ADU question

[Robny]

So...I'm finally gettig a dedicated camera AA 183m pro tec.  I've seen lots of comments regarding adu.  Is there a SIMPLE guide/explanation to what this is and how to best use/monitor it.

I have bits of broken information in my head of what it kind of is and what it kind of does but I cannot piece it all together and not sure how it relates to what I should be doing or looking for.  

Any help would be awesome.

Thanks

Rob

[symmetal]

Hi Rob,

ADU  (Analogue-Digital Unit) is a convenient way of expressing the digital output from the camera. Most Astro cameras give the output in 16 bit resolution, and 16 bits can be used to express any integer from 0 to 65535, which is 65536 different values (2 ^ 16). 0 represents no output ( which is black) while 65535 represents maximum output (white). The output used to be expressed in LSB (Least Significant Bits) which is the smallest value by which the output can change but this can be more confusing than ADU to those not familiar with digital systems.

You often see questions relating to the optimal ADU for 'flats'. You just want the flat output not to be clipped (that is not to have any value at 0 or 65535) as that means data is lost and can't be recovered. So you aim to have the peak of the flats to be around the half way point of 30000 ADU. The histogram output is a graphical representation of the ADU output with 0 being at the left end and 65535 being at the right end of the histogram. A peak near the middle of the histogram will be around 30000 ADU.

CCD camera A-D converters are 16 bit while CMOS camera A-D converters are only 12 bit. However the CMOS cameras add 4 zero value bits to their output to make the output a 16 bit value for convenience. This means their output steps are in multiples of 16 ADU, that is (2 ^ 4) while CCD camera output steps are in single ADUs.

So ADU is just a number and has no actual defined units in which it is expressed.

Here's  a more detailed explanation of ADUs if you want to learn more. 

Alan

[RolandKol]

I simply imagine sensor as a Solar Panel which accumulates electricity and which  is divided into tiny 65535 slots, each of then can collect solar energy in amount of 1 volt.

So once your camera is directed into the light it starts filling up each slot with electrons, - accumulates electricity,tiny slots do not become full instantly once hit by light, it takes time.

Your set Exposure time directs sensor how long it should accumulate electricity before releasing it.

Let's say exposure time was 1sec, and after 1 sec accumulated electricity is released from sensor to other part of camera, which converts electrical signal to Digital. And ADU is artificial figure created to represent the amount of energy received, but in Digital Format.

At Unity Gain, imagine, 1 electron = 1 ADU. So if you Collect 65535 ADU, it means All Solar Panel's Slots were full with electrons, - in other words, sensor was completely lit = overexposed and any additional electrons which fell on it, were not absorbed and not converted to digital value, lost/"clipped",

Analogically with 0 ADU, - none electrons received, no energy to convert to data.

So flats should be somewhere in the middle of MAX ADU range or a bit lower, between 15 - 30k, depends how many flats you want to collect and stack.

The lower ADU value, the more Flats to stack. On the opposite side, the Higher ADU value, - less to stack, however, keep in mind, some areas are not lit equally and the brighter parts may be "clipped" fast, usually central area.

So going above 30K is not recommended As 30K which is given as example for Flats, is Median Value for the whole image.

So if you go for 30k ADU Flats, once done, Check ADU value and central area, which should be somewhere around 36k ADU maybe 40k, as it is the brightest area, 

However, you will not want to point all parts of the image to check if any parts are clipped or not, 

So 

The main data source should be histogram as it easily shows if are there any parts of the image"clipped", histogram should not touch Right or Left side.

For flats peak, around Middle.

For the DSO somewhere just after the first Quarter, 25-35% from the left Side, 

 

 

[Robny]

Thanks for explanations guys, it actually makes sense to me now (I think).  I'm used to looking with and working with histograms, so essentially this is just a numerical way of looking at it?

Thanks again

Rob

[noah4x4]

Atik publish an excellent video on its website explaining Gain and ADU as regards it's Atik Horizon CMOS. The clever amplification technique adopted completely changes the traditional paradigm. Well worth a watch.