Smart histogram gain recommendation

[hindsight2020]

I ran the sensor analysis and then smart histogram. It was just a test so I had the telescope pointed to an area of the sky that didn't have any galaxies or nebula. Camera is a Player-One Poseidon-M Pro which has HCG gain at 125. This is on an f4.6 scope with 920mm focal length and lum filter. I had Aim For Unity Gain set. Min exposure 1, max exposure 10min, read noise limit 10%, stacking time 60m. I'm at a location with a sky brightness of 21.76 mag/arcesc^2 though with the moon phase being a bit over half full last night it was probably brighter (I had the scope pointed away from the moon as much as possible).

To my surprise, it recommended gain 5. All the math I've done in the past has suggested it doesn't make sense to use anything below HCG gain of 125.

When I ran this on my other telescope that is f6.4 2000mm focal length and has a Player-One ARES-M Pro, it suggested a gain value 2 points above HCG gain. This sort of made sense - it made sense that HCG gain would be selected though I thought it odd that it picked 2 gain points above (125 is HCG on that camera and Smart Histogram suggested 127).

I'm sure the math supports the gain 5 recommendation but any help understanding it would be appreciated. Maybe if I had it aimed at a galaxy, galaxy cluster, or nebula of some sort it would have been very different?

[admin]

Hi,

The recommended gain in this situation is being selected because of the 'aim for unity gain' selection - a gain of about 5 is unity gain on the Poseidon cameras. To be honest, I probably should have never included the 'unity gain' option in that part of the software.

At the time it seemed like a reasonable thing to do, but since then I have tried - and failed - to work out any valid reason to use a unity gain during imaging. It seems like a nice idea that one electron corresponds to one unit of the readout value, but the fact that there is always noise involved means that it doesn't really gain you anything.

You will get much better suggestions by changing the option to aim for 'Max Dynamic Range'.

cheers,

Robin

[hindsight2020]

Very helpful thank you for the response. That makes perfect sense. I was not aware that unity gain on that camera was 5!

Will try again at max dynamic range.