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Sky map of Calgary by AstroViewer^®

PRICES SHOWN ARE US DOLLARS - ADD 5% TO GET CANADIAN DOLLARS

Designed by Dr. Doug Welch and Anthony Tekatch
Made in Canada

Finally, an affordable meter for measuring sky brightness for astronomers!

The Sky Quality Meter from Unihedron, measures the brightness of the night sky in magnitudes per square arcsecond. Unprecedented sensitivity in a handheld meter, it is available in 3 configurations, one of which is sure to suit your needs. A handy, pocket sized item that will be used with your visual observing or astrophotography - possibly every time you step out into the night sky.

USES:
• Find out how good the night or site REALLY is.
• Compare the sky brightness at different sites quantitatively.
• Document the evolution of light pollution in your area.
• Monitor sky brightness through the night, night-to-night, and year-to-year.
• Determine which nights show the greatest promise for finding the "faint fuzzies".
• Calibrate the effect of sky brightness on qualitative measures such as the Bortle Scale.
• Investigate how sky brightness correlates with the solar cycle and month-to-month sunspot activity.
• Help provide local ground truth for future sky brightness prediction with the Clear Sky Clock.
• CCD users can make a correlation between the SQM reading and when the background reaches some ADC level.
• Set planetarium dome illumination to mimic the skies people are likely to experience elsewhere in the city.

FEATURES:
• Infrared blocking filter restricts measurement to visual bandpass.
• Precision readings at even the darkest sites.

These are some real trick items. No equipment junkie should be without one. Is your birthday coming up?

Sky Quality Meter Part # SQM IN STOCK $120

Sky Quality Meter with Lens Part # SQM-L $135
(Narrow Field of View)

                                          • Audible signal while measurement is in progress.
                                          • Sky brightness displayed in visual magnitudes per square arcsecond.
                                           • Temperature in Celsius or Fahrenheit and model number/serial number can be displayed with
                 different button press sequence.
           • Power-saving features designed in for maximum battery life.
           • Reverse battery protection.
         • The Half Width Half Maximum (HWHM) of the angular sensitivity is ~10°.
                The Full Width Half Maximum (FWHM) is then ~20°.
                The sensitivity to a point source ~19° off-axis is a factor of 10 lower than on-axis.
                A point source ~20° and ~40° off-axis registers 3.0 and 5.0 magnitudes fainter, respectively.               • Operates from 9V battery (included).
                                             • Size 3.6 x 2.6 x 1.1 inches.
                                             • Maximum light sampling time: 80 seconds.

                                                Specifications are subject to change without notice

Ethernet Sky Quality Meter with Lens Part # SQM-LE   IN STOCK $250
                                      (Narrow Field of View)

                                • Monitor sky brightness through the night, night-to-night, and year-to-year.
                                • Investigate how sky brightness correlates with the solar cycle and monthly sunspot activity.
                                • Sensor temperature, model number, and serial number are provided.
                                 • Field-upgradeable firmware.
                    • Ethernet connectivity (6ft Ethernet cable included).
                    • SQM-LE users manual, CD with PDF manual and example software.
                        • Calibration sheet indicates unit MAC address, and calibration values
                        • Included applications for reading data in Java, C, Perl, Python.
                   • The Half Width Half Maximum (HWHM) of the angular sensitivity is ~10°.
                            The Full Width Half Maximum (FWHM) is then ~20°.
                     The sensitivity to a point source ~19° off-axis is a factor of 10 lower than on-axis.
                           A point source ~20° and ~40° off-axis registers 3.0 and 5.0 magnitudes fainter, respectively.
                                      • Operates from 5-6 VDC adapter (included).
                                   • Size 3.6 x 2.6 x 1.1 inches.
                                   • Maximum light sampling time: 80 seconds.
                                   • Minimum light sampling time: 1 second.

                                                Specifications are subject to change without notice

Frequently Asked Questions

Is the SQM-LE waterproof?

No, the SQM-LE is not weatherproof. For permanent mounting outside, it should be mounted under a weatherproof dome.
For people using the unit only during telescope observations, it can be stowed away with the telescope.

Some considerations for preparing a high end weatherproof enclosure:
• The enclosure should be thermostated and heated to keep condensation off the top of the dome.
• There should be some airflow inside the enclosure to prevent condensation.
• Airflow from inside to outside usually means insects are a factor and that means a screen would be required.
• Circulating the air may require a fan.

Here are some sources for plastic domes:
Global Plastics
Electronic Goldmine

What is the difference between the SQM and SQM-L?

The main difference is the field of view. The SQM-L (with lens) is an improvement over the SQM.
The lens collects more light from a smaller cone so that the meter is not affected from lights or shading on the horizon.
The SQM-L is better suited for astronomy and dark sky enthusiasts.
It has a lens to narrow the field of view so that street lights and buildings or trees do not affect the reading very much.
If you expect to always take readings at dark sky sites in an open field then the regular SQM will do fine for that task.

Do you have any benchmarks for linking magnitudes per square arc sec with the Bortle scale?

We believe that if you check this Light pollution report by Richard Berry, the descriptions associated with each
magnitude per square arcsecond are sufficiently detailed that you could draw up a pretty decent correspondence.

What are "Magnitudes per Square Arc Second"?

"Magnitudes" are a measurement of an objects brightness.
For example a star that is 6th magnitude is brighter than a star that is 11th magnitude.
The term "arcsecond" comes from an arc being divided up into seconds.
There are 360 degrees in an circle, and each degree is divided into 60 minutes, and each minute is divided into 60 seconds.
A square arc second has an angular area of one second by one second.
"Magnitudes per square arc second" means the brightness in magnitudes is spread out over a square arcsecond of the sky.
For example, let's say the SQM provides a reading of 20.00.
That means that a light of a 20th magnitude star brightness was spread over one square arcsecond of the sky.
Quite often astronomers will refer to a sky being a "6th magnitude sky".
In that case you can see 6th magnitude stars and nothing dimmer like 11th magnitude stars.
The term "6th magnitude skies" is very subjective to a persons ability to see in the night.
For example I might say "6th magnitude skies" but a young child with better night vision might say "7th magnitude skies".
You can use this nifty calculator created by SQM user K. Fisher to do that conversion, or this chart.

The "magnitudes per square arcsecond" numbers are commonly used in astronomy to measure sky brightness.
Here is a link to such a comparison.
See the third table in section 10 to see how these numbers in magnitudes per square arcsecond relate to natural situations.

Each magnitude lower (numerically) means just over 2.5 times as much more light is coming from a given patch of sky.
A change of 5 magnitudes per square arcsecond means the sky is 100x brighter.
Also, a reading of greater than 22.0 is unlikely to be recorded and the darkest we've personally experienced is 21.80.

Reading accuracy

The value produced by the sensor in the SQM is affected by temperature.
There is a temperature sensor in the SQM that compensates for this effect.
However, when the SQM is first powered up, the light sensor is colder than when the power has been on for a few seconds.
Depending on the ambient temperature this will result in the first reading being slightly higher than subsequent readings.
For the most accurate results, it is best to take many readings and disregard the very first reading.

Holding the Sky Quality Meter

Point the SQM directly above (at the Zenith). The SQM sensor is on the same side as the display.
The SQM should be held at or above head level so that shadows or reflections from your body do
not interfere with the reading. Avoid using in areas that are shaded by trees or buildings.
After you press the button to take a reading, the SQM will beep while it is accumulating photons.
When the beeping stops, face the SQM towards you and note the reading.

Links of Interest

                                                 • Surface Brightness by Tony Flanders

                                                               • GLOBE at Night

                                                   • International Dark-Sky Association

                                                               • SQM-LE Reader

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Last modified: 06/26/09

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