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Author Topic: Adu  (Read 462 times)

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Offline clive

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Adu
« on: 22:01:46, 20 November, 2017 »
Wild weather here  ok here we go  good evening what does adu mean in sg pro  why do we need to adjust it 

Offline chris.bailey

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Re: Adu
« Reply #1 on: 22:28:31, 20 November, 2017 »
Evening Clive

ADU is a measure of the number of photons to hit a pixel site after conversion to a number by the Analog to Digital convertor (which turns a voltage into a number).

What in SGPro are you trying to adjust? The only time I am aware of any need to monitor it per se is in taking flats.

Chris

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Offline clive

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Re: Adu
« Reply #2 on: 12:16:47, 21 November, 2017 »
Hi chris i was wandering what adu   what its purpus was  it for because  my camera needs about 20000 adu and over cameras need lower adu or higher adu

Offline chris.bailey

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Re: Adu
« Reply #3 on: 13:06:13, 21 November, 2017 »
Clive

In very simple terms the detector (CCD) counts photon arrivals and converts those into electrons. The rate of this conversion is the Quantum Efficiency (the higher the better). The electron charge is then amplified to an analog voltage which is then converted into a digital signal (ADU - Analog Digital Number). For a 16 bit device that number can be one of 65,536 values. So with a Quantum Efficiency of 70% (0.7) and a gain of 0.25, 10 photon arrivals becomes an ADU of 10 x 0.7 x 0.25 = 1.75 which rounds to 2. So far so good but as the charge increases (number of photons into number of electrons i.e. charge) the whole device can become non-linear so 250,000 photons should be 250,000 x 0.7 x 0.25 = 43,750 ADU but may only be 42,000. This is very important for flats. Flats are normalised (values of 0-1 only) before being applied so linearity errors will tend to be multiplied and the flats will tend to over or under correct.

Now CCD's have much better linearity these days than they used to so it is less of an issue BUT to keep on the safe side and more by trial and error, different CCD's tend to behave better with flats where the peak is limited to certain values. It would be very easy to assume that low peak ADU flats would avoid the problem altogether BUT low ADU = low signal = low signal to noise so you would need to take a lot more flats with low ADU values to achieve the same signal to noise ratio of fewer flats taken with higher ADU values.

20,000 peak ADU in my experience is a pretty good starting point for flats. Decent ADU so decent signal to noise ratio and pretty much safe in terms of linearity.

Hope that makes sense.

Chris
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline clive

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Re: Adu
« Reply #4 on: 13:46:44, 21 November, 2017 »
Well explained chris  top job

Offline Roger B

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Re: Adu
« Reply #5 on: 18:11:54, 21 November, 2017 »
I couldnt get my head around that. Lol.   

Rog
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Offline clive

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Re: Adu
« Reply #6 on: 19:18:41, 21 November, 2017 »
Ha  init 

 

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