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Old June 19th 07, 04:06 AM posted to alt.binaries.pictures.astro
Richard Crisp[_1_]
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Default ASTRO: Common object uncommonly processed

here's my point;

you actually want dynamic range compression and that's what you get with an
ABG: the more signal the less "gain" you have in the system.

effectively that increases the dynamic range of the system, but the
nonlinearity can be an issue in at least one non-photometric scenario:.

in a linear ccd, there can be a very small signal "swing" (low contrast
image) sitting atop a tall "pedestal" (bright background: a spicule on the
surface of the sun shot through a tight Ha filter for example)

since the response is linear: 100ADU difference in the signal at the top of
the Pedestal is the same as 100ADU difference at the lower end of the
sensor's dynamic range.

in the case of an ABG sensor, what can happen is that a 100ADU signal at the
lower end of the range may compress to a 25ADU signal sitting atop a tall
pedestal.

that's really the only issue I see.

in the case of the Cat's Eye, the core is reasonably high contrast; but the
darn thing is just bright compared to the outer halo

my way of thinking is that you use an ABG to advantage by:

1) use [OIII] and Ha filters and or [SII]
2) take longish unbinned exposures (see how long you can go without
saturating a 2x2 binned and then multiply that by four) that don't saturate
the core
3) forget binning and luminance, do it straight up eline (except maybe Blue
in lieu of [SII] which isn't blue anyway). The ESO shot you showed used
[NII] and [OIII]. You could accomplish nearly the same thing by using Ha and
[OIII]

you can do that shooting even in a full moon which is pretty nice



"Rick Johnson" wrote in message
...
I'm sure the ADU counts I listed for the core are more reflective of the
efficiency of the AB gate on that pixel than of the actually photon count.
By 50k the count is completely unreliable. I try and limit the brightest
parts of the object I'm taking to less than 30k for this reason. It is
quite linear below 30k but above that the individual ab gates seem to
start adding noise. It's so small as to be meaningless to the photo, not
photometry however, until you hit about 45k. Above that all bets are off.
Most of the time only stars reach that level so its not a problem but the
core of the Cat's Eye is so bright it hits that level in about 2 minutes
at 2x2 binning even through a color filter (not narrow band). At that
time the outer shell would be barely out of the noise. It could be done
in one shot but I doubt it would look as good as it would with separate
short shots for the core. Your type of chip would be far superior for
this, even the ST-7's chip being non abg would do better though it would
barely fit the FOV. I'll go the two exposure route.

Rick


Richard Crisp wrote:
it says composite but composite can mean different things to different
people

it can be a composite made from different filters....

the abg camera makes is far more doable than an NABG in my opinion

deeper wells help too.

my sensor used was the KAF6303E.... 100K wells NABG


"Rick Johnson" wrote in message
...

As Stefan and Richard mention this is the Cat's Eye Nebula NGC 6543. The
bright blue part of it to the right is IC 4677. The Cat's Eye nebula is
about 3000 light years away. There are two famous Hubble shots of it:
http://antwrp.gsfc.nasa.gov/apod/ap061112.html
http://apod.nasa.gov/apod/ap070513.html

A composite of the eye and the distant faint shell I shot, as Richard
wants to avoid, taken by earth based scopes is at:
http://apod.nasa.gov/apod/ap020904.html






If I ever get super seeing, I'll try a similar compost. To do it in one
shot isn't possible with an ABG camera like mine. With a long enough
exposure to capture the outer shell the inner part is reduced to a
handful of intensity levels by the ABG gate. In my shot the core was
57126 to 57214 ADU units showing no useful detail.

The galaxy is NGC 6552 and is some 325 to 350 million light years
distant. For it to appear this large at that distance it is a giant
galaxy!

Rick