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Some 2"-1.25" adaptors are threaded for 2" filters and this is where you place the 2" filter. However, you must be very careful when using 1.25" eyepieces that have a long barrel since you wil put a nice hole straight through your 2" filter.
If you enjoy low power wide fields with low power wide field 2" eyepieces then get the 2" filter. If you are looking for small planetary nebula then the 1.25" O-III is all you need. Note you can still get a nice wide field with an eyepiece such as a 24mm Panoptic which is 1.25".
You may find you end up with both 1.25" & 2" versions of you favourite filters.
I'm not an imager and cannot comment on the photo useage other than to say the O-III has a very narrow bandpass and exposure times would be quite long.
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