Last week I presented two classic methods of determining sky quality for a given evening, called seeing and transparency. This week we’ll look at more methods.
It’s all about the Bortle?
There have evolved other scales that help us understand how good a particular evening is for astronomy. One that gets a lot of attention is the Bortle Scale. I’ll get into that soon enough but let’s first take a look at some other methods.
NELM (Naked Eye Limiting Magnitude). NELM is a form of transparency estimate, incorporating a guide chart of stars visible to the naked eye. The key is you need to be outside in the dark long enough for your eyes to be “dark adapted”, your irises open as wide as possible. Using the chart (with orange or red light to prevent loss of dark adaptation) you compare the dimmest stars visible with the chart (usually high in the sky, near zenith). The chart lists star magnitudes. The dimmest star visible is your NELM. . Because an object’s magnitude number decreases with brightness, the bigger the number the better. 7 or higher is excellent, 6 or lower is poor.
SQM (Sky Quality Meter). A SQM is a digital light meter specifically designed and calibrated to measure sky luminance. The most often used metric comes from aiming a sky quality meter straight up (zenith) and taking readings. The range is 17-24.5 where 17 is bad and 24.5 is perfect. A number greater than 21.5 indicates a dark sky.
Mean All-Sky. This requires a calibrated camera and lens, and analysis software. A mosaic image from multiple images is made and analyzed. This takes light pollution into account.
Bortle. Created by amateur astronomer and telescope maker John Bortle. His measuring technique and resulting scale were published by Sky & Telescope magazine in February 2001. Also check AstroBackyard on line for details. It does not use a photometric device but is better defined than other eyeball scales. It uses zodiacal light visibility, light pollution affect, how clouds look, Milky Way visibility, how the Moon appears if present, naked eye visibility of galaxies M33 and/or M31, and the limiting magnitude in a 12.5-inch reflector telescope. The Bortle scale goes from 1-9, where 1 is a very dark sky in the boonies, and 9 is inner-city where only bright planets, the Sun and Moon are visible. That’s a lot of parameters to consider! What if you don’t have a 12.5-inch scope? You can get close using a few parameters. Just based on Milky Way appearance, in Canyon Lake our sky is around a Bortle 4.5-5 (suburban). The Milky Way is visible overhead but dimmish even on a good night. We unfortunately are impacted by light pollution. How about New Braunfels?
What’s in the Sky?
March 4; after sunset; high in the southwest: See Mars and the Pleiades sharing the sky.
March 5; before sunrise; east-southeast: See Mercury, Jupiter, and Saturn above the horizon.