I’ve been reading and re-reading this now for a couple of months and it seems like I need to lengthen my exposure times beyond what people talk about here in order to get the needed ADU’s and histogram separation from the left side.
According to Bortle maps, I’m on the orange zone (#5) and the iPhone Dark Sky Meter app tells me my SQM is 20.3. I’ve pumped up my settings to 139/21, yet 3 min narrowband subs are not enough to give me any left-edge separation or needed ADU’s. The scope is a f/6.5.
ADU + Histogram.JPG
I too have struggled with the recommendations. I've asked several times, and for the life of me I can't rectify the "optimal" ADU values and the desire to separate the histogram from the left side (which I take to mean starting to raise the noise floor).
In the end you are trying to balance two things:
1. Getting the signal above the noise floor so that you know you aren't loosing SNR due to noise. (getting the histogram to move from the left side)
2. Not clipping the brightest portions of the image due to a lack of dynamic range. (not blowing out your stars)
It is entirely possible that the two goals are incompatible, that if you move the histogram away from the left side, you'll blow out stars. Or vise versa.
I don't buy these median ADU charts, I don't see how they could possibly account for different content in the images. Take imaging the Orion Nebula vs a dim galaxy cluster for instance, median ADU can't tell the whole exposure story.
Gain is inversely proportional to dynamic range, so one way of combating this problem is to lower your gain. However at least with CMOS cameras, read noise increases with lower gain so there is a bit of tail chasing.
I'm still very much in the learning by experimenting phase however I've had my best luck by prioritizing maximizing target SNR over clipping of the brightest stars.
My technique is to expose at the absolute minimum time to safely separate the noise floor from the left side of the histogram reliably. Anything past initial separation and you are not gaining any additional SNR but you are blowing out more stars so you need to tune that as best you can. This often means I'm exposing well past the recommended ADU readings suggested here.
Edited by kingjamez, 12 December 2017 - 10:58 AM.