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Hi Greg. No, it's nothing at all to do with the autoguiding. These are plots of the how the HFR star shape parameter varies from sub to sub. I used "FITs image grader" to analyse the final subs - for example, each point in the lower graph on the second image shows the HFR value for one of the 183 individual subs in that multi-night sequence (ie these 183 points represent measurements from about 10 hours of imaging). I think that FITS image grader works somewhat similarly to CCD inspector, analysing the shapes of multiple stars across a sub - however, it reports a single average star shape parameter (HFR) for the sub. HFR is similar to FWHM/2, so each data point represents what the seeing was like over the 3-4 minutes when the sub was taken (at my image scale, an HFR of 1.2 pixels would be pretty close to a FWHM of about 2.2 arc seconds).
Regards Ray[/QUOTE]
I see. But still how possible is it to remove the effects of autoguiding from an image so the balance is wind, seeing.
Add to that the fact you may get additional flexes at different angles as the night progresses to further complicate the data.
So even though an algorithim has been used to try to say this is the seeing part I doubt it can be done very accurately. Further evidence that this is the case is the need to do a huge TPoint model then a huge CPU intensive calculation to work out a supermodel, get rid of the outliers, try different possible flexes in the system.
Even the tracking rate of the mount would not be correct at lower alititudes than near the zenith due to atmospheric effects (AP mounts enable different rates of tracking at different altitudes to compensate for this).
After having spent endless hours watching the guide errors on many many nights of different conditions I think it would be very hard to differentiate between simple PEC and seeing variability.
Planetary imagers probably know what the seeing is like the best as they fight it all the time. I wonder how often the seeing is quite stable and how often it isn't.
Regardless though if its seeing, PEC, a wind gust, balance of the mount, flex, cable drag, boundary layer on the mirror, mirror cooldown times, ground effects, polar alignment I suppose the idea of shorter subs giving sharper images is still true. But I don't think its scientific to say its 100% seeing related.
I wonder even if any mount works the same all night given there could be slight shifts in voltage from the power supply, efficiencies at one angle are not as efficient at other angles, etc etc. You could go on and find other minor factors that influence the result. Hence TPoint super models.
You can't be sure of that with your data input. What percentage the data is affected by seeing would be an educated guess and probably varies by location a lot.
At my dark site I don't think seeing varies much at all. Especially 3 hours after dark onwards.
Balance would be a big one. Scopes are often top heavy and what is in balance horizontally can be badly out of balance at 75 degrees - the common imaging angle.
It would be interesting to see FWHM numbers from the really long subexposure strategy versus SNR versus stacked short subs for the same duration.
I predict it would come up with the same result as you have shown and it boils down to the read noise of the camera as the major determining factor at a dark sky site exposure length (the usual advice is to go longer on subs at a dark sky site using Kodak chips).
Greg.
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