ADU & SNR in simple terms…

  • Asatron
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  • The Tea Boy
  • The Tea Boy
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ADU & SNR in simple terms… was created by Asatron

Not all but a lot astronomers perform checks on either individual subs or perform batch checks in programs like Pixinsight, The results you will receive are reactionary numbers to outside factors/conditions some of which I will list below. So whilst we always aim to deliver the best data at all times, Your data may be taken over many nights with ever changing sky conditions and moon phases, rather than in the course of one night only.

So whilst the sky may be clear above and we are collecting data for you, outside factors like …

The moon

We use moon avoidance to mitigate our very own natural lightbulb in the sky but you can never get away from the fact that the moon will always have an effect on your data however slight. In an ideal world we would always image at new moon or when the moon is in one of its smallest phases but this would mean missing out on 2/3rds of a months imaging time. From an astronomers perspective as well as from a business point of view it is unacceptable not to image whilst the moon is up. None the less we all have to accept the following.  

 • Small to moderate change in background ADU (analog-to-digital unit) as well as an effect on your SNR (signal to noise ratio)
 • The Moons Highlighting of high atmospherics which in turn exasperates overall background sky brightness
 • We may start your data set before the moon rises and continue to image after it has risen, depending on what filter its being used at the time. This may result in a ADU & SNR varience
 •  The larger the Moon phase the further our algorithms place the sensor away from it until some nights we may not image at all. For example on day 15 (full moon) with a HA filter, the closest we image to the moon is 90º away which is much further away than most people choose. None the less the quality of the data will be affected and different to HA imaged on day 5 of the moon phase.

Seeing and transparency

 • Every sub we ever take will be different because the sky conditions are always in a state of flux so no two subs will ever be the same
 
Sky Position

As a rule of thumb the higher an object is above the horizon the better it is placed in the sky to image and you will get better data. This is why we have pier limits in place in order to optimize when our equipment take images.

 • Some shorter focal length piers can image as low down as 20º  
 • Others like pier 12 which being almost 3000mm in focal length is more affected by atmospherics so starts at 35º

As a side note - Gradients

Even with superb skies like we have, you will never get away with not getting them at times. For example pier 4 is able to image very low down because of it’s very short focal length and low sampling rate but due to its very wide field it is far more susceptible to sky gradients than piers 1, 9, or 12 for example
Please ignore my dylexia wherever possible, just be thankful I can control my Tourettes ;)
11 Sep 2020 11:05
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