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Metric(s) to measure if my stars are filling up the wells half-way

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#1 jerahian

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Posted 16 March 2019 - 01:07 PM

I feel like this is going to be a silly question, but I just want to be clear.  I would like to request help in understanding how to find out, from one sub using my capture tool (SG Pro) or via PixInsight, whether I'm "filling up the [sensor] wells half-way."

 

Using the 1600MM-COOL and capturing in SG Pro, the target is comprised of mainly a star field with no nebulosity.  Is there a value or set of values I can look at during capture that would indicate to me if my "wells are filled halfway?"  I imagine the stars are going to be the filling up the wells the most, so is this information I can find in the SG Pro Image Statistics view at time of capture?  Would I look at the Maximum ADU value for a sub to determine this?  Given the image is 16 bits per data pixel, the max ADU is 65,536, so would I be looking for a maximum value of 32,768?

 

Moreover, if my assumption is correct, is there a way to look at my sub stats to target all of my stars to average at half full, not just a maximum?  The mean and median are pulled down due to the background, so it's hard to tell if my stars are half-way, with it being OK if some stars are clipped.

 

I hope I'm making sense...



#2 Dynan

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Posted 16 March 2019 - 01:32 PM

You are correct, Sir.

 

You can open an image in SGP and scroll over a star and get that pixel's reading.


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#3 DuncanM

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Posted 16 March 2019 - 01:38 PM

The sensor's FWC can only be reached at Gain = O. At higher Gain the Well is read out before it reaches capacity. However 1/2 the max ADU should always be 1/2 the effective FWC at whatever Gain you are using.


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#4 roofkid

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Posted 16 March 2019 - 01:46 PM

Hi Ara,

 

well astrophotography is always a compromise and stars require the highest dynamic range (from complete signal flowing, to nothing, e.g. background).

To answer your original question. Yes that is the way to go. But it is potentially just ONE pixel that gets saturated. If you go by this rule, which I would not advise you will miss out a lot of signal in the faint areas. In my opinion it's better to blow out a few stars to get to the optimal exposure in terms of faint signal recovery than to watch out for all the stars. I'd rather do HDR exposures to restore the blown out stars than compromise based on the full well.

 

Cheers,

Sven


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#5 jerahian

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Posted 16 March 2019 - 03:02 PM

Thank you all.  I should have mentioned this is for an exercise in aperture photometry, so the stars are the only things I am interested about in the subs I'll take.


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#6 bobzeq25

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Posted 24 March 2019 - 10:51 PM

Another way is to pass the default PixInsight cursor over the stars (zoomed in as necessary).  At the bottom it will show the degree to which the wells are filled, the default scale is 0-1.


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