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Yes, I agree that this is off topic. But there is the moral of the story... you
should try to take your AP GTO mount to a dark site as it can make a huge
difference in image S/N, even when using narrowband filters. Case in point is
that even a very portable 6" refractor and an "inferior" camera (shallow wells
and lower QE) can out perform a much larger scope from the city. This is one of
the primary reasons why remote dark sites are flourishing these days.
From: ap-gto@... [mailto:ap-gto@...]
On Behalf Of George LaBelle
Sent: Tuesday, August 07, 2007 11:23 AM
Subject: [ap-gto] Re: suburban imaging: an argument for deep
wells and high dynamic range sensors
I think this is a good post, however does it not belong in one
of the imaging forums and not AP GOTO. I look to this specific group
for info on my mount.
--- In ap-gto@...
<mailto:ap-gto%40yahoogroups.com> , "Richard Crisp"
am in the san francisco bay area near the Oakland airport (less than
I shot M27 from my backyard in mag 3 to at best mag 3.5 skies. I
10 miles away).
too which adversely affects my transparency.
I have a lot of urban air pollution mixing with nearly-nightly fog
line filters help improve contrast but bright skies are just that:
But the biggest problem is that the skies are bright. Emission
I want to take as long of an exposure as I can without saturating my
the faint halo of M27 is low contrast. When I image such an object
sensor. That's to get as much signal over background noise as I can.
The low contrast signal is riding atop a very high pedestal: the
bright sky background. So the problem is how to extract a low
contrast signal from a high average level background?
you will saturate before you get as much signal as you would like.
Long exposures are how to do it but if you don't have deep wells
So that's why I say you can benefit from having a high dynamic range
sensor. The 24x24 micron pixels of the TK1024 are big and gather a
lot of light. Being back illuminated, they have good quantum
efficiency too: in the 80-85% range over a lot of the visible
spectrum. But a major thing they have going for them is the
datasheet 200K well capacity offered by the large pixels.
50K wells with the 7 electron read noise gives a dynamic range of
But the KAF3200ME has the 80-85% QE too, so why not use it it? The
50,000/7 or about 7,000 to 1 or 77dB.
noise gives a dynamic range of 200,000/11 of nearly 18,200
The 200K capacity of the TK1024 coupled with the 11 electron read
or 85.2dBtime you can integrate: the shallower wells simply saturate with
The extra headroom makes a big difference in the maximum exposure
less exposure when imaged at the same image scale.
24x24 micron pixels used at 3366mm yield 1.47 arc-sec/pixel versus
In my case i traded off resolution for s/n and dynamic range: the
the KAF3200ME in the CM10 giving me 0.42 arc-sec/pixel But I got a
lot more signal with the same exposure time: at least as much as the
ratio of the respective pixel areas.
time, the fact that I was able to capture the faint halo in mag 3
So even though 29 hours is a bit over the top in terms of exposure
skies speaks volumes to the importance of high dynamic range when
imaging in less than optimal conditions. And taking over 20
exposures with each filter gave a very low noise image capable of
being aggressively stretched to reveal faint structural details that
would have been buried in the grain in a shorter exposure from my
backyard. That let me avoid the excessive post processing that I
darks, stacked and had ddp and a mild unsharp mask applied along
The data was simply calibrated with good low noise flats and
with the usual levels and curve adjustments. No high pass filtering,
no deconvolution, no background smoothing, no stars cut and pasted
from other images. Pretty much a straight up simple process the way
I like doing it.