Sunday, April 26, 2009

Am I imaging Mercury as a disk?

Last night I did a little experiment to try to determine if the circular appearance of Mercury in my images is actually due to the extended nature of Mercury as a planet, or if this was some sort of glitch with my Nikon Coolpix L4. I worked on the assumption that any such glitch would not be consistent at different zoom levels. So I decided to take a sequence of images at a series of progressively higher magnifications, using the tripod to maintain the same image and the self-timer (which has a ten-second delay) to eliminate any vibration caused by clicking the shutter button.

I took a series of nine images, one of which I displayed in the previous post. This is the original, unzoomed image, taken at 8:47 PM. Mercury is barely visible in the larger version you'll get when you click through.

I zoomed in a step, pressed the button again for the delayed shutter release, and took the next image. I repeated this process with each step. As the process went along, the sky grew darker and Mercury sank lower and farther to the right. This is the fifth image, taken at 8:48 PM:

The ninth step was the maximum zoom I would get without kicking in the "digital zoom." I'm not quite clear on how the zoom feature works - while my camera claims a "3x optical / 4x digital" zoom, and through this sequence I believe I only used the optical zoom, this final image is about 10x the size of the first image. That is, while the unresized version of this image covers 31.556" x 23.667", a crop of the first image covering approximately the same area (using the branches and wires as a guide) is only 2.986" x 2.181". This last image was taken at 8:50 PM:

Here is the whole sequence, cropped and resized to only include enough of the branches to give a reference for comparison. It is easy to see the darkening of the sky and the downward / rightward motion of Mercury from the first image to the last.

It is also apparent - to me, anyway - that the disk of Mercury remains roughly consistent throughout this sequence, despite the differing zoom levels throughout.

Which still proves nothing. Only one test will really be valid. Mercury subtends a known amount of sky at any given time - 8.7" on April 30, according to Astronomy magazine. The Moon also covers a known span of sky, though I don't have that figure at my fingertips. Tonight, if conditions permit, I will get a series of pictures of the Moon and Mercury together. Will I be able to use these images to directly compare the angular sizes of Mercury and the Moon? If I can, then this will tell me if I'm actually imaging the disk of Mercury - or if I'm just seeing a glitch in my camera. Stay tuned!

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