[APBeta] Sky Tab NELM Contour Issue

[Paul Rodman]

On Jan 24, 2010, at 1:38 PM, Sean McGivern wrote:

> I have found what is all but certain to be a bug, but I'm not sure how I managed to get it.
> 
> In one of my plan files, the Sky Tab NELM contours should a NELM maximum of 7.83, which is probably a bit high for just about anywhere :).  It popped up as I was playing around with setting the NELM (mag/sq.arcsec) of my site.  However, I can't at the moment reproduce it from a new plan.  I'm happy to send the plan in if that would be helpful.
> 
> I have also noticed that the Sky Tab magnitudes do not appear to respect the Bortle setting on the site settings.  I manually set the mag/sq.arcsec to 20.5 as an estimate of my best NELM of 5.7 (this was somehow what caused the above, I think).  However, the maximum NELM in the contour plot is 6.32 (when I set the date to around the new moon after astronomical twilight -- plot is nicely symmetric).  Even if I reset my Bortle to a super-dark site and restart the program and/or create a new plan, I get the same behavior.  I just confirmed that this doesn't change if I set my site to Inner City skies and reopen the program and start a new plan.
> 
> The contour plot looks like it is refreshing (I see the swirl on the lower right side) whenever I change that value, but the plot doesn't change.  It does change sensibly when I change the date (based on presence of sun and/or moon).
> 
> Is there a behavior I'm missing?  Shouldn't the NELM contour respect the site?

The NELM calculation assumes a perfectly dark sky site (i.e. no light pollution). It ignores the Bortle rating/sky brightness for the site. When I get round to writing the manual, I will emphasize this.

The computations depend on the positions of sun and moon, moon phase, site latitude, temperature, altitude, time of year, and observer's eyesight. I believe I default the eyesight and temperature to reasonable values.

This is based on an article by Bradley Schaefer, Sky & Telescope May 1998, pp 57-60.

I'll probably look at this further in the future in order to take into account sky brightness at the given site. It might just be a matter of imposing an upper limit of sky brightness equal to the Bortle rating.

> BTW, VERY cool feature.

Would be if it worked the way you think it should :^)

Paul R.