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Venus As A Banana
CREDIT: Alan Hodgson ASIS HonFRPS

On the shoulders of Giants

An exercise in capturing stuff

Just over 400 years ago Galileo turned his telescope on the planet Venus. He observed that Venus exhibited phases just like the moon. These are easy enough to see with a modern telescope, and to capture if you fit your DSLR onto one. But that would be just too easy...

So this became a challenge for my Aperture project. Can I ride on the shoulders of Galileo and record the phases of Venus with a standard camera lens and DSLR? Off we go.

Like many photographers I like to sit back and study stuff before I dive in. Which lens should I chose? What sort of settings will work? What is the best time to do this?

The first thing to consider is that Venus is a really small object in the sky because of the distance. At the present time it is a thin crescent - think banana at 1km and you have the scale about right. So we are talking about a telephoto lens to see it as anything but a point of light.

My Nikon D750 DSLR has a pixel pitch of 6 microns. I will save you the maths but a banana at 1km will be only 11 pixels long with my trusty 300mm lens. Now we get to the difficult question of lens settings and the question of aperture. And the fact that not all apertures will deliver the results I require.

The resolution of a lens is determined by the smallest spot of light it will deliver onto your image sensor, known in the text books as a point spread function. In terms of your lens settings it is determined by the aperture with small f/numbers delivering the smallest point spread function. Again I will save you the maths and state that an aperture narrower than  f/5.6 makes for spots of light bigger than my 6 micron pixels. I know from past experience that this lens has difficulties fully open so with the lens set to f/5.6 we can step outside and take some pictures.

From the UK Venus is currently visible to the WNW just around sunset, looking like a really bright star. Try and catch it before it gets too low as the seeing tends to get worse. Here is a sample image, taken at ISO 100 and 1/1250 seconds; a short exposure to reduce any image blooming across any pixels. Venus as a banana; an approximately 15% lit crescent cropped to 25x15 pixels.

Pretty close to the calculated 11 pixels length and great when a photographic project comes together. Now to see how thin a crescent I can record in between clouds…