Pickering "Seeing" scale - a very nice website

[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 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]

Originally Posted by davidpretorius

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

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

Cheers

Dennis

[Don Pensack]

Originally Posted by 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

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.