Quote:
Originally Posted by codemonkey
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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that sounds to me like normal seeing variation with wavelength, not CA.
Yes, the paper provides a way of estimating the best possible seeing for a given altitude - a median of 2 arcsec would seem to be a good estimate for most of Australia and if we are not getting that it may be worth trying to find out what is stuffing things up.
I also get better results with wind - seems to break up the turbulent columns of air that (I assume) develop if it is still.
Quote:
Originally Posted by Paul Haese
That is vastly different from what I got last night and demonstrates how different seeing can be from region to region. While you got 4 last night I was getting 1.86. Did you have north westerlies last night Ray? I did not have any wind until about 6 am. Air stream and cloud flow was moving from that direction. As I understand it seeing is good at your site when you get south easterlies, am I right in thinking that. That would mean a high pressure must be located in the centre of the Bight for you to get 1.5" seeing. Is this right? I thought the seeing whilst pretty good was influenced by the air stream coming down from the north.
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It was still here, but after a sunny day, this site gives poor results in still air. I put that down to thermal columns developing over the roadways and houses in the town, but that is no more than a hunch. With onshore sea breezes, everything cleans up and seeing can get below 2 arcsec. But sea breezes around here can be 20+ knots, which brings another set of problems.