Tweaking Camera's Brightness and Gamma with SharpCap 3

oopfan

My motto is "Do more with less". In keeping with that slogan I try to maximize my ASI120MC's performance when capturing galaxies and nebulae.

The impetus for this post began last week when I discovered an image posted on Astrobin. The galaxy had incredible detail in the dust lane. I looked at the technical card and saw that he was running his cooled (non-ZWO) camera with a gain of 8.0! Usually, from what I have seen, cooled-camera users push the gain to >>100. They acquire the image in less time but at the expense of severe quanitization of the wisps of nebulosity.

So I looked at the user manual for my camera and saw the graph of Gain versus Dynamic Range. I decided to decrease my gain from 50 to 25 which should give me an extra stop of DR, plus I switched from RGB24 format to RAW16. All other settings I kept the same (i.e. Brightness=0 and Gamma=50). Unfortunately I have only had one night of partly cloudy skies and a Full Moon to test it out, so I don't have enough data to really say what the net effect is.

Which brings me to the topic of this post. Yesterday I was going back-and-forth with another user on the SharpCap forum. He described how he uses SharpCap's Histogram Tool to offset the histogram so that the subframe is neither underexposed nor overexposed. His test images were of M27. He decided to set his camera's Brightness (i.e. Offset) to a value of 10. I like it!

Next, I thought to myself, I almost always use PixInsight to stretch the Histogram, so I asked myself: Is there a way to ask the camera to do some stretching itself? That is when I discovered the Gamma setting for the camera in SharpCap. So I decided to begin doing test exposures with Gamma of 75.

My question to the fine engineers at ZWO: Does this sound worthy of experimentation or is there something wrong with my reasoning? Just to repeat myself: I am trying to achieve three objectives:

1. Increase dynamic range by one stop by changing Gain from 50 to 25.

2. Prevent under-exposure by shifting histogram to the right by changing Offset from 0 to 10.

3. Reveal more detail in faint nebulosity by stretching the histogram by changing Gamma from 50 to 75.

I apologize for this long post. Frankly I tend to give up reading posts if they extend more than a couple paragraphs, so I don't blame you!

Thank you for any help or guidance.

oopfan

Just my thoughts on Gamma:

Wikipedia states that Gamma correction originated back in the days of the Cathode Ray Tube (CRT). I am old enough to remember CRT's, sad to say! Even before CRT's were used in computer monitors they were used in television sets. Take my word for it -- there was no such thing as "digital" in television sets back then. Historically, Gamma correction was performed in the analog circuitry of the amplifier that deflected the electron beam across the phosphor.

Fast forward to 2017 -- the trend these days is to do everything in software -- which leads to the question:

In the ASI120MC does Gamma correction happen in the analog amplifier before the signal is digitized? Or does Gamma happen in software after the signal is digitized?

If the answer is "in software after the signal is digitized" then that would be counterproductive and a shame.

oopfan

Well, I looked at the datasheet for the AR0130CS from ON Semiconductor. The chip has a built-in A/D converter and 12 bits of digital output. There are inputs for gain for each channel but no mention of gamma.

So it looks to me that gamma is being applied after digitization by either ZWO firmware on the camera or within SharpCap. (It is never easy to track down where the magic happens! No one is very upfront about it sadly.)

The bottom line is that there is nothing to be gained by tweaking gamma for DSO capture. It is best to do that in PixInsight. It is a shame.

Sam@ZWO

you need to keep the gamma to 50, this is done by software

you need to get the raw16 data

oopfan

Thank you, Sam.

Do you see any "danger" or have any concerns about setting Offset to 10 (from the default of 0) for DSO imaging?

By the way, when Offset is applied is it an arithmetic "addition" of a constant or "multiplication" by a constant?

Sam@ZWO

oopfan wrote:
Thank you, Sam.

Do you see any "danger" or have any concerns about setting Offset to 10 (from the default of 0) for DSO imaging?

By the way, when Offset is applied is it an arithmetic "addition" of a constant or "multiplication" by a constant?

offset is an "addition", you need to turn it up if you wish to capture in RAW16 format