Another M51

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nexusjeep
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Re: Another M51

Post by nexusjeep » Mon Apr 13, 2020 4:52 pm

Even with longer exposures dithering will still help one is not exclusive to the other. Also as long as the actual short exposure is long enough so that the sky background level swamps the level of the read noise from the sensor then that should be sufficient.

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Nick

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oopfan
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Re: Another M51

Post by oopfan » Mon Apr 13, 2020 4:57 pm

Please quantify swamp? How many ADU above bias?

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oopfan
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Re: Another M51

Post by oopfan » Mon Apr 13, 2020 5:16 pm

The answer is, it depends on your Read Noise.

One hard fact: if your sky background equals your bias then you are in deep do-do.

For my CCD having Read Noise of 5.3e-, I am satisfied when my sky background is 500 ADU above bias. My bias is 220 ADU so I am happy with anything over 720 ADU but I am happier to measure 1000 ADU.

For my 290M CMOS with 1.5e- Read Noise, I like to see 200 ADU above bias, so if my bias is 400 ADU then my sky should come in at 600 ADU or greater.

These rules have never failed me.

nexusjeep
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Re: Another M51

Post by nexusjeep » Tue Apr 14, 2020 11:26 am

swamp was just a generic term not going to get into a long conversation over CCD/CMOS and short vs long exposure. I have used the brain function and it looks to get the sky noise level about the sensor noise level and general in my bortle 4 - 5 sky this will calculate out at 15-20s of exposure with gain 0 - 100 on my CMOS colour camera which would be fine if I was just live stacking but I process outside of SharpCap so tend to go for a longer time on the exposure to reduce the number of frames and I only image one target in an evening. Dithering will help immensely if the sensor has any form of fixed pattern noise.

I image for the fun of it and do not spend large amounts of time analysing the data and ADU levels I basically just use the histogram to make sure that the spike is in from the left and that I am not over saturating and take it from there and I am happy with the results and images I achieve.

Clear Skies
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Nick

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oopfan
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Re: Another M51

Post by oopfan » Tue Apr 14, 2020 12:49 pm

CCD image quality is higher than CMOS due to its use of a single A/D converter. CMOS sensors utilize dedicated A/D's on the columns therefore the 'fixed pattern' you observe. CMOS is great for high-speed applications like lunar and planetary but for deep-sky the high-speed has little utility. The advantage that CMOS has is low Read Noise and higher QE which cannot be denied. Though if you ever get a chance to try CCD, do it. You will be amazed at the quality.

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