Titan’s Shadow on Saturn

Dr. Donald G. Bruns (dbruns@stellarproducts.com)

San Diego, CA 92129

In 2025, Saturn was just at the right point in its orbit to allow its moon Titan to cast a shadow on the planet for a few hours, several times over a few months. This orientation occurs about every 15 years, and this time, the transit was centered near midnight in the western hemisphere; Europe and Asia could not see the event. While Saturn was a little low in the sky, and the humidity in San Diego was a challenge, I was able to get some nice images in good seeing for both the September 3 and September 19 events.

I have been working on a DIMM (differential image motion monitor) and was able to use it before I started imaging Saturn on September 3. At about 9:30 PM PDT the seeing was 0.8 arcsec at the zenith. Since Saturn was 44 degrees during my best imaging near midnight, that means the seeing around Saturn was about 1.0 arcsec (assuming the seeing did not change much during the night).

I used a C11 telescope with a ZWO678 monochromatic camera with a red filter, giving 0.15 arcsec/pixel plate scale. I imaged for about 50 seconds, recording 1000 images, at a gain of 200 and exposure time of 0.05 seconds. I tried longer and shorter exposures with other gains, but this combination seemed the best.

I then used Autostakkert! to measure each image quality, then had that program stack the best 100 images. I used the wavelet feature of MaxIm DL to sharpen the image just a little bit, to get the final images. I combined four of them to make the movie. In the best stacked image here, taken just about midnight, you can just see Titan to the upper left of Saturn. If you left-click on this image, a short movie will be downloaded, showing Titan’s motion across the planet for a few hours.

The next imaging opportunity was September 19. The skies were hazy and very humid, so I was able to image for only about 30 minutes. I used the same telescope, camera, and filter, so the images could be compared. Because Saturn was close to opposition, the rings were supposed to be much brighter (the Seeliger effect), and that is evident in my image. Titan itself is visible just overlapping the lower left of its shadow, a slightly darker spot. Left clicking here also shows a short movie.

To get an idea of what the seeing looks like, left-click on the next image below. That will download a movie made from the first 50 images captured by the camera on September 19. It also shows how well stacking software and sharpening software works!

All text and images are owned by Stellar Products, 1992-2025. Any use by others without permission of Stellar Products is prohibited. For information on commercial use of any of these images, click here.

Web page last updated September 22, 2025.