SharpCap Forums

Last night I experimented with the smart histogram to select an optimal exposure/gain imaging session for a given time. I tried finding information on the "Read Noise Limit" setting that is set to 10% by default. The only direct information I could find on the web site is that contribution from noise in the green region of the histogram is small (<10%).

Last night I played around with this setting and I first set it to 1% and then 3%. It kept the gain at 5, yet the exposure time was 6 subs of 80 seconds with 1% and 16 subs of 30 seconds with 3% read noise limit. After processing these sets, I found very little difference in image quality.

Do you have any additional recommendations what to set the read noise limit to and how it affects image quality?

Thanks

Hi,

if you want the long read, check this out - viewtopic.php?f=35&t=456 - it explains the maths/theory behind the smart histogram.

The short version is that there is a minimum level of noise that is unavoidable due to random fluctuations in the number of photons hitting a pixel (if you expect 10000 photons then there is an unavoidable uncertainty of +/- 100 due to these random fluctuations). The % limit that you set for the read noise is how much the read noise is allowed to add to that limit. If you set the read noise percentage to 10% then that would mean that the brain would allow the read noise contribution to increase the total noise from 100 to 110 in the example above. As you can imagine that's not a really big difference to the nosie level!

Some people prefer to keep the read noise to an absolute minimum and might select low values, others are more pragmatic and are happy with 10-20% if it gets them shorter exposures, less guiding issues, etc.

cheers,

Robin

Thank you! That was a long read, but it gives me a good feel of how that setting fits into everything. Probably somewhere between 5 and 10% is fine in most cases.

Eric