Last night Linda and I joined with members from a small photography group at Ashbridges Bay to watch the fly-over of the International Space Station (ISS). NASA offers a Sighting Location web site announcing up-coming local fly-overs.
Last night’s fly-over was unusual for two reasons: [1] the ISS was high in the sky at 73° and [2] it was visible for 6 minutes. If I were to add a third it would be that the sky was fairly clear; only a few high clouds that didn’t interfere.
I was surprised by the brightness of the Station and the pace it travelled across the sky. It was certainly brighter than any star in the night sky, comparable to Venus. Coupled with the pace, it could easily be confused with a high-flying aircraft. In the images below, the streak is the Space Station.
ISS Rising
ISS Setting
There are a few challenges facing the photographer in taking a shot like this.
First, the length of time a digital camera’s shutter can stay open is often limited. In my case, 60 seconds. Therefore, it is impossible to keep the shutter open for the full course of the Station, which in this instance was 6 minutes. What this implies is that capturing the track of the Station needs to be done in not one, but several individual shots, that are subsequently combined together. In this case it was 9 shots.
Second, even if one could keep the shutter open over the duration of the fly-over, the stars would no longer appear as points of light in the sky, but rather as trails. Such a composition could turn out to be artistically interesting in itself, but that’s not the shot I was looking for.
My objective was to keep the stars as points of light, not trails. This would highlight the track of the space station. To accomplish this I needed to take certain steps when taking the shots and then subsequently in post processing.
In capturing the images I had to consider: [1] stars will begin to blur after keeping the shutter open for an extended period (which varies according to the focal length of the lens); the general rule is to keep the exposure under 20 seconds for a wide angle lense. I kept mine open for 8 seconds using an 18mm lens. To cover the full 6 minute fly-over, I needed to stand by the camera and press the shutter after each shot completed [2] The camera has a noise reduction step which is executed after each shot of 2 seconds or greater. Unfortunately this step can’t be turned off. The duration of the noise reduction step is equal to the duration of the exposure (i.e., 8 seconds in this case). This meant that there would be gaps in the ISS trail between images.
In post processing I had to merge all the individual shots into one composite and resolve the gaps in the trail introduced by the noise reduction step. This would show the track of the ISS as a single trace across the sky. Typically one would use Star Trail software to automate the task. However, as the stars are moving too, simply combining the shots would result in star trails as well, not the single points of light I was looking for. To avoid this problem: [1] rather than using star trail software to stack the images and create the trace of the ISS, I manually stacked each image as separate layer using On1 Layers [2] to keep the stars as points of light, I selected one layer as the background sky and masked out everything from the other layers except the track of the ISS [3] I used the Lightroom cloning brush to fill in the gaps.
The connections between the tracks isn’t perfect (there are a few misalignments noticeable when viewed at 200%) but it is sufficient for this purpose.
For those interested, some technical information on the capture:
- Lens: 18mm
- Camera sensor: full frame
- Aperture: f/3.8 (that was the widest opening for that lens)
- ISO: 1000 (at this speed noise should not be a problem)
- Shutter Speed: 8 seconds (determined by camera)
- Settings: Aperture priority
- Focus: manual, at infinity
- Tripod
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