[AZ-Observing] Re: urban SQLs?
- From: BillFerris@xxxxxxx
- To: az-observing@xxxxxxxxxxxxx
- Date: Fri, 11 Sep 2009 11:02:36 EDT
A 0.2 mag/arcsec^2 difference between two units seems pretty good to me.
That would indicated each unit is consistent to +/- 0.1 magnitude, which is
at the threshold where most experienced visual observers are able to notice
a difference in brightness.
A big limitation of SQM devices is that they don't measure transparency or
seeing, which are two critical factors in evaluating the quality of a night
sky. Another limiting factor is that there is always a subjective
component to any measure of quality. Something is always lost in the
translation
of qualitative evaluations to numerical values. This isn't to say that a
numerical record isn't of value, rather that the numerical value will never
fully encompass the qualitative experience. No single number--no collection
of numerical ratings, for that matter--will ever fully convey the
qualitative experience of standing under a pristine night sky and drinking in
the
naked eye view.
The subjective quality of that experience is one reason I don't rate sky
conditions. Now, I know a lot of observers value the information collected
over years of rating sky conditions at their favorite sites. And I'm not
minimizing the value of that approach for them. Personally, subjective
numerical ratings for darkness, transparency and seeing don't come nearly as
close
to capturing the observing conditions as, say, a careful drawing or
detailed written notes made over the course of 20-30 minutes at the eyepiece.
There's a lot of information to be found reading between the lines of an
observation or studying the delicate detail in a sketch. And that's
information
that a single number will never fully convey.
Bill in Flag
In a message dated 9/8/2009 6:00:03 P.M. US Mountain Standard Time,
KenGSikes@xxxxxxx writes:
I purchased a SQM a couple years ago in the hope that useful data could be
gathered to help in planning observation based on the SQM reading. As was
mentioned the readings of mag/arcsec^2 is also the measurement of surface
brightness. There also is an equation ::
objB_mpas = objMagnitude + (2.5 * (Log10((PI() / 4) * objMajorAxis_arcsec
*
objMinorAxis_arcsec)))
that when put into a spreadsheet allows one to try different V mag and
major & minor axis to calculate just how dark of a sky one needs to see
an
object based on surface brightness.
There is also the comparison used on the Clear Sky Clock sites using the
mag/arcsec^2 and Bortle scale. The problem I encountered was the same one
everyone else has there is no consistency in the readings. Beevo and I
put
two SQM meters back to back and pointed at the same part of the
sky....straight up and the readings would vary as much as .2 difference.
If someone could devise some sort of standard as to what the readings
would
mean I think there is enough people and enough interest that we can all
justify the $ 119.00 we spent on the darn thing.
just my thoughts
Ken Sikes
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