by Chris Peterson » Fri Jun 18, 2010 2:19 pm
WildGuruLarry wrote:Based on the length of the star trails (about 45 degrees), it looks like it was a 3 hour exposure. With digital cameras, quite often you can take multiple shorter exposures and "stack" them, but I never could figure out how you could stack star trail images without getting gaps in the trails. If anyone can explain that to me, please do. :)
You can definitely take an image like this with your 40D. The simplest way would be to get a shutter cable (one that can lock the button down), and lock it down for 3 hours. For an exposure of that length, you will need a really dark place no matter what aperture or ISO settings you use. :)
You can't take a 3-hour exposure with a 40D, or with any DLSR. The dark current is too high, and the pixels will fill up from that and obscure any actual image. DSLRs give the best long exposure results for 5-10 minute images. An image like this is made as you first suggest- a lot of short images are stacked together. You do sometimes see star trail images that show gaps in the trails. However, newer DSLRs like the 40D are capable of writing an image so quickly that there is less than a second between the end of one frame and the beginning of the next. That's short enough to make gap too narrow to see in a wide field image like this (stars along the celestial equator only move 15 arcseconds in a second; nearer the pole it's less; this image has a scale of around 200 arcseconds per pixel).
I'd make an image like this at ISO 200 or ISO 400, with the lens reduced one or two stops from maximum (to reduce aberrations), and I'd use a smart cable release, that can be programmed to shoot a sequence automatically (an inexpensive Canon accessory). Then I'd stack the subframes in Photoshop.
[quote="WildGuruLarry"]Based on the length of the star trails (about 45 degrees), it looks like it was a 3 hour exposure. With digital cameras, quite often you can take multiple shorter exposures and "stack" them, but I never could figure out how you could stack star trail images without getting gaps in the trails. If anyone can explain that to me, please do. :)
You can definitely take an image like this with your 40D. The simplest way would be to get a shutter cable (one that can lock the button down), and lock it down for 3 hours. For an exposure of that length, you will need a really dark place no matter what aperture or ISO settings you use. :)[/quote]
You can't take a 3-hour exposure with a 40D, or with any DLSR. The dark current is too high, and the pixels will fill up from that and obscure any actual image. DSLRs give the best long exposure results for 5-10 minute images. An image like this is made as you first suggest- a lot of short images are stacked together. You do sometimes see star trail images that show gaps in the trails. However, newer DSLRs like the 40D are capable of writing an image so quickly that there is less than a second between the end of one frame and the beginning of the next. That's short enough to make gap too narrow to see in a wide field image like this (stars along the celestial equator only move 15 arcseconds in a second; nearer the pole it's less; this image has a scale of around 200 arcseconds per pixel).
I'd make an image like this at ISO 200 or ISO 400, with the lens reduced one or two stops from maximum (to reduce aberrations), and I'd use a smart cable release, that can be programmed to shoot a sequence automatically (an inexpensive Canon accessory). Then I'd stack the subframes in Photoshop.