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Phil's Rambling Rants
September 23rd, 2010
02:03 am


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S00j win, astronomy fail
I went over to Indy today for s00j's concert.  Getting there was a bit of a pain.  First there was a bill I remembered I really needed to get in the mail, so I was a little late leaving.  Then, just before the Crawfordsville exit, traffic came to a complete stop and everyone who could was bailing out at the exit.  I decided to do so also and go through Crawfordsville rather than sit, but it ended up costing me another 15 or 20 minutes.  Then coming into Indy there was a really bogus work zone, (I'm a bit worried that I may have gotten a photo ticket), and then I had to get through construction on 38th street which can only be described with an 11 letter word that starts with C, ends with K, and might upset a few readers.  Thus my plan of being half an hour early turned into being a few minutes late, so I missed most of the intro, but I didn't miss any songs.  And I certainly wouldn't have wanted to miss any.  I didn't get the new album because they didn't have any, but by way of apology, we got to hear several songs even newer than the album.  I can get the album at OVFF.

When I got home (which involved less stress than getting there), I decided to take a night walk and admire the full moon and Jupiter.  It was just past local midnight, and when I was half a mile away from my house, it was all clear and I got the crazy notion to try to use my honkin' big telephoto lens as a telescope and find Jupiter's moons.  Unfortunately, by the time I got back to my house, clouds had rolled in.  There were a few breaks, but not many.  It was very pretty, but I didn't have the ambition to switch to a less extreme lens and figure out a good exposure.  I could easily see Jupiter as a disc and I think I saw one of the moons, but there was never enough of a gap in the clouds to set up a long exposure.  Maybe I'll be inspired to try it again and it will actually be clear.

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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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