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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
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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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