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2013-11-03

Partial Solar Eclipse 2013-11-03 13:25 UTC

I'm back on a rig, for a different bunch to the previous people in Tanzania (who are continuing with their programme there).
This time I'm on the West coast of Africa. Several tens of miles off the coast, actually. This map shows (approximately) where. The boats are not exactly un-obvious, but there's no point in making the pirate's lives any the easier.

Google Map

Today, in between doing things in the office, we've had a bit of post-prandial (after eating, for those who don't speak "Classics") excitement trying to follow a partial solar eclipse. We're actually several hundred miles from the totality region (which made landfall in Gabon, between Port Gentil and Libreville), so we only got a partial eclipse, but it was still pretty damned good.
Unfortunately, I'd not done my homework, and so hadn't brought decent filtering equipment for solar photography with me. Mea culpa. So I had to bash together a filter to protect (one) eye, and the sensor of my camera. Sometimes the warnings about "eclipse eye safety" are a bit overblown, but it is true that you can hurt yourself, so a reasonable degree of precaution is sensible. (And apply rule #1 : if it hurts, stop doing it!)
So ... what had I got? The black rubbish bin liners on the rig ... are too opaque and the plastic is loaded with biodegradable starch grains, so they're no use as filters. (Pure plastic sheeting, if thin enough, can do a reasonable job, and you can stack layers to adjust the dimming. It's a bodge, and only a poor substitute for a proper filter. As you'll see.)
Then I realised that I'd got a pair of those cardboard "spectacles" for viewing "3d pictures" with.
Now, those filters by themselves don't do a lot of filtering - maybe a stop or two (transmitting a third to a half of the light falling on them). But they're complementary colours :
So the combination is much darker - 6 or possibly even 8 stops (transmitting a tenth to a twentieth of the light incident on them). It does impart a rather unhealthy hue to the photos, but it got the light into the right sort of range of intensity that the camera's auto-exposure could handle it. With a little help from being set into low-sensitivity mode by the user. (Yes, I did RTFM.)
So, results :
Lesson # 1 : flash is a waste of battery in these circumstances. And it's still (just) too bright. By eye I can see the bite out of the Solar disc, but it's washed out in the sensor. On the other hand, the sensor hasn't started streaking (charge leaking from one CCD cell to the adjacent ones, resulting in vertical or horizontal (depending on the wiring of the sensor) streaks of overexposure).
The netting around the helideck is obvious, and those blobs of cloud promise considerable dimming of the light as they pass across the Solar disc.

My next attempt to handle the overexposure : prop myself up against a convenient piece of metalwork, and zoom the telephoto lens to it's maximum. (It is left as an exercise for the reader to realise why I didn't bother trying to use the additional "digital zoom" in the camera.) It didn't really help much. The cloud probably helped more.
This is the same image, but with the contrast cranked up all the way, and the brightness scaled back to reveal the top couple of bits of the intensity range. The zone of most over-saturation is noticeably off-circular, and that is in approximately the correct orientation for the "bite" that the Moon was taking out of the Sun's disc. All I need is another stop or two.
The moral of this story : carry your bloody filters! I'm there in a technical sense, just. But, to be honest, the photos are crap.
So what else can I do? Wait for cloud. Take some photos of our little flotilla out here.Look to see if we can see the shadow bands of the totality away to the south.
Oh, look. Cloud!
And the cloud is dialling the exposure down.


I processed this image (offline ; you don't have time to do this in the real world. You do your exposure calculations in your head.) in exactly the same way as the previous one (contrast up by 127 bits ; brightness down by 126 bits) and even the eye of faith isn't needed, because there's clear evidence of the wolf Fenrir attacking the Sun.
And the cloud thickens more.And off comes the makeshift filter.
Time to blow trumpets and bong gongs to scare the wolf away!

Not the best photography in the world, but not too shabby for stuff scrabbled together at the last moment.

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