Re: suburban imaging: an argument for deep wells and high dynamic range sensors

George LaBelle <george.labelle@...>

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@..., "Richard Crisp" <rdcrisp@...> wrote:

I shot M27 from my backyard in mag 3 to at best mag 3.5 skies. I
am in the san francisco bay area near the Oakland airport (less than
10 miles away).

I have a lot of urban air pollution mixing with nearly-nightly fog
too which adversely affects my transparency.

But the biggest problem is that the skies are bright. Emission
line filters help improve contrast but bright skies are just that:

the faint halo of M27 is low contrast. When I image such an object
I want to take as long of an exposure as I can without saturating my
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?

Long exposures are how to do it but if you don't have deep wells
you will saturate before you get as much signal as you would like.
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.

But the KAF3200ME has the 80-85% QE too, so why not use it it? The
50K wells with the 7 electron read noise gives a dynamic range of
50,000/7 or about 7,000 to 1 or 77dB.

The 200K capacity of the TK1024 coupled with the 11 electron read
noise gives a dynamic range of 200,000/11 of nearly 18,200
or 85.2dB

The extra headroom makes a big difference in the maximum exposure
time you can integrate: the shallower wells simply saturate with
less exposure when imaged at the same image scale.

In my case i traded off resolution for s/n and dynamic range: the
24x24 micron pixels used at 3366mm yield 1.47 arc-sec/pixel versus
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.

So even though 29 hours is a bit over the top in terms of exposure
time, the fact that I was able to capture the faint halo in mag 3
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
often see.

The data was simply calibrated with good low noise flats and
darks, stacked and had ddp and a mild unsharp mask applied along
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.

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