Recently, an image depicting the transit of the International Space Station over the planet Saturn was posted on different websites and a lot of fame came to the so-called author. It was later proved that the image was a fake, and some prestigious websites removed it altogether from their pages. The author stated that (after a lot of discussions on different forums between amateurs that knew how difficult it is to get such an image) that he was actually depicting a simulation of such an event. Well, I can now prove just how difficult such a transit is to image…
The transit ephemerids were computed using the CalSky website.
Together with my wife and her brother we’ve traveled for around 40 kilometers from home to witness just such an event, a transit of the ISS over the planet Jupiter. From experience, I knew that the great brightness difference between the two objects will give me some problems; Jupiter has a rather low surface brightness compared with the ISS. If some astro-photographer wants to image both objects at the same time, he must either underexpose Jupiter at such a level where Jupiter will be barely visible in the images, or, he must overexpose the ISS, and doing so loosing some of the structural details.
For this transit I chose a half-way method, with just a bit of underexposure for the planet, and a bit overexposure for the ISS.
The following shots were acquired using the 355mm F/5 Newtonian, a 3x Barlow lens, and the ASI174MM camera with a Green filter. All this was placed onto the EQ6 mount. I do believe that in order to get the ISS sharp (with no motion blur due to its high speed) and with a decent level of details, one must use a rather large scope in order to cope with the great brightness difference of the two objects (such as Jupiter and Saturn). The camera setting were: gain at maximum, and exposure time at 0.5 milliseconds (1/2000sec in photographic terms). This allowed me to have a frame rate per second of 90, which was good enough to capture the Station in seven frames.
And before I start with the more or less processed result, I shall post the most-processed image of the event, a 1800-frame stack for the planet and two satellites, with the ISS position from the seven frames placed in a different layer and blended with “lighten” in PS CS2. The image shows some nice details on the planet (like the Great Red Spot) and also the two satellites towards the right (Io is the one closer to the planet, and Ganymede is the other).
The shot also shows just how difficult it is to get the ISS over such a small apparent diameter: despite being on the predicted “totality” band, the ephemeris errors are always large enough that the event is practically impossible to catch. I think that 100 meters away the ISS would have been at least partially transiting the planet.
And now only the seven frames which captured the ISS, again blended using “lighten” in PS CS2. Note how few details are visible on the planet, and the satellites are almost impossible to spot.
To have a “true” sense of the actual data, here is a raw frame of the event, with no processing (only .bmp to .jpg conversion). The hour of the event is present in the file name.
And now two animations. The acquisition rate was at around 90, while the first animation presents the event at 20fps:
And a slower 3fps version:
The first shot from this post was published on the SpaceWeather site.