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Thanks everyone! Sounds like I'm in the ballpark. I have noticed, given a sample set so tiny as not be even close to significant, that there's seems to be a correlation between bandpass and FWHM.
Given too few samples and too many unaccounted for variables, I can see a median of 2.8" in L, 2.7" in Ha and 3" in OIII. This may be consistent with the wavelengths captured, or it may be related to chromatic aberration as Ray suggests, I'm not sure which... maybe both!
Thanks for the link to the study Ray, always great to back things up with studies like this! One thing that does have me question the application of this study in a more general sense is that sampling sites were specifically selected for astronomy: On the other hand, sites selected for astronomical observatories are naturally free of features that may have strong negative effects on seeing. As such, they should be expected to have seeing components close to minima and to show less scatter about any model than for randomly selected sites.
Having said that, it seems likely that it'd be a good general rule.
I suspect a lot of it here has to do with moisture in the air. We get thick fog almost every day in winter. I've noticed better results on windy days where there's less moisture in the air, even though I would have expected worse results with the greater wind.
I wasn't measuring FWHM until recently so it'll be interesting to see what I can get when it starts warming up.
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