SharpCap Forums

Hi, Sorry if i have got this question in the wrong section here.
I am just trying to find out some information or advice on two things in the smart histogram please.
The read noise limit:
I see it is set at 10% at the default setting, when i do my sky darkness test with an asi2600mc an Skywatcher Esprit 150, get get a result of around 75 seconds for Max Dynamic Range, i am in around Bortle 2-3 area.

So with this Read noise limit, is it actually worth while going below 10% to say 5% or will there not be really much difference when the images are all stacked, I am talking about 2 hours of data.

The second question is the Optimal Black Level, in all my runs, it comes back recommending 12, do this sound about right, or dose it seam a tad low and should be more like 24?

I may be doing things a bit odd here, but i do like to do my imaging with another program, but i like to use SharpCap Pros smart histogram to get the values first

Any advice would be appreciated,
Cheers.

Hi,

it really will make very little difference... There is a minimum possible noise level that you cannot go below that comes from the natural statistical variation in the number of photons collected over a given total exposure period (say 2 hours). The read noise from the camera adds a bit to this minimum noise level. If you select 10% then you are allowing the read noise contribution to be 10% of the minimum noise, so a total of 110%. If you select 5% then you are going to take longer exposures to bring the read noise contribution down to 5% for a total of 105%. As you can see, not a big reduction in total noise going from 110 to 105...

You can always use a higher black level if you want. The SharpCap calculation sets the black level to put the histogram peak at least three standard deviations clear of the zero point, which should limit the fraction of pixels impacted by black level clipping to a very small number. In practice, since the calculation does not include the effects of thermal noise (for darks) or sky background (for lights) the histogram will be even further clear of the zero point as both of these will tend to shift it upwards.

Hope this helps,

Robin

Very much appreciate the reply and advice,
I did notice that the exposure time jumped up quiet a lot if i selected 5% for the Read noise limit
Seams best to stick with the 10% value then, that way you get more subs which i am guess will result in less noise when stacked anyway.

I will try next time just setting the Optimal Black Level at the number that SharpCap Pro recommends and see how it runs.
Lots to learn, but getting there slowly.