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Tracking the ISS


Tyneside, UK
2018 Jan 20
Saturday, Day 20

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(new Window/Tab, Flash... "sorry iPad")


Visual Flare from the ISS

This piece of work was done by way of an expeiment to see if software can be used to obtain a light curve from a satellite that is varying in brightness. The ISS is an easy target and was going through a period of evening visibility at the time. The starting point is a straightforward trailed image of the ISS passing over Lincoln, UK

STS

The image was obtained using a Nikon D-80 digital camera fitted with a Nikon 18-70mm zoom lens set to the shortest focal length and aperture f6.3. The exposure was 68 seconds long. The ISS moves from right to left during the exposure.

Measurements from Image

Using a a piece of software called 'Makali'i Image Processor', it was possible to measure the relative brightness of the trail across the image. The result is below.

Brightness Curve

The technique used was to allow the software to sample the trail and produce a dataset that consisted of a brightness value for each pixel group along the length. Although not particularly obvious from the image, there is a slight dimming due to an encounter with light cloud to the left of the flare. The cloud shows as a faint red glow as it reflects local urban lighting. The sky also brightens towards the bottom of the image frame where the trail starts. Both of these affected the raw readings taken from the trail.

These effects were taken out of the equation by creating what was effectively a 'dark slide' of a thin slice of sky immediately adjacent to the trail and deducting the results from the trail measurement.

The result is a plot that describes the flare very well. There are some features to note on the plot. The slightly rhythmic section at the left of the trace and the two spikes prior to the flare maximum (ie to the right of the peak) are stars within, or very near to, the trail. The two on the right are very obvious in the image.

The flare itself is not a smooth event, with a step occurring on the rising slope of the flare curve.

The technique is good for relative measurement but does not provide absolute values of brightness.
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