(IAAC) [amastro] Re: limiting magnitude at WSP

Alex Langoussis writes:
>Your mileage may vary.  When comparing skies at different observing
>sites, I would think the best results would be obtained by  the same
>observers using the same scopes at the different sites.
This is key, Alex: using unaided-eye Limiting Magnitude as an analog, two
observers on the same night at the same location, with the same full (40+ min)
dark adaptation, can differ in their LM as much as a full magnitude.
And in fact, for the same INSTRUMENT and conditions, I'd be interested to hear
what relationship holds between unaided and telescopic LMs across a sample of
several different observers. With all-important individual variations accounted
for, I suspect these two numbers are pretty well linked.
BTW there is one problem with the sorts of charts published in S&T, and in the
excellent guidebooks of some of our 'amastro' participants: unintentional bias.
Particularly with estimates using intermittent averted vision, there seems to be
a tendency for HONEST and EXPERIENCED observers to "see" what they know is
there, even if strictly speaking it is not really visible.
To completely avoid this, you have to sample multiple preselected, well-defined
star fields in a session, and to do "double-blind" estimates: either plotting of
the field without prior knowledge, or simply counting stars in the field.
Lew Gramer
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