Question on bit depth

I understand the meaning of bit depth but am having trouble visualizing the effects on an image and have a couple of questions.

1. Can you obtain a greater bit depth by combining images of different lengths? For example if I use an 8 bit camera (e.g. SAC8) and combine 2 exposures of say 30 and 60 seconds will I have the equivalent of a single 16 bit image?

2. Is the "dynamic range" of an imaging device synonomous with its bit depth, or are there other factors that play into the dynamic range.

In the background of these questions is that I am contemplating several entry level devices and do not want to save a few bucks now only to find out that I want more out of the device after a few uses. I am thinking about the SAC 8 and Starlight xpress cameras (5 and 7 series). Any thoughts on these is also appreciated.

Thanks everyone.

John

1. No. Bit depth is the number of brightness levels in the image. An 8-bit image has 256 brightness levels. Adding two of them together, you still only have 256 brightness levels, not the 65,535 levels of a 16-bit image.

2. Dynamic range is the number of steps from the smallest possible value to the highest possible values. The size of the step is equal, roughly, to the noise of the system. An 8-bit camera can have no more than 256 steps in its dynamic range; a 16-bit camera can have no more than 65,535 steps. To determine the dynamic range of a camera, divide the full well capacity by the read noise. If these numbers are not available, you can't calculate the dynamic range.

To get a camera with good dynamic range, you need a 16-bit camera with a large well depth and low noise. The ST-7E is probably the entry level of cameras with a large dynamic range. The payoff for a bigger dynamic range is better S/N in your images, and more detail for a given exposure time.

Ron Wodaski
author of "The New CCD Astronomy"
http://www.newastro.com