Quote:
Originally Posted by Slawomir
Longer exposure for narrower filters is a myth, at least when considering mono CCDs, maybe except for 5nm vs 3nm Halpha filter.
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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.
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