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I initially thought ... What tha? ... Mike (and you know I loves ya mate) is stating the bleeding obvious. But then I realised that we (people in this forum) are not all experienced imagers and seeing seeing (hehe) illustrated like this is beneficial.
My advice to any up & coming imager who doesn't already do so is to monitor FWHM on all their subs!! While FWHM does depend on the star field (number of bright stars vs fainter stars & exposure duration), it is an excellent indicator of seeing. I check every sub using CCDInspector and I've come to know the best and worst seeing conditions at my site. Typically I expect to see FMHM ranging from 1.5 to 3.75 arcsecs (and much worse sometimes), with the average "good" night around 2.5. I automatically discard ANY sub greater than 4 (even at F5.5) and if most subs are significantly less than that I lower that threshold because I strive for fairly uniform FWHM in my image stacks. If I'm imaging at F11.7 (1.06 arcsec/pix) then the threshold is also lower.
As Mike's example shows, and considering I obsess over detail, there's simply no point imaging beyond a certain threshold - especially for Lum and Ha. Of course this threshold will vary depending on your site and your image ratio so if your rig has a ratio of say greater than 4 arcsec/pix (the super WFers among us) then lucky you - you'll be able to image even when the seeing sucks!!!
Cheers, Marcus
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