Do these values look ok

[blinky]

I am trying to get the settings perfect on my QHY8 for the offset & gain. I upped the offset up to 120 from 115 as a 3 min exposure had a minimum of 0 - is that the correct thing to do?

Here is what I got after a 500 second exposure with the following gain and offset - does it all look ok?

Gain 45%

Offset 120

[blinky]

Woops..... I forgot I had zoomed into the screen stretch window - how's this?

[SteveL]

That looks good enough

[blinky]

Cool! I suppose, on looking at it again, the gain should maybe be upped as after a 15min exposure I was not hitting full well capacity?

[Tim]

Craig, read Craig Starks explanation about gain/offset in the Nebulosity manual. It explains the theory far better than I could, and will save you worrying about it.

Gain, Offset, and Bit Depth | Ask Craig | Stark Labs

[blinky]

Now I am really confused.... I have read that article a couple of times and it says that for a 16bit image the max value is 65,535 or 2^16 but according to the screenshot my max value is 66,190 how is that? Now I am more confused than before!

[Beyond_Vision]

I suspect the axis on the graph goes slightly higher than the maximum count. There is no data higher than 65,535

[blinky]

Ah, OK I see Kevin, it's just the axis that goes higher than the 16bit value and not the actual data. Yeah that kind of makes sense.

[martin_h]

For the offset take a bias frame and look at the min value, it needs to be between 100 and 1000, I've got my QHY8 set at 121 which gets me 1/2 way at 500(ish)

With the Gain at 0.

"So, how do I set it? (man, you ramble a lot when you get going!)

1) Take a bias frame and look for the minimum value in it. Is it at least, say 100 and less than a thousand or a few thousand? If so, your offset is fine. If it's too low, boost the offset. If it's high, drop it. Repeat until you have a bias frame with an offset in, roughly 100 - 1000. Don't worry about precision here, it won't matter at all in the end. You now know your offset. Set it and forget it. Never change it. "

[SteveL]

Cool! I suppose, on looking at it again, the gain should maybe be upped as after a 15min exposure I was not hitting full well capacity?

Dont touch the gain, it doesnt help anything. If the data is there in the image with the gain set high, then the data is still in there with the gain set low, its just dimmer... but you havent lost any dynamic range. High gain means higher noise, and for longer expsores will just over saturate and bloat your stars.

If the scale of data values at 0% gain is:

0,1,2,3,4,5,6,7,8,9,10,etc

then bumping up the gain just makes those values

0,2,4,6,8,10,12,14,16,18,20,etc

you havent gained anything by bumping the gain value, you just need to tease the data out during post processing.

Gain just another name for DSLR ISO values. The difference is that with DSLRs, the ISO value allows you to take the same properly exposed image with shorter exposures for fast moving targets. the image is always slightly more noisy becauise of the ISO bump. With DSLRs, the camera is trying to adjust the various options to get an image that looks OK straight from the camera.

With Astroimaging, you want low noise images with as much dynamic range as possible. You then create the image you want in post processing. This means keeping the gain (i.e ISO) low. I bet the very center of your brighter stars are white in that image. Higher gain would make the over saturated area of the brighter stars bigger.

Get your offset/bias values nailed, and leave the gain alone.

[martin_h]

I only ever use mine with Gain = 0