The following definitions are proposed by Bertrand Laville, a french Deep Sky observer from Ciel Extreme.
T = transparency
(0->excellent, 5->very bad)
S = seeing
(0->excellent, 5->very bad)
P = light pollution level, varies on the direction we look at the sky (city light halos, etc ...)
MvlonUmi : faintest star magnitude (naked eye) on Ursa Minor.
MvlonZ (const.) : faintest star magnitude (naked eye) toward the zenith (and the constellation it belongs to)
V = direct vision
V1 bright, obvious at the first glance in the field,
V2 luminous, easy to detail after an appropriate adaptation,
V3 hard to see in direct vision, one immediately tends to averted vision.
VI = averted vision (indirect vision)
VI1 object seen easily 100% of the time, without interruption (but not in direct vision witch is V=3).
VI2 object seen 75% of the time, almost continuously.
VI3 object seen 50% of the time, disappears as often as it appears, at an average of several seconds periods
VI4 object seen 25 % of the time : only short, irregularly spaced glimpses.
Other abreviations :
L60 : lunette de 60mm de diamètre
Mb : magnitude dans le Bleu
Mv : magnitude visuelle (en général tirée du catalogue Tycho dans le cas des étoiles).
S : les étoiles du champ notées "S" sont des étoiles du Sky Atlas 2000.0.
Lucida : terme usité chez les amateurs aux US pour désigner l'étoile la plus brillante d'un amas.
a/b : rapport longueur / largeur de l'objet.
L60 : 60mm refractor
MB : blue magnitude
MV: visual magnitude (for stars : from the Tycho Catalog). Mv if estimated by the observer.
S : field stars labelled "S" are on Sky Atlas 2000.0 .
Lucida : the brightest star in a cluster.
a/b : lenght / width ratio of an object.