The Pleiades, Dust Clouds and Observing With Longer Exposure Times
March 09, 2017
The image above showing the easily recognizable Pleiades star cluster was taken from the New Forest Observatory in England on January 20, 2017. This image totals approximately 15 hours of exposure time and includes about 7-hours worth of 40-minute sub exposures from the Sky 90 array telescope. Using longer sub-exposures (subs) we're able to bring out the very faint dust clouds filling the entire Pleiades region that are completely missing from most older images. For comparison see the 2007 Earth Science Picture of the Day that totaled 5 hours and 48 minutes of exposure time, mostly using 10-15 minute sub-exposures.
This image therefore dramatically shows just how much deeper you can go in imaging deep-sky objects by significantly increasing the sub-exposure time. So why don't we simply go for very long subs all the time? There are several good reasons, but a major one is that unless your equatorial mount is very well polar-aligned, you'll start to see the effects of polar rotation coming through -- stars will appear egg-shaped.
Photo Details: Image by Greg Parker and processed by Noel Carboni. Photo composed of 18 x 40-minute subs on the Sky 90 array telescope.