Sean's blog

2010 January 15th Solar Eclipse (not) seen in Malaysia

Solar Eclipse January 2010, as seen in Henan, China

I was so excited when I heard about there was going to be a partial solar eclipse visible in Malaysia today. It was an annular eclipse in some places, such as this photograph taken from The Malaysian Insider of the eclipse viewed from Henan, China.

It was only going to be a partial eclipse, but even so, I was looking forward to capturing it for my blog. Before you read any further, I should add the video here. Don’t bother watching it, unless you really want to feel as I felt today. It was cloudy. For the whole duration. Here’s the video:

It’s quite nice to see the clouds roiling above my house. Or is it?

USB Camera Obscura

The bit where you can clearly see two layers moving in different directions isn’t bad. If it looks like poor quality, it has been reduced from 640×480 to 320×240 prior to uploading to YouTube, and it was captured by my USB Camera Obscura! Here it is, sitting outside my house today.

The plastic table and chairs came in very handy! My Dell Mini9 was on one (covered in rubbish most of the day – I was worried someone would see it). The table served as a sun-shade to allow me to see the screen (almost impossible otherwise). The other chair was my ‘observatory’.

The cardboard box is the camera obscura. I didn’t actually test whether pointing the webcam directly at the sun would work. The way I see it is that if it didn’t work, there’s a slim chance I’d destroy my webcam, so I played it safe and made the camera obscura. It’s really just a box with a hole in. The image of the sky is formed on the side of the box facing my PC. The hole is facing the sky. I put a single sheet of white paper on the image side of the box to improve contrast. The webcam is rather clumsily taped to the lid of the box at the same end as the hole, but pointing towards the other end (where the image is).

If it’s not clear what’s going on – the video is of the image projected through a small hole in one end of the cardboard box onto a sheet of white paper at the other end – inside the box. The webcam is also inside the box. The dark edges are not the walls and roof of my compound – they’re the sides of the cardboard box, seen from inside. The webcam doesn’t ‘line up’ with the sheet of white paper because I only found out about the eclipse an hour or so before it was due to start! Everything was rushed!

Then all I did was run the magic mencoder command line:

mencoder tv:// -tv driver=v4l2:width=640:height=480:device=/dev/video1 -nosound -ovc lavc -fps 1 -o solar.avi

That tells mencoder (part of MPlayer) to use the second webcam (the first is built into the lid in my Mini9) and create an AVI movie with no sound and 1 frame per second. The low frame rate is still much higher than needed for sky-watching, but was a compromise between not wanting to miss anything, and not filling up the tiny flash drive in the Mini 9. I think I ended up recording just over an hour of video, occupying nearly 2GB on the PC.

And then it was cloudy all afternoon ~:( and now I’m really, really disappointed! Never mind, it was good to go through the whole procedure. The camera obscura seems to work a treat. I think the Logitech webcam I’m using may not be the most sensitive in the world. It’s really hard to tell – maybe I should build in a light-proof eye-hole so I can check in future! There was a bit of light leaking in. You can see in the picture where I’ve tried putting black fabric tape over the corners of the box to cover up the holes. There’s a bit of an orange blur over the video: that’s light leaking in.

Just one last thing: the 1frame-per-second video is very annoying to replay! I changed it into a 20fps video, with this command line:

mencoder solar.avi -vf filmdint=io=25:1,scale=320:240 -fps 500 -ofps 20 -ovc lavc -idx -o solar2.avi

The filmdint filter drops 24 out of 25 frames (so I end up with one frame for 25 seconds of ‘action’). I keep staring at this command line and failing to understand how I end up with just over 1 minute of video from well over an hour’s worth of data. It’s very late now, and I’m a bit sad about the result of my ‘hard work’ today, so I’ll leave that last bit as an exercise for the reader.