H beta emission is so close to the OIII emission that depending on the bandwidth of the filters being used, you may find little or no difference between these two images.
Added:
An easy way to check is to fit one of them, look at the image and then add ( ie double them up) the other filter; if the initial image disappears then the cut-off of the second filter is OK
Last edited by Merlin66; 16-11-2008 at 11:29 PM.
Reason: explaination
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