Tuesday, 26 June 2018

Capturing a pass of the X-37B OTV-5, and imaging an ISS transit over the Sun

click to enlarge

Yesterday evening was very clear, and the moon low in the south no real hindrance. I observed a very fine pass of the X-37B secret space plane OTV-5. It was an easy naked eye object. The photograph above (10-second exposure with an EF 2.0/35 mm lens) shows it ascending in the southwest, through Bootes (Arcturus is just above the open window).

The next morning (26 June) at 10:17:21 local time (8:17:21 UT), the International Space Station ISS was predicted to make a transit over the solar disc as seen from my house in Leiden.

I set up the Celestron C6 telescope in the courtyard, put a Baader Solar Foil filter in front of it, and hooked up the Canon EOS 60D to the prime focus. Instead of photographing at rapid burst, the technique I used for imaging with previous transits, I this time put the camera in HD movie mode. While this yields a lower resolution image than photography, the upside is that it yields more images showing the ISS silhouetted in front of the sun. And the ISS is big enough that the reduced resolution is not a real problem, the solar panels of the ISS are still well visible.

The image below is a composite of 21 frames from the resulting movie:

click to enlarge

Here is the movie itself, showing you how rapid such an ISS transit over the sun is (the total duration was only 0.8 seconds - it is over in a blink of the eye). The ISS had an apparent size of 45.8" during the transit, with the sun at 41 degrees elevation in the east:

The movie was made in the prime focus of a Celestron C6 (15-cm, F1500 mm Schmidt-Cassegrain, equiped with a Baader Foil solar filter) with a Canon EOS 60D DSLR in HD movie mode at 25 frames/second, with each frame having an exposure time of 1/4000th of  a second to avoid blurring the ISS. The track and time of the transit had been checked before the observation by loading the latest orbital elements for the ISS into Guide.

The biggest challenge with this kind of imagery is always to focus properly, certainly when the sun is spotless as it was this day. I always find focussing on the sun cumbersome. The focus this time turned out to be reasonably good though.

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