c (Bill Ferris) wrote in message ...

Thanks for your clarifying comments. So when using Bartel's ODM
program, discussed in your August 2003 Sky & Telescope article against
a series of low-contrast extended objects, you recommend using the
following brightness numbers, based on local observing-field
conditions:

NELM.(+/- 0.5)..===..Sky Brightness (mag./sq. arc sec.)
........8.0............22.0
........7.0............21.0
........6.0............20.0
........5.0............19.0
........4.0............18.0

Your article suggested a series of galaxy DSOs to practice with.
Where I got thrown was its suggestion to use 21 or 22 as a default
brightness number for a "good rural sky". (I may not be remembering
your article correctly, not having it immediately in front of me.)

This threw me, since I experience more nightly variation in
light-pollution at my observing location and wanted to fine tune use
of Bartel's program a little further. I am a small aperture observer
and wanted to experiment with brighter objects across more light
polluted skies than the list of galaxy DSOs in good or excellent skies
suggested in your article.

( Bartel's ODM program: http://zebu.uoregon.edu/~mbartels/dnld/odm.zip
)

Also, it should be pointed out that there really is no controversy over the
surface brightness of the darkest sites on Earth. That limit is 22.0 MPSA
(+/- 0.1-mag.), which has been derived from photometric data taken
over decades from sites all over the planet.

As to the top end of Schaefer's and Clark's curve being at 24 MPAS,
above the empirically measured sky brightness of 22, I assume that is
because Blackwell was measuring brightness in an artificially-darkened
controlled-laboratory setting. Your brightness table suffices for my
immediate needs and I'll leave for another day the details of how
Schaefer's and Clark's brightness formulae is used internally in their
models.

The inability of beginning amateurs, like myself, to distinguish
between when an extended object might not have been resolved because
light pollution was too high (and viewing the object should be tried
again), and when the object could not be resolved because it is simply
to faint for the aperature and magnification being used, is one of the
more frustrating aspects of getting started in hobbyist observing.

The traditional method of evaluating the visibility of extended
objects, by their integrated magnitude compared to naked eye limiting
field magnitude or to zenithal limiting magnitude (which seems to
emphasize the bright core of an extended object, e.g. the Andromeda
galaxy), verses an object's dispersed-average brightness (in MPAS)
compared to the background sky brightness (which seems to emphasize
the object's average brightness across its entire area) both have
their strengths and weaknesses. Neither method seems to fully capture
the effect of the dispersion of the brightness of an extended object
between its central core and less-bright outlying oval and its effect
on visibility. (This is probably less true with respect to the list of
distant galaxies of small angular size suggested in your article.)

I found the MPAS-ODM based approach to be a useful adjunct to the
traditional approach of using integrated magnitude, when trying to
decide whether an object could never be seen with the current scope or
might be seen in a future session in better skies. It increased my
understanding of and observing skill with respect to an object's size,
its brightness, the background brightness of the sky and the
magnification employed.

Thanks - Kurt

P.S. -

The following is a csv file I threw together that contains the Messier
objects sorted by descending MPAS brightness and that lists the
corresponding traditional integrated magnitude:

http://members.csolutions.net/fisher..._Mag_to_Ba.csv

The object brightness in MPAS was computed using Clark's estimate of:

B_mpas=V_m+2.5*(log(2827*Size_x_arcmin*Size_y_arcm in))