This website shows video images of how a star may appear visually, in seeing conditions ranging from Pickering 1 to 10.
I was surprised to discover that seeing conditions which I had previously been reporting as 7 or 8 during some of my imaging sessions, are closer to the " Pickering 6" clip.
That's a good site. Looking at that reminded me that in my youth I used a different scale which was popularised in Nortons Star Atlas (the original but now defunct astronomers bible).
Here's the abstract from my 1981 edition:
Antoniadi Scale. (a leading planetary observer apparently)
I. Perfect seeing, without a quiver;
II. Slight undulations, with moments of calm lasting several seconds;
III. Moderate seeing with larger air tremors;
IV. Poor seeing, Constant troublesome undulations;
V. Very bad seeing, scarcely allowing the making of a rough sketch.
I rather like it - it's not very scientific, but easy to apply.
Nortons is only defunct in the sense it is no longer the universal point of truth for all things Astronomical - I think it's still in press, but not often seen in the shops.
Pickering should have lived where I live. I've seen seeing WORSE than #1 on his scale, where star images looked out of focus 100% of the time and approached 1 *minute* of arc in width. Even 30 power on my scope was essentially unusable.
Antoniadi would have added:
VI. So that's what a 2-wave optical surface looks like! What's on TV? :-)
Am i right in thinking, i should get a single diffraction ring in great seeing on a focussed star?
I seem to remember the Taks with diffraction rings, but i can't remember my newt ever having then?
Dave this is why the pickering scale is referenced to a 5" refractor.
In the ed80 I had, diffraction rings were visible in anything but the most horrible seeing, whereas for my 10" newt it takes excellent seeing to see a diffraction ring. Its related to the aperture and the way light behaves.
You cant quote out of 10 seeing on the pickering scale in a meaningful way unless you have a 5" refractor, or have enough experience to know how the seeing would look in a 5" refractor on a given night.
I do hope I didn't kill your thread with my comment Dennis, but for what it's worth I do use this particular scale to gauge the seeing conditions - One has to go by something! yes?
Dave, are you saying you have NEVER seen diffraction rings in your newt???
I do hope I didn't kill your thread with my comment Dennis, but for what it's worth I do use this particular scale to gauge the seeing conditions - One has to go by something! yes?
Hi Asi
Not at all. I missed it the first time round and have just learned something new from Starkler re the 5" refractor - I hadn't considered the role that an instrument plays, but maybe this is because I have a 4" refractor so the scale still has some meaning with that.
So basically we're all guessing the seeing conditions. I guess we all know the difference between good & bad seeing. At one stage I was getting a jupter AVI in front of me & counting how many crisp, infocus frames compared to frames that were not lol..
very interesting, I remember Stu's 5" tak with rings, but I am sure Asi, that I have never seen diffraction rings around stars when focussed in my 10"
doesn't it bugger up the view?
is it a certain magnitude star that shows up the diffraction rings?
ie does sirius show the brightest diffraction rings?
great thread by the way!
Hi Dave
I find Sirius too bright so I use a mag 3 or 4 star but yes, Sirius would show very bright rings but there would be so much light in the centre dot, it could easily overwhelm the 1st or 2nd rings, especially in mediocre seeing.
In good seeing, I can see diffraction rings just slightly inside and outside of focus with my Vixen 4" refractor at x180 (5mm eyepiece) and My C9.25 at x235 (10mm eyepiece). They are certainly there at lower magnifications, and even more visible when you grossly defocus the image.
The attached image shows an example before the tube had cooled down.
Hmm, must have been asleep at the time. 
