🚀 go-pugleaf

Some times even well-known authors using telescopes and digital capture devices confuse how signal to noise increases with increased exposure and come to believe that 300 one-minute exposures equate to one 30 minute exposure (ten to one ratio).

Ouch! That's wrong.

Signal to noise increases in proportion to the square root of the exposure whether this is a single exposure or a stack of multiple exposures.

The single exposure is actually better, because it has a single read-out noise component, whereas the 100 one-minute ones will have 100 read-out noise components.

Of course, DSLRs are not cooled devices, so you don't want to leave the shutter open for hours. The sensor gets really, really hot.

I've been doing multiple night-time exposures of the night sky for a long, long time. When shooting deep sky (galaxies, nebulas and so on) digitally, I try keep each exposure to less than 240 seconds, occasionally even 30 seconds, usually on a motorized equatorial mount.

One thing to remember is to switch off auto noise reduction (dark-frame subtraction), if for no other reason than to save imaging time.

The image below was taken with a 300mm Pentax manual lens on a Pentax *istD, in Western Oregon a few years ago. Dozens of shots were stacked, I'd have to dig my notes to be sure, but I seem to remember it was in the order of 70 or 80 stacked exposures. I used a Canon cable release with an intervalometer, adapting it for Pentax contacts.

The sensor/chip in the *istD, which was also used by Nikon in their first DSLRs, just happen to be the one also used by the the Starlight Xpress dedicated cooled CCD camera, which sold for about $8,000 US Cy. at the time. In those days, Nikon had trouble with noise reduction that could not be switched off which deleted faint stars. Bad karma. ;) Canon was not in the running for those purposes yet.

 <http://www.pixentral.com/show.php?picture=1Bsn1LuIuXj1XL8Tkc48qpwaskIBb0>