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Offset when lucky imaging?


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Hi All,

Having read what I've found on the subject, the offset value clips the black end of frames removing a degree of typical noise.

Particularly for long exposures this sounds great, however before I do some testing I'm hoping you can save me some time.

It sounds like most stay with the standard offset values while some do calibrate this for their specific conditions.

I'm trying to go light bucket and short exposures (lucky imaging) particularly for galaxies and having only just bought my first semiproper camera this setting has reared it's head.

My image files are coming out practically the same size for each exposure and I think this may be clipping some data that I want to go through to stacking.

I absolutely agree I need to try longer but need to understand this better first.

I'm thinking of doing some tests to understand how the standard offset changes/if it does, depending on gain, exposure time, temperature, etc.

Nothing is detailed in the manual (zwo).

Would appreciate the benefit of your experiences please particularly on short exposures (lucky imaging) in order to try to alleviate seeing as much as possible.

Thanks very much.

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Offset adds a fixed amount to every pixel to prevent black clipping. 


So having a high offset value is probably good for lucky imaging as it will guarantee no pixel is black clipped. Your range of signal won't be great due to the short exposure so don't worry about loss of dynamic range. 

Edited by paul
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Thanks very much Paul,

I had this the wrong way round in my head thinking it was to get rid of noise and that it may also clip faint data off also, and in some ways it could with wrong setting but not how I imagined it.

So I actually need a higher offset at higher gains to get the faint data away from the noise.

Doing more reading I've found a few mixes up so I'll read up more to get it clear in my head.

Does sound very relevant especially at both gains/short exposures so not to blow stars out but beefing up the faint data to stack while not getting too noisy.

Much appreciated,


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Really sounds like I should be looking at the histogram more than the image.

I hadn't quite got my head around how it could clip the faint data but found a good explanation finally showing how at higher gains the faint data can actually get pushed too far down, thought gain should only multiply but not the case.

So next time will check its not being lost at high gains with the standard offsets and will increase of necessary.

Much happier now thanks very much and think this explains some of my data not being there, even though I also need to try longer exposures out.

Just going from 80mm to 12" so quite a jump.

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As you say increasing gain does make a bigger signal overall. But the downside of increasing gain that noise can grow a lot more quickly than signal to swamp some of the fainter detail. 


[There are camera sensor design strategies that can mitigate this, one is cooling the sensor to reduce the noise overall. Another is to switch between amplifiers with different characteristics (this is why you see some astronomy gain versus noise graphs with a step change in performance.)]


As you increase the gain the uncertainty for the zero point increases due to a larger noise component, that is why some people increase the offset value from default when they increase the gain above the unity value. Because you are doing lucky imaging (i.e. very short exposures) it is probably a good idea to increase the offset value a bit because most of your signal is going to be very small and just above zero.


So, the general principle is to increase the offset a bit and have a high enough gain so that you have enough range to spread the signal from weak to strong in the image making the processing easier. But don't set the gain so high that strong signals are clipped and weaker signals are swamped by the increasing noise caused by the higher gains. 

Edited by paul
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Really appreciated thanks,

My main thing is understanding it.

Going forwards I'll be sticking with the standard offset values first, but paying more attention to the histograms and will see first before I start tweaking offsets.

I need to do some longer exposure for comparison.

Also the heatsink paste came today to mount my tweaked heatsink with fan and Peltier cooler.

Then couple the temp controller and temp sensor and finally insulate the camera body.

So getting there slowly.

Cheers and thanks very much again.


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