Quote:
Originally Posted by clive milne
It is probably worth qualifying that statement.
The signal that you will record through a 5nm filter will often be higher (than the 3nm exposure) because the passband of the 5nm filter will be broad enough to include more of the Nii lines (654.8 & 658.3) which straddle the Ha line at 656.3.
However, when the frame is shot noise limited, the signal to noise advantage afforded by the 3nm filter should result in a reduction in exposure time for a pure Ha source to a ratio of 0.36 : 1
ie) The 5nm filter will need ~2.8 x more exposure to achieve the same result.
c
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And this is where the "you need more exposure for narrower bandpasses" statement comes from. If you are saying that the shot noise is reduced by a factor of 2.8 (signal surely isnt going up by changing a filter) then it stands to reason that to make a single sub shot noise limited, the 3nm filter will need each sub to be 2.8 times longer than the 5nm.
I think the thing that trips people up is that the 3nm filter image will be read noise limited, which will still be lower than the total noise in the 5nm image, even if they are exposed for the same duration. So the statement that it "needs" more exposure is not valid, the 3nm filter would benefit from more exposure, where the 5nm would not.