Capturing the ISS Solar Transit: A Celestial Ballet in My Backyard

Last Tuesday, I had the incredible opportunity to photograph the International Space Station (ISS) transiting across the Sun. While I previously shared a static image of this rare event, I've now animated the entire sequence to give you a better appreciation for the speed and scale of this celestial dance.

The Moment of Transit

In reality, the entire transit lasted a mere 1.36 seconds. While I couldn't adjust the speed in Photoshop to reflect its true duration (resulting in a slow-motion effect for the animation), the visual is still breathtaking. It’s a testament to the precision required to capture such fleeting astronomical events.

ISS Sun Transit

Sun & ISS Data:

Date: 12.08.2025
Time: 07:01:40 UTC
ISS angular size: 32.46"
ISS distance: 851.22km
Angular separation: 0.1'
Azimuth: 96.6°
Altitude: 26.3°
Transit duration: 1.36s
Transit chord length: 31.6'
R.A.: 09h 29m
Dec: +14° 51'
Parallactic angle: 39.9°
ISS velocity (angular): 23.2 '/s
ISS velocity (transverse): 5.74 km/s
ISS velocity (radial): 4.68 km/s
ISS velocity (total): 7.40 km/s
Direction of motion relative to zenith: 117.8°
Sun angular size: 31.6' (58.4 times larger than ISS)

Equipment:

Celestron NexStar Evolution 8” EdgeHD with Mount
Baader Digital Solar Filter OD 3.8
Canon EOS R5 MarkII

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 journey from raw data to the final animated GIF involved several meticulous steps:

Adobe Lightroom Classic: Exported all CR3 files as TIFFs for further processing.
AutoStakkert!4: Stacked the best 75% of the frames to improve image quality.
RegiStax6: Applied wavelet sharpening to enhance details.
Adobe Photoshop: This is where the magic happened. I inserted all photos featuring the ISS as separate layers, meticulously masked the ISS, and then combined them into a single file. To achieve the vibrant colors of the Sun, I desaturated the image and used curves adjustments: a primary curve for overall brightness/contrast, followed by individual curves for RED (Input 84, Output 139), GREEN (Input 95, Output 20), and BLUE (Input 218, Output 65). Finally, each layer was animated into a GIF to showcase the sequence of the transit.