Re: A beginner question

On Mon, 11 Jun 2007 21:06:25 +1000, "Peter Webb"
<webbfamily@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Essentially then digital ISOs are analogous to push-processing on film. To
somewhat extend the analogy, its like a "digital zoom" - no extra
information is actually added.

I don't think the analogy is precise, but I'd agree that digital ISO is
more like push processing than native film ISO. However, as I noted
earlier, if the gain is applied in the sensor, before the readout stage,
it's possible to see a slight improvement in S/N because the readout
noise contribution isn't amplified (or isn't fully amplified).


Sure. My issue was that I had two different "scales" in use in my head, and
now way to calibrate between them. I know its much the same principles for
astro and terrestrial photography (of course, no depth of field issues,
point sources only, some others), but I had no way "in my head" of equating
the two scales. When I looked at the question concerning a magnitude 5.8
object on a terrestrial camera, I had no scale to equate this to daytime
photography. Unlike, for example, photographing the moon which is directly
illuminated by the Sun. I don't even know how to work out the exposure etc
for photographing the Sun directly (and nor would I try!); if I did, I could
pull out an inverse square law and work out what would be needed for the Sun
as a distant point source to be "resolved".

Yeah, I don't think there's any way to really equate the calculation of
exposure for terrestrial imaging with the calculation for astronomical
imaging.


Presumably the camera optics are not much better than are needed to resolve
to physical pixel size on the CCD array (what would be the point)? So
presumably also the point source can easily spill over into adjacent pixels.
Most digital cameras will let you reduce resolution - I wonder whether this
improves sensitivity (as it does for film grain size). This is presumably
whether they sum the baseline data from adjacent pixels to produce the lower
resolution image. If they do, halving the total image size should provide a
massive 1.5 dB gain.

Of course, this can as easily be done in post-processing, I guess?

There is only one noise source that is pixel dependent, and that's
readout noise. For short exposures (typically less than a few seconds
with ordinary digital cameras, and up to many minutes with cooled
cameras under dark skies) the readout noise is dominant, so there is a
value to having less pixels involved. However, this advantage is only
realized when the camera is capable of binning the pixels within the
sensor, so a single readout operation is involved. If you reduce the
resolution after the sensor is read, either in the camera or during post
processing, there is no reduced noise.

For exposures long enough that readout noise is fairly insignificant, it
makes no difference how many pixels are involved.


Is the intent to capture in each JPEG frame the "dark background" in
sufficient resolution to add multiple images and decrease noise by summing?
So is the JPEG really better considered as a map of the background noise
rather than the foreground point sources, and the digital artefacts don't
matter as much?

And if the JPEG contains the data to exactly reconstruct the original
bitmap, then its hard to see how it could be much smaller than the bitmap.
You have got to be chucking information away somewhere.

Well, a lossless JPEG compression (which isn't an option with any camera
I've seen, although the standard supports it) could still be highly
compressed with respect to a bitmap for many astronomical images, given
that much of the background may be essentially zero. Naturally, if you
fully encode the noise, the JPEG will have to be the same size as the
bitmap.


I assume that RAW is just that, the pixel values. Every camera I have owned
in the last five years has it, wouldn't that be heaps better? (Although I
have never used it, and until recently was prohibitively expensive in
storage).

That's what you'd like RAW to be, although with most cameras there's
still a certain degree of processing that has occurred before the raw
image is saved. But it's close enough to raw pixel values in most cases
that typical image processing operations such as stacking can be
performed. Besides the lack of compression artifacts, what's very
important with RAW is that the original dynamic range of the sensor is
largely preserved. The data will be 10-12 bits deep (60dB-72dB),
compared with at best 8 bits (48dB) with JPEG encoding.

_________________________________________________

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

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