 32.46"; distance: 851.22km
• Transit duration: 1.36s
• Sun angular size: 31.6' (58.4 times larger than ISS)

Acquisition Details:

• Focal length: 2032mm
• Focal ratio: f/10
• Frames: 215 (41 with ISS)
• Shutter speed: 1/8000s
• ISO: 400

Location: My garden, Illnau, Switzerland

Processing:

The captured images underwent a detailed processing workflow:

1. Adobe Lightroom Classic: Exported CR3 files as TIFFs.
2. AutoStakkert!4: Stacked the best 75% of the frames.
3. RegiStax6: Applied wavelet sharpening.
4. Adobe Photoshop: Inserted ISS frames as layers, masked the ISS, and composited them. Color adjustments were made using curves to achieve a vibrant representation of the Sun.
5. Adobe Lightroom Classic: Final crop and adjustments.

Future Endeavors

This successful first attempt has certainly fueled my passion. The precision of the predictions is astounding, and the experience was incredibly rewarding. My sights are now set on the next challenge: capturing an ISS transit across the Moon. Stay tuned!

I hope you enjoyed this glimpse into the world of ISS solar transits. I welcome any constructive criticism and feedback!