Re: Comet like Ring nebula, or a planet

On Mon, 29 Oct 2007 14:31:05 -0600, Greg Crinklaw
<theskyhoundyoureye@xxxxxxxxx> wrote:

While I agree that a dark sky will be critical to visual observers,
isn't this a little over stated for imaging? I mean, the sky background
can always be removed from the signal as long as the signal to noise is
great enough can't it? Isn't that why imaging can be done from light
polluted locations? Easier maybe. But I think critical, no.

You can subtract out the skyglow signal, but that glow produces its own
noise (equal to the square root of the skyglow signal) which, like any
noise, can't be removed. It sets a fundamental limit on S/N, and if we
assume that the most interesting stuff to be seen in the comet is either
very faint, or very low contrast, sky noise will prevent it being seen.
So yes, I'd say that dark skies are very important if the goal is to
tease out very subtle detail.


I would think the larger issue would be the enormous dynamic range
involved given how much fainter the outer envelope is than the comet itself.

That problem is reasonably dealt with by taking multiple short images
(although readout noise becomes a potential limitation as well). The
images I've been taking have utilized exposures from 10-30 seconds. A
stack of 250+ such images (which is what I've been using) results in a
final image with something over 120dB of range- better than 20 bits.

_________________________________________________

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

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