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What's the advantage with using ISO200 with this camera?
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Good question.Let me try explain it.
1 - Canon has a tendence to saturate the bright stars.
2 - Canon has a tendence to make all stars white because of the saturation in RGB values.
3 - My Canon (both 350D and T3) has much noise in Red channel.
With low ISO, for some DSOs like clusters, these problems are less intense.
I give attention to the histogram. If 120 seconds with ISO 800 is the same than 240 seconds with ISO 400, I use the ISO 400.
Analysing the first image, if the stars are saturated or all are white ...I change ISO to 200.
With nebula and galaxy it is more complicated. They are or have areas very faint. But I try to follow the same procedure.
There is an amount of photos (from Canon) that is the ideal to stack in DSS. Use more than it can saturate the RGB values of stars. But you need at least 10 photos to get good performance with DSS, for noise reduction.
With low ISO you can take largest number of photos without the RGB saturation issues.
Four photos of 60 sec has the "same" signal than one photo of 240 sec. But the four photos of 60 sec will have more noise than one photo of 240 sec. With low ISO I can use largest time of individual exposition.