Long exposure vs. multiple exposure

Hi all,

I bought a Canon PowerShot S45 digital camera a year or two ago before I
got into astronomy. It's a decent camera, I was thinking of rigging it
up to try my hand at astrophotography.

I've looked through the docs and experimented with long exposures, but
this model only goes up to a maximum of 15 seconds. I was initially a
little disappointed, but then got thinking about using multiple
exposures of 15 seconds (or less) and stacking the images.

Maybe my logic has taken a wrong turn somewhere but, given the accuracy
of your average, run-of-the-mill mount, wouldn't the use of several
images allow some manual correction of tracking errors? Surely there is
some major advantage of a single, longer exposure that I've so far
overlooked? (Apart from the obvious elimination of the stacking process
itself.)


Just wondering,

Steve

On Thu, 19 Aug 2004 02:11:26 +0200, Steve Maddison wrote:

Hi all,

I bought a Canon PowerShot S45 digital camera a year or two ago before I
got into astronomy. It's a decent camera, I was thinking of rigging it
up to try my hand at astrophotography.

I've looked through the docs and experimented with long exposures, but
this model only goes up to a maximum of 15 seconds. I was initially a
little disappointed, but then got thinking about using multiple
exposures of 15 seconds (or less) and stacking the images.

Maybe my logic has taken a wrong turn somewhere but, given the accuracy
of your average, run-of-the-mill mount, wouldn't the use of several
images allow some manual correction of tracking errors? Surely there is
some major advantage of a single, longer exposure that I've so far
overlooked? (Apart from the obvious elimination of the stacking process
itself.)

There are different sources of noise in digital images. Dark current noise
increases with time, and is the same for one image or a stack of equal exposure.
Readout noise occurs with every image however, and for short exposures is a
significant part of the total noise. The advantage of long exposures is that the
readout noise becomes insignificant.

As you note, there is an advantage to using shorter exposures in order to
minimize tracking problems. Most serious imagers find a compromise position,
using subexposure times ranging from between 5 and 30 minutes, and then stacking
as many as required, usually for total exposures of one to several hours.

With a digital camera, your options are much more limited. In most cases, you
will simply collect a great many images at the longest possible exposure. Noise
will be large compared to what you would have with a cooled, long exposure
camera, but that doesn't mean you can't get quite nice results.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com