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ASTRO: a case for high dynamic range sensors and deep wells



 
 
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  #1  
Old August 8th 07, 02:43 AM posted to alt.binaries.pictures.astro
Richard Crisp[_1_]
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Posts: 985
Default ASTRO: a case for high dynamic range sensors and deep wells


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).

http://www.narrowbandimaging.com/m27...2hao3_page.htm

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: bright.

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.


  #2  
Old August 8th 07, 07:01 AM posted to alt.binaries.pictures.astro
Rick Johnson[_2_]
external usenet poster
 
Posts: 3,085
Default ASTRO: a case for high dynamic range sensors and deep wells



Richard Crisp 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).

http://www.narrowbandimaging.com/m27...2hao3_page.htm

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: bright.

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.


I think the main difference is you have 7 hours of H alpha and I have
only 1.5. The shell was in the image but barely above the noise. Not
enough to make a smooth image so I didn't pull it up. My shot was taken
July 30 one of the nights you were imaging. I had to shut down as the
night got worse and I was losing the guide star. I went from 20" to 5
minutes on the integration of the guide star image to get enough light.
That's how far the transparency dropped that night. But I did use 30
minute sub frames to your 20. My pixels were 18 micron and I was only
reaching about 12,000 count. At about 35,000 the anti blooming gate
starts to draw down the wells. Saturation is about 58,000 though a
super star can hit 60,000. In any case in my M27 shot 12,131 was the
highest reading on the nebula itself. A star went to 22,000 in the
nebula. So I could easily go to 1 hour subs but too many satellites and
clouds to make that a crap shoot.

I'd love to have 29 hours of clear weather a month. Hasn't happened for
two months now.

I'm getting paranoid. But considering we've seen nothing from Doug he
must be having even worse conditions so I shouldn't be complaining.

Rick

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

 




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