RE: (IAAC) Obj: Jonkherre 900 (PK 194+2.1) - Inst: 51cm f/10 classical Cassegrain

Steve and Art,

I also use star mags estimates when I rate a night but suspect it is
neither efficient nor objective. I have found that differences of
transparency (i.e. visibility of faint objects) are not proportional to
differences of magnitude limits. The skies (when conditions are good) at my
usual site are ranging from 6.2 to 6.4 (on a Polaris sequence, always at
the same altitude above horizon) but I feel that these small differences
are not enough to make the difference in transparency. I guess that a site
could offer 6.0-6.2 (a little less) and be a lot worse than the one above
in transparency.

I believe that stars have more resistance to haze than fuzzy objects. Maybe
the usual Milky Way rating or a faint cluster of galaxies with well sparse
surface brightness decrease are better for rating a night's transparency.

clear skies, Yann.

Yann Pothier	tel: 01 43 41 43 29
11 impasse Canart, 75012 PARIS, FRANCE
Email: ypothier@abi.snv.jussieu.fr
Site : http://pegase.unice.fr/~skylink/publi/cielextreme

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