A simple dark-sky meter.
AS NOTED BY JOHN BORTLE ON page 126, sky quality is one of the most persistent topics of discussion among amateur astronomers. Some claim to enjoy skies that are pitch black. However, those who have experienced truly excellent skies may regard the same skies as dark, but not exceptionally so. Most amateur astronomers use limiting stellar magnitude as an indicator of sky quality. But this method provides, at best, only a crude estimate of conditions since it can be easily skewed by variations in the individual's state of dark adaptation or visual acuity. Far better would be some kind of device that would allow amateurs to directly and accurately measure the brightness of the night sky.
Introducing the Pitch-Black Meter
To facilitate the measurement of sky brightness, I have designed an easy-to-build optical device that I call the Pitch-Black Meter (PBM). The PBM allows the user to make measurements by comparing the brightness of a light source with a given area of the night sky.
The PBM consists of three components: a sighting tube, a variable power supply, and an ordinary digital multimeter. The main parts of the sighting tube are a length of tubing, a green light-emitting diode (LED), and a green glass filter. The LED is centered in the sky end of the tube. In front of the LED (facing the viewing end) is the filter that is included to minimize color differences between the sky and the LED.
When you aim the PBM at the night sky and look through it you will see a dark spot (the LED) surrounded by a green-tinted area of sky. By simply adjusting the brightness of the LED until it matches the background sky, you can make a repeatable and accurate measurement of sky brightness by reading the current value off the multimeter.
Building the PBM
The length and diameter of the PBM tube determine the amount of sky viewed. In my PBM I used a piece of PVC plumbing tubing 310 millimeters (12 1/4 inches) long, with an inside diameter of 42 mm (1 2/3 inches), which produces a 6field of view. Almost any kind of rigid plastic or cardboard will do for the tube. The 30-mm-diameter glass filter I used was from my optical junkbox. You may already have a suitable filter too --a Wratten 58 (green) eyepiece filter used for planetary observing will work fine. I recommend adding a 90telescope star diagonal to the viewing end of the PBM tube for greater comfort when making measurements of the sky near the zenith.
The power supply is nothing more than a few resistors, a transistor, two switches, and a battery housed inside a plastic project box. None of the necessary components is even slightly exotic --everything should be available at your local electronics parts store. Make sure that the wire that connects the LED (mounted on the sight tube) to the power supply is long enough for you to make brightness adjustments while looking through the sight tube. In addition, don't forget to provide a pair of sockets or contact points for connecting the multimeter.
The momentary "twinkle" switch makes distinguishing the LED from the surrounding sky easier, especially when you're using averted vision. It is very handy, but if you want the simplest possible design it can be omitted.
The only requirement for the multimeter is that it be capable of reading current in the microamp range. Because the brightness of the LED varies logarithmically with voltage, a small change in voltage will cause a great change in LED brightness. For this reason, you can achieve greater accuracy by measuring current instead.
Using the PBM
To ensure that your measurements are repeatable, wait until your eyes are dark adapted. Then calibrate your PBM by taking measurements with the sky end of the tube closed. Write down the value of the current when the LED first disappears or you can barely see it (choose one of these methods). Although the individual measurements can vary, the mean "dark" value should be fairly consistent over time.
To use the PBM, all you do is aim it at the region of sky you wish to measure. A camera tripod makes the procedure easier. Adjust the current from the power supply so that the luminosities of the LED and the sky appear equal. Read off the current (sky-brightness) value from the multimeter and record it. To get reliable results, repeat this measurement three to five times and then average.
The graph at the top of page 140 shows two sets of observations (separated by one month) for the sky above one of my favorite observing locations--a pier next to the Baltic Sea that features an unobstructed horizon from east to south. For critical astrophotography, I generally use the segment of sky from the south horizon to the zenith. As the graph shows, this is the darkest part of my sky. For visual observing any direction can be used except the western sky because of the light dome from the city of Stockholm, some 35 kilometers away.
The November chart shows what can be expected under dark skies--note that the Milky Way shows up clearly in the numbers! The darkest region was the Great Square of Pegasus, which was positioned in the best part of the sky that evening. The December chart shows the situation one month later. The constellations have shifted two hours (30) to the west and, as a result, both the sky within the Pegasus Square and the region of Milky Way were slightly brighter due to their proximity to the light-polluted western horizon. (Notice that the "dark value" is the same for both observations. I have checked this reading on other occasions--including indoors--and found it to be remarkably consistent.)
The PBM can be easily assembled by anyone with a minimum of electronic and mechanical skill, and it can be used for a wide variety of sky-brightness-measurement applications (see the sidebar below). I encourage all readers to give this project a try and begin measuring the sky above their favorite observing sites. Who knows--perhaps the PBM will help you find truly the pitch-black skies you desire. It may even become your favorite observing accessory!
GOTE FLODQVIST is chairman of the Stockholm Astronomical Society and an electronic engineer at the Huddinge University Hospital in Stockholm, Sweden. He may be contacted via e-mail at firstname.lastname@example.org.
Applications for the PBM
By Gary Seronik
Gote Flodqvist's simple device opens up a whole new world of possibilities. Instead of relying on anecdotal guesstimates, you can now measure sky brightness accurately. Here are a few projects to undertake with your PBM:
Sky-brightness contour map: What part of your sky really is best? By making a series of measurements at regularly spaced intervals of altitude and azimuth, you can build up enough data to construct a sky-brightness contour map--something very handy for planning observing sessions that demand the darkest available sky.
Monitoring light pollution: Did that new mall down the street really brighten your sky? Before-and-after measurements will help you figure this out.
How does the amount of light pollution at your favorite observing site change during the course of a night? If PBM readings show that the sky darkens noticeably after midnight, you might want to schedule your critical observing to coincide with those hours when the sky is darkest.
Rating observing sites: Most observers have a number of sites to choose from. With a PBM you can find out which site really is darkest. However, the choice is typically more complicated than this. One site may have a great south, while another might offer a superior west. PBM measurements will allow you to select the site that best suits your observing program.
Observers who regularly travel to various star parties can verify which locations have skies as dark as claimed by party organizers and which do not.
Monitoring auroral activity: As discussed in the March 2000 issue (page 48), the brightnesses of auroras are gauged on a five-step scale. With a little effort it should be possible to calibrate a PBM to this scale for a handy and unambiguous means of measuring auroral brightness.
Light-pollution contour maps: Ambitious amateurs can map the extent of light pollution around their town or city by taking PBM measurements at various distances from the city center. Such a map will answer the very practical question of how far and in what direction one need travel to find dark skies.
These are but a few of the more obvious activities anyone with a PBM can engage in. Without doubt, a good number of this magazine's industrious readers will think of many more.
Floqvist's PBM Readings November December Dark 3.6 3.6 Pegasus 8.3 9.8 Milky Way 10.4 11.9 South 9.9 Zenith 8.7 West 17.3 East 10.8 The author made these PBM measurements at his favorite observing site. Note how the dome of light pollution from Stockholm dramatically brightens the sky to the west. Note: Table made from bar graph.
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|Title Annotation:||includes related article|
|Publication:||Sky & Telescope|
|Date:||Feb 1, 2001|
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