Quote:
Originally Posted by Ukastronomer
To me a great image but can I ask a question of you (professional) imagers please
How does RJ or anyone work out how many images to take at what exposure
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Thank you Jeremy,
It's far from perfect hence the critiques but it's a step up in my journey.
The choice of number of subs in my experience is really dependant on a lot of factors not least of which is the object itself. The main reason for multiple subs is to increase the signal to noise ratio in the image. Digital photos suffer from noise with the length of subs, temperate of the sensor and ISO. If you have a feint object, to capture enough signal ( photons ) you need to either take longer subs or increase the ISO. This makes the image noisier so to compensate for this you take more subs to increase the signal verses the noise. The more subs you take, the better this ratio but it doesn't equate linearly. If 20 subs is twice as good as 10 subs then another 10 doesn't make the ratio three times as good , it would take 40 subs . As you can see, it gets to the point where 100 more subs makes only a small difference ( all things being equal ) but it is up to you where you stop. Obviously 100 subs at even 90 seconds takes a long time to capture and if you've already got 300, you've got to weigh up weather it's worth it.
As far as the varying lengths on his particular target, that is in an attempt to not blow out the core. M42 has a high dynamic range ( really bright bits and really feint bits ). If you take all long subs, you'd capture lots of feint areas but the core just get washed out to pure white ( highlight clipped ). Taking the shorter subs and combining them balances this out.
Hope this explains a few things for you.
