2004-06-08/

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Venus crossed the face of the sun this morning for the first time since 1882. I spent a few hours constructing a home-made helioscope to see the transit. There was a lot of trial and error involved. The first photos (about 8.30am, starting with IMG_2022.JPG) are very indistinct and have spots due to dirt that are clearer than that due to Venus. By IMG_2037.JPG there's just the one spot (top and slightly to the right), and it is Venus, but I don't know that for sure yet. Then I found a way of improving the contrast and the photos got much better. The best is IMG_2056.JPG (about 10am). After that the sun left my balcony and I stopped experimenting, although the transit continued for a few more hours.

This set of photos includes several of the apparatus. Not being equipped with any suitable lenses, I used a pin-hole camera. I made a small hole (initially 1mm diameter, later enlarged to about 3mm) in the middle of an A4 piece of card, and wedged it under the arm of a garden chair. This cast a small round spot of light on the floor of my balcony surrounded by a large shadow.

The spot is an image of the sun, but it is rubbish for two reasons. First, it's only about 1m behind the card, so it is only about 8mm across, and the resolution is about 1mm (the size of the pinhole). Second, the rest of the balcony is in bright sunlight, so there's a lot of ambient light, and the spot is barely brighter than the surrounding shadow. 

I therefore propped a mirror against a piece of wood to reflect the spot in through the window. I used a wooden drawing board to cover the other parts of the mirror that were in direct sunlight. The ambient light in the room is much lower, and I reduced it further by drawing the curtains as much as I could, leaving just a small slit to let the beam in. This improved the contrast.

The beam length is now about 7m, so the image is about 60mm across, still with 1mm resolution. That's good enough to see Venus in theory, whose image is about 3% as big as that of the sun.

The image is cast on the bedroom door, and moves constantly but slowly to the left as the earth rotates. I held up a sheet of paper as a screen and took my first photos.

I also took some photos of the apparatus. The most spectacular is IMG_2024.JPG. In this photo there are five genuine spots of light, plus numerous reflections inside the camera. The genuine ones are: 1. the dust on the mirror where the beam hits it; 2. the dust on the window where the beam hits it; 3. the reflection in the mirror of the frayed cardboard around the pinhole; 4. and 5. direct sunlight creeping in to the top-left and bottom-right corner of the mirror round the edge of the card (the wooden board needs adjusting).

Later I opened the bedroom door to let the beam into the bedroom. My bedroom has much better curtains and I managed to make it quite dark. The remaining ambient light is mostly from the white-painted balcony wall in the sunlight just behind the mirror, and from the reflection of the sun off the wooden board. Both these contaminants shine in right next to the beam, so there's not much I can do. The beam length is now about 12m, so the spot is about 100mm across. That's big enough that I can afford to enlarge the pin-hole to 3mm diameter to let more light in. That alone immediately improved the contrast by a factor of ten.

IMG_2065.JPG, IMG_2060.JPG and IMG_2059.JPG together show the whole apparatus in its final form. This was the setup with which I got the best photos.

The movie MVI_2076.JPG shows how fast the image moves across the wall as the earth rotates. In the 30 second movie, the sun moves about a quarter of its own diameter, as you can see by the pattern on the wall-paper. With a 12m beam length, a quick calculation predicts that the image moves about 0.9mm per second. IMG_2072.JPG, IMG_2075.JPG, the movie and IMG_2079.JPG were all taken with the mirror in the same place within a few minutes of each other. As you can see, keeping the beam where you want it requires continual adjustment of the mirror.

MVI_2076.AVI