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suburban imaging: an argument for deep wells and high dynamic range sensors
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Richard Crisp
i agree, however given the tone of several posts appearing in this group it appeared that an explanation was warranted irrespective of the topic to which it was addressed.
George LaBelle <george.labelle@...> wrote: 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. George --- In ap-gto@..., "Richard Crisp" <rdcrisp@...> wrote: am in the san francisco bay area near the Oakland airport (less than 10 miles away). http://www.narrowbandimaging.com/m27_mk1sn2_dm_geg_cs4_s2hao3_page.ht m too which adversely affects my transparency. line filters help improve contrast but bright skies are just that: bright. 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? 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. 50K wells with the 7 electron read noise gives a dynamic range of 50,000/7 or about 7,000 to 1 or 77dB. noise gives a dynamic range of 200,000/11 of nearly 18,200 or 85.2dBtime you can integrate: the shallower wells simply saturate with less exposure when imaged at the same image scale. 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. 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. 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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Joe Zeglinski
Or ... Ray,
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As the years wear on, and you can no longer carry all that luggage to a dark site, and very soon, not even find one, within the cost of gas, squeezed out of your life savings - it will be far simpler and cheaper to get a large scope with a deep well camera, to produce identical results. Same thing, but each has it's advantages. Joe ----- Original Message -----
From: "Ray Gralak" <rgr@...> To: <ap-gto@...> Sent: Tuesday, August 07, 2007 4:40 PM Subject: RE: [ap-gto] Re: suburban imaging: an argument for deep wells and high dynamic range sensors Hi George, |
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Ray Gralak <rgr@...>
Hi George,
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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. -Ray -----Original Message-----page.ht <http://www.narrowbandimaging.com/m27_mk1sn2_dm_geg_cs4> _s2hao3_page.ht> mtoo which adversely affects my transparency. |
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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. George --- In ap-gto@..., "Richard Crisp" <rdcrisp@...> wrote: am in the san francisco bay area near the Oakland airport (less than 10 miles away). http://www.narrowbandimaging.com/m27_mk1sn2_dm_geg_cs4_s2hao3_page.ht m too which adversely affects my transparency. line filters help improve contrast but bright skies are just that: bright. 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? 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. 50K wells with the 7 electron read noise gives a dynamic range of 50,000/7 or about 7,000 to 1 or 77dB. noise gives a dynamic range of 200,000/11 of nearly 18,200 or 85.2dBtime you can integrate: the shallower wells simply saturate with less exposure when imaged at the same image scale. 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. 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. 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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Richard Crisp
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_mk1sn2_dm_geg_cs4_s2hao3_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. |
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