Pickering "Seeing" scale - a very nice website

[Dennis]

Hi

This website (http://uk.geocities.com/dpeach_78/pickering.htm)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.

Cheers

Dennis

[h0ughy]

Nice find Dennis.

[ving]

cool! thanks. it explains alot in pictures :)

[AstroJunk]

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.

[Starkler]

Sounds like a good excuse to buy a 5" refractor ;)

[Dennis]

Is it now defunct? That’s sad - I’ve got the Twentieth Edition dated 2004. Lovely reference book.

Cheers

Dennis

[AstroJunk]

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.

[asimov]

Hmmmm. I thought you were an IIS member when we were all discussing the pickering scale & Damians site many moons ago Dennis? Maybe not.

I gotta laugh Geoff! thats almost the same comment you gave last time!:D

[Dennis]

Hi Asimov

I’ve just done a search and yes, I found a post by Robert (http://www.iceinspace.com.au/forum/showthread.php?t=11081)- and its only 6 months old. :shrug: Hmm, must have been asleep at the time. :whistle:

Anyhow, that was then, this is now! ;)

Cheers

Dennis

[Don Pensack]

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? :-)

[davidpretorius]

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?

[Starkler]

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.

[asimov]

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???

[Dennis]

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.

Cheers

Dennis

[davidpretorius]

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!

[asimov]

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..

[asimov]

I have seen one diffraction ring in a few scopes, the C9.25/Stu's(now Lesters)Tak/a 152mm achro.

Yes, great thread.

[Dennis]

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.

Cheers

Dennis

[davidpretorius]

so are diffraction rings visible when sharp in focus or only slightly out / in focus?

thanks dennis.

I still like Asi's counting the frames....he might joke at it, but i reckon it is pretty spot on!

[matt]

Dennis

Are you talking about the brightest outside "fuzzy" fresnel rings in your star test images or the fainter couple of rings closer to the airy disk?

[Dennis]

With the Vixen 4" f9 refractor at x180 and above, in good seeing, I can see usually the first diffraction ring even when the star is in focus.

If I remember correctly, the 4 stars in Orion's Trapezium were showing this faint 1st ring when I last looked a couple of weeks ago. Generally, the rings are easier to see on dimmer stars as the central spot is not too bright, which can otherwise overwhelm the 1st ring.

Cheers

Dennis

[Dennis]

Hi

Here is a website (http://calgary.rasc.ca/seeing.htm) that shows both the Antoniada (I think?) and Pickering Scales.

Cheers

Dennis

[davidpretorius]

Ok, I have had a good chat with Paul over on SCP re seeing. I must admit to over rating the seeing and to a certain degree confusing tube currents with atmospheric conditions to give a feel for seeing.

I know that this pickering scale was done using 5" refractor, but I am happy with myself if i throw in the 4mm TMB (= 300x) and finding a mag 1 / 0 star near my object of interest and then compare against this scale.

The mirror must be spot on to the ambient to be accurate.

I know on saturn the other night, the mirror was absolutely equal to the ambient...hence the shape of Saturn did not move at all....what was lacking was the detail and sharpness of the image ie I had totally isolated the "seeing" and could rate it.

Definately a lesson learnt for me

[Dennis]

A very interesting and useful observation - thanks for the explanation Dave.

Cheers

Dennis

[Don Pensack]

This could be an indication of a lesser-quality mirror, OR it could be an indication of less-than-perfect collimation. I was amazed how the seeing improved once I mastered the art of collimation and found the Catseye collimation 2" tools. That led to modifying the scope to prevent collimation changes with movement and generally led to superior images.
The 3 C's are: Cooling, Collimation, Correction. When the first two are near perfect, all that's left is the third.
The good news is that the first two have greater effect than the 3rd.