[APML]: Re:evaluating the seeing

[Alfredo Zanazzo <alferd@mbox.lol.it>]

1)Measuring the seeing in arc seconds.

2)Measuring the seeing by reading ST4 average error.
3)How much is the seeing influencing your photos.
These are topics i'd like to discuss on this list.
On "Astrophotography II", a guy called Patrick martinez indicated a
method:
Using a very long focal lenght, let say ten meter at least, let a star
traikl unguided and measure the drift on both sides of the path.
He does not specify if you have to subtract the star diameter trail from
the sum of the two drifts, which seems to me obvious.
The method implies a calculation of the telescope image scale.
Personally I measure the seeing evaluating the smallest star diameters
of my negatives.
If for instance I have stars of 14 microns, and my image scale is 
7 microns per arc second, I know for sure that the seeing must have been
LESS than 2 arc second, because guiding, optics and mechanical problems
for sure had an influence on the star diameters.
I'come back on this later.
About point 2 I have found out that factor 2 is the maximum allowed to 
get a decent photo.
But for a top one, you need factor 0 to 1.