"Southern Hemisphere Challenge: Crux and Omega Centauri" Fri 17-May-13 11:10 PM by nrothschild
This is only for the Southern Hemisphere because these objects are not visible from the USA and other Northern Hemisphere mid-latitude sites.
For Marjani in Cocos/Keeling, I think you should try to shoot Crux with either your 85/1.8 at f/2 or the 70-200, wide open, at about 100mm. If you are interested in this end (the longer end) of things, I would try the 85 to see how you like the images. That's a big stop if it produces clean stars at f/2. If it needs f/2.8 then it's a push which to use, assuming the 70-200 is clean or as clean wide open.
For anyone else, you generally want to use the longest fastest lens available, up to about 100mm. Beyond that star trailing becomes counterproductive.
Crux fits very tightly into a 200mm frame but a 100mm frame will be easier to shoot and lessen star motion problems. At 200mm you lose a stop of exposure due to star trailing so no real gain there.
The best time to shoot, right now, for Marjani in Cocos, is 8:38pm, +/- a half hour or so. That is when it is highest in the sky. That optimum time drifts backward at the rate of 30 minutes per week, and in about 3 weeks you run into dusk.
For anyone else, we need your latitude and longitude to compute exact times, but it should be within 30 minutes or so of Marjani's time, simply based on where you are, in longitude, in your time zone. And these are standard times, in the case where you observe DST add one hour.
So if you want to do this, you should do it in the next 3 weeks if possible. Thereafter you have to shoot it just after dark at increasingly lower altitudes. Or wait until next year
Your shutter speed, for about 3 pixels of drift, would be about 3s for the 70-200 at 100mm or about 4s at 85mm. That for a 12mpx FX sensor such as the D700 or D3. For 12 mpx DX that would be about 50 and 70mm.
I would push the ISO as high as you can tolerate; for a D700, 3200 minimum and I would suggest trying 6400.
This may not be "art" but it *is* a scientific experiment .
If this works, I have another idea, to see if anyone can image Omega Centauri from a fixed tripod. This is a huge globular cluster, the brightest in the sky. We have nothing like it up north. It is one of the top observational spectacles visible anywhere.
Omega Centauri is the size of the full moon, in principle. On a fixed tripod you will only image the center so it will be somewhat smaller.
Omega is located about 12-16 degrees above Crux, at the optimum time I mentioned, in Centaurus. If you have binoculars, try locating Crux with them, then move straight up, more or less, about 16 degrees, or about 2-3 binocular field widths. Scan around, and if you can find a fuzzy ball, you should have the target. It is probably fairly faint but it should stick out as obviously non-stellar .
If you can locate it, then try to get the camera framed in that area and do the same exposure as for Crux, same lens and focal length.
The optimum time to shoot Omega Centauri is 9:44 right now, or about an hour after Crux's optimum time. So it is a nice one night project.
If this works you may find yourself taking pot shots at the brighter parts of the Milky Way. Have at it
And of course, if you have a motorized equatorial mount, by all means use it . Then your exposure is limited to the quality of the mount and your ability to control it, guide it, and align it.
Feel free to shoot at wider focal lengths, such as a fast 50 prime. I just suspect 85-100mm is the best compromise between imaging Omega Centauri with reasonable resolution and star trail issues. For Crux, that would be beautiful at any focal length.
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