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What telescope are you using Bart?
The reason I'm asking is because OIII signal is usually significantly weaker so it often gets stretched more than Ha consequently making halos more visible. A telescope which has noticeable chromatic aberration (achromat, ED doublet) will produce halos around bright stars due to the out of band signal slowly leaking though the filter, even with the best narrowband filters.
Last edited by Slawomir; 16-08-2018 at 07:28 AM.
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