[irwjager (Ivo)]

Originally Posted by JohnH

Nobody has mentioned setting the GAIN correctly. Can this not help S/N?

I am pterrty sure it does on my ccd where the signal can be bosted mare rapidly than noise (well it looks that way in the graphs I have seen) - the compromise will be saturation occurs faster for the bright elements.

Very good point!

Gain in digital systems comes down to multiplication of the signal (and thus also multiplication of the noise). When using gain settings that are higher than unity (e.g. a factor > 1.0), as you're suggesting, saturation is reached sooner and there is less time to average noise events, resulting in noisier images.

Gain factors that are lower than unity (e.g. < 1.0) may help, in that there is more time before saturation occurs, so there's more time to collect light/expose and average out noise events.

In the real world, however, your mileage may vary depending on your hardware's characteristics (triggering voltages, thermal noise, circuit noise, rounding errors due to gain multiplication, etc.).

The comment about h'ware binning is the opposite of what I have heard - s'ware binning is not binning at all really - it is a 2x2 or 3x3 averaging.

Actually, the term 'binning' does not define how the binned data is used. Binning is pretty cool; you can use binning to increase exposure (e.g. add the pixel values), use binning for noise reduction (e.g. add the pixels, then divide the result by the number of pixels you binned), use binning to increase bit-depth (e.g add the pixels and promote the image to a higher bit-depth at the same time), or any other exotic operation such as median filtering, adaptive binning, fractional binning (1.75x1.75, 3.14x3.14), etc.

It doesn't matter whether this is done at the hardware level, or the software level; the operation is *exactly* the same and there's no difference. Hardware binning is still software binning, only that's it's performed by the software in the firmware, instead of the software on your computer.

EDIT: Though they are almost the same, RickS points out that the latter statement is nevertheless untrue (it is done at the hardware level) and there is one important benefit to hardware (summed) binning; read out noise reduction.

'Software' binning allows more flexibility in two ways; 1. It allows you to manipulate the image before it is binned (ex. subtract light pollution - stuff that you don't want binned). And 2. It allows you to use more advanced types of binning than your hardware supports (for example, StarBright determines how much a group of pixels needs to be fractionally binned for maximum contrast and no over exposure, reducing resolution only by the smallest amount possible).

H'ware binning 2x2 will reduce the required exposure time by a factor of 4 (at the cost of resolution) for a given signal level and is used to great effect in LRGB imaging where L is 1x1 and colours are taken at 2x2.

And that's exactly what I use binning for most of the time. After I have taken my shorter than normal exposed image without binning (e.g. dimmer, but at full resolution). Then using a binning algorithm on my computer after I have taken out the light pollution.