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S00j win, astronomy fail - Phil's Rambling Rants — LiveJournal
September 23rd, 2010
02:03 am


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S00j win, astronomy fail

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Date:September 23rd, 2010 11:06 pm (UTC)
Jupiter gets about 1/25 the sunlight that earth gets, which is a reasonable basis for figuring out how to get the planet well-exposed. However, with a telephoto lens, the moons are going to be smaller than a pixel, so you'll need to overexpose the planet to get the moons to appear.

I've never tried photographing Jupiter's moons with a telephoto lens, but I've got some sense of how to do astrophotography. Exposure for the moons as essentially point sources depends as much as the absolute aperture of your lens as f/ratio. I think that you would want a large fraction of a second--in the range of 1/4 sec to 1 sec to get something with a telephoto lens. More than that and the moons will start to trail or streak, depending on your focal length. For 1200 mm focal length, 1/4 sec is about were the moons start to streak and the planet turns visibly oval. I recall that my dad's longest telephoto lens was about 300 mm, so presumably up to a full second or maybe two would be worth trying.

Summing it up, I'd start with the aperture stop fully open. I'd start in the range of 25 x the time you'd use for a daylight exposure at the same ISO and F/ratio, and then try longer exposures up to about 2 seconds.

But keep in mind I haven't done serious astrophotography with an SLR since the pleistocene film era.
Date:October 9th, 2010 01:17 pm (UTC)

Example DSLR photo



Taken last night in Carlsbad, NM (el 3100')

420 mm f5.6 0.5 second ISO 400

Much longer exposures and it will blur.

Good tripod, no wind, remote shutter release, mirror lock-up, and most importantly, clear dark sky.
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