sharkmelley
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Here's a side by side comparison of dark frames (undebayered green channel). They are 5minute darks taken at ISO 800 at an ambient temperature of 20C using a camera that had acclimatised to ambient for at least a couple of hours.
ISO 800 5minute darks at 20C ambient temperature
I've converted the noise to electrons using the gains I have calculated.
It can be seen that the Canon 200D is much noisier than the Nikon D5300. But to be fair it should be pointed out that the Nikon does apply spatial filtering to longer exposures. Many people don't use dark calibration at all on the D5300 because of the low noise levels.
However I would definitely want to use dark calibration on the noisy Canon 200D and the good news is that it works very well to reduce the fixed pattern noise. The bottom two panes of the above diagram show the result of subtracting one dark from another and scaling the result by 1/sqrt(2). But even with dark subtraction the 200D is still not down to the low noise levels of the D5300. The Nikon spatial filtering does not remove all fixed pattern noise, so there is still a benefit to be obtained from dark calibration.
As for calculating dark current, my preference is to calculate dark current from calibrated dark frames, so the fixed pattern noise (FPN) is removed leaving just the random thermal noise. In my experience this give more consistent results and a good indication of what can be expected from calibrated exposures. The read noise must also be adjusted for. Doing this I obtain a figure of 0.125e/pixel/sec for the Canon 200D at 20C ambient and the Nikon D5300 is less than half that.
You might prefer to calculate dark current from a single dark frame, even though it includes the FPN. If so, be my guest - you'll end up with a shockingly high figure of 0.79e/pixel/sec at 20C ambient for the 200D.
Of course I omitted to mention that in continual use, taking long exposures typical of deep sky imaging, the internal camera temperature (and therefore the thermal noise) climbs quite sharply. Again the Canon is worse than the Nikon. The rear panel of the Canon (behind the articulating display) becomes quite warm to the touch. I'll produce graphs of this increase in thermal noise in my final review - it's an important factor that many people overlook.
As for the Canon 200D horizontal banding, I posted an example elsewhere but here it is again - a single light frame opened in PS/ACR:
Canon 200D ISO 200 2min exposure at f/6 with PS/ACR neutral processing
The wide horizontal stripes are fairly obvious and they are positioned randomly from exposure to exposure so they can't be calibrated out. The battery had been fully charged.
Although it might appear that I'm doing a hatchet job on the Canon, this was not my intention. I bought the camera as a backup thinking that Canon might have performed a paradigm shift with their entry level cameras.
It was CentralDS that drew my attention to the "new technology" sensor on the 200D, 80D and others:
http://www.centralds.net/cam/?product=astro-200d-cooled-200d-sl2-kiss-x9
Quote: "Recently the was a big development in Canon’s CMOS technology. That is moved A/D converter into CMOS wafer (= On-chip ADC)."
This may or may not be true but I'm certainly not seeing any big performance increase from this "big development". In fact it's the reverse, performance has gone downhill.
Mark
--
Takahashi Epsilon 180ED
H-alpha modified Sony A7S
http://www.markshelley.co.uk/Astronomy/
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