Can i measure Maximum ADU form an image?

[bluesilver]

Hi, I have been doing a bit of research on exposure times, also been using the Brain function to get a measurement of sky darkness and also recommended exposure times for both Unity and max dynamic rage.

What i would like to do if possible, is to actually find out what the Maximum ADU value is for the image i have jut taken.
The idea is to use the recommended settings and just tweak them a bit just to see what the effects do, I am trying to just find out if i am exposing for too long
The camera is a asi2600mc, so the Maximum ADU value should be less than 65534 ADU if i have that correct.
I think i am correct in saying that with the histogram, the sliders show the Mean or Median ADU value in that test sub?

[admin]

Hi,

you can find this out by using the histogram and hovering the mouse over the brightest point registered by the histogram.

Set the histogram to 'Logarithmic' mode, this will make sure you can see the highest points even if there are only 1 or 2 pixels with that value.

Capture.JPG (37.08 KiB) Viewed 866 times

Here you can see the sort of readout that you get when hovering the mouse. I'm using a camera with a 12 bit sensor here, so the ADU goes up to 4095 as a maximum, hence the readout of 3878 at the selected point. Note that the readout for pixel count applies to the height of the horizontal crosshair line - I suspect that the peak I was hovering over was just a single pixel of that brightness, but moving the crosshair down to that level made for a poor screenshot.

cheers,

Robin

[bluesilver]

Appreciate the reply,
I have just ran a single image from M83 that i was imaging a few nights ago to see if i can work out what is going on,
I got a bit of a strange looking graph.
Camera is asi2600mc, gain is 100, exposure is 180 seconds, offset is 50
I see that i have way too much offset, after running the brain function after all this was done, i got a recommendation of an offset of 12
exposure of 111 seconds
So a couple of things i can change.
But it looks like a lot of signal bouncing of the bottom of the histogram for some reason.
Not sure about the number on the histogram either, but looks like it is well away from what the camera ADU value is capable of.

SharpCap histogram.jpg (185.26 KiB) Viewed 836 times

[admin]

Hi,

I suspect that the pixel counts for most of that stuff stretching out to the right is '1' - ie there is only a single pixel on the image contributing to each of those little peaks. For a 16 bit camera, SharpCap does not display all 65536 levels of the histogram (you couldn't see them anyway, and trying causes performance problems) - what happens instead is that SharpCap reduces the histogram down to a few thousand levels, meaning that it really doesn't take too many 'warm' pixels on the image to get the sort of histogram you are seeing.

Overall, the conclusion is that most of the stuff from 30% or so to the right hand side is just noise from warm or hot pixels... You probably want to look at a higher threshold of 'pixel count' to identify the rightmost bit that is really part of the image.

cheers,

Robin