• Stephen Strum

ISS Transits the Sun

I've tried to image the ISS a couple of times before, but with limited luck. I tried again on Tuesday, August 10th when the ISS would be passing in front of the sun relative to my location in Oklahoma. I used the tool https://transit-finder.com/ to determine the exact time of the solar transit, which only lasted for about 2.4 seconds.

I set up my 8" EdgeHD telescope with my mono ASI camera, and my TV60 with my Canon SL2, both mounted on my Celestron Evolution mount. I didn't have a solar finder on my telescope, and you can't use a regular finder to locate the sun, so I used the telescope shadow method to get everything aligned. If you look at the shadow your telescope is making on the ground, the shadow will be the smallest when the telescope is perfectly aligned with the sun. This works well at low magnification, but it was still be challenging to get everything perfectly aligned in the C8 with the ASI camera I used. In retrospect, what I should have done was to use my DSLR in the C8 and the ASI camera in the TV 60 since the Canon has the larger field of view of the two cameras I used.

Anyway, I was able to get the transit in the TV60, but my shutter speed wasn't fast enough and so the ISS was still blurred in each frame, plus it was very small since the transit was only a half-hour or so before sunset. That means the ISS was about as far from my location as it can be while still being visible (just above the horizon), and so small in apparent size. I haven't had an opportunity to observe a transit closer to midday yet, but hope to eventually.

In my C8, I failed to take into account the mirror reversal and had the camera focused on the wrong segment of the sun. So, I only captured the ISS in the corner of a few frames.

You can watch the video I captured of the ISS transit with my Canon SL2 and Televue 60 refractor below. Not great, but I learned a lot trying to do the capture, so I should be able to achieve better results the next time around.

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