Unihedron SQM, Sky Quality Meter User Manual

Thank you for purchasing a Sky

Quality Meter (SQM) from

Unihedron!

Features

The SQM has the following features:

 It is sensitive only to visual light

(there is a near-infrared blocking
filter in front of the sensor).

 The effects of temperature on the

“dark frequency" of the sensor are
removed.

 The effects of temperature on the

microcontroller oscillator are
removed.

 It is protected against accidental

reversal of battery polarity.

 Each SQM is calibrated using a NIST-

traceable light meter. The absolute
precision of each meter is believed to
be ±10% (±0.10 mag/arcsec²). The
difference in zeropoint between each
calibrated SQM is typically ±10%
(±0.10 mag/sq arcsec)

 The brightness of the numeric LED

display has two (automatic) settings.
Under dark skies, you won't have your
dark adaption ruined by use of your
SQM! Under urban skies, the display
will be correspondingly brighter.

 A repeating audible beep indicates

when a measurement is in progress.

 Any kind of 9V battery is usable. The

SQM contains a voltage regulator to
power the sensor, microcontroller and
other components.

 After reading is taken and displayed,

the meter automatically turns itself off.

 The Half Width Half Maximum

(HWHM) of the angular sensitivity is
~42°.

 The unit operates from a 9V battery.
 Size: 3.8 x 2.4 x 1 in. (97 x 61 x 25mm)
 Maximum light sampling time: 80

seconds.

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Quick Start

The SQM is very simple to use. Point the
SQM so that sensor/faceplate points
toward the zenith. Press the red button
once and release. Under urban skies, a
reading will be displayed almost
immediately. Under the very darkest
conditions (no moon in the sky, far from
civilization) the SQM may take up to a
minute to complete its measurement.
Please ensure that you maintain the
orientation of the SQM until the reading is
displayed.

The SQM's reading is indicative of the sky
brightness with a cone of half-angle 40
degrees centered on the faceplate
perpendicular. There must be no direct
illumination of the faceplate sensor by a
terrestrial light source if the reading is to
be meaningful.

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Typical Readings

Magnitudes per square arcsecond is a
logarithmic measurement. Therefore large
changes in sky brightness correspond to
relatively small numerical changes. A
difference of 1 magnitude is defined to be
a factor of (100)

(1/5)

in received photons.
Therefore a sky brightness 5.0
mag/arcsec² fainter corresponds to a
reduction in photon arrival rate of a factor
of 100.

The following schematic gives a rough
idea of of how to interpret the readings:

At the darkest sites, natural variations in
conditions such as airglow and the
brightness of the zodiacal light are
limiting factors.

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Temperature reading

The temperature in °C then °F are
displayed when you press and hold the
button a second time. Also, the model and
serial number are displayed after the
temperature.

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Care of your SQM

The SQM is a fairly simple and robust
device. Avoid dropping, immersing, and
compressing it and it will give you years
of dependable service. Keep the faceplate
clean and ensure that the battery still has
useful capacity. If you have left your SQM
for a long period of time (i.e. years) and
see a white, powdery substance around
one of the battery contacts, your battery
will need to be replaced and the contacts
cleaned before you can expect reliable
operation.

The SQM should not be negatively
affected by dew during normal operation
EXCEPT for the reduction in received
light by the sensor. Make sure that the
sensor faceplate has been wiped before
making measurements.

During storage, make sure that the push­button is not being continuously pressed
since the meter will draw current from the
battery and drain it in that situation.

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Troubleshooting

After I push the button, no reading is
displayed.

Are you in a very dark location?

Yes → The Sky Quality Meter may
take up to a minute to acquire a
reading when the sky is very dark. If
your meter is operating properly, you
will here a soft beeping sound while
the measurement is in progress.
When complete, the sky brightness
will be displayed for a fixed number
of seconds.

No → Your 9V battery may need to be
replaced.

OR

The connector to your 9V battery may
be loose.

If, after you have checked for both of
these possibilities and your SQM still
won't display a reading under normal
operating conditions, contact
Unihedron for further information
and a possible replacement.

I don't know how to make sure the SQM
is off.

The SQM functions in such a way that
it is only temporarily on and turns
itself off automatically. This is a
design feature to maximize battery
life.

The readings don't repeat exactly.

Are you pointing the SQM in the same
direction each time? Under dark
conditions, you must keep the SQM
pointed in the same direction until the
reading appears on the LED display.

Your SQM must be pointed at an
altitude greater than 40 degrees above
the horizon or you may detect light
directly from surface light sources
(cars, buildings, streetlights). It is
normally the zenith sky brightness
which is measured.

The readings do not change when
pointing to various parts of the night sky.

Each SQM reading must be initiated by
pressing the button. The displayed
reading will stay on for 10 seconds
before shutting down. After the unit
has shut down, press the button to
initiate another reading.

The readings are lower (brighter) than
expected.

Make sure that no stray light from
street lights or other sources is getting
into the sensor, the sensor has a full­angle response of 120°.
Also, check that the IR filter is in place.
This light blue glass can be seen from
the display side. A missing filter will
reveal the clear sensor.

When I use the meter during the day, all I
see is a on the display.

The SQM has a fantastically large
range over which it will report accurate
sky brightnesses. However, to be
sensitive in the darkest conditions, it is
necessary to sacrifice the ability to
record daytime sky surface
brightnesses. Normal lux meters can
be used in such circumstances once the
effective solid angle for the lux meter's
sensor is known. The indicates
that the sensor is saturated.

All I see is a on the display.

The indicates that the sensor
was unable to produce a reading. This
can occur in a light-tight dark room or
if the sensor is faulty.

Sometimes the first reading is different.

As the temperature of the unit changes
slightly due to being powered up, the
very first reading may be slightly
higher than the following readings.
Ignore this first reading and average
the following ones for the most
accurate value.

Other scales

To convert the SQM mag/arcsec² reading
to cd/m², use the following formula:

[cd/m2] = 10.8×104 × 10

(-0.4*[mag/arcsec2])

Unanswered Questions

Help us to inform you and other
customers better by forwarding
unanswered questions about the SQM and
measuring light pollution to:
[e-mail]

Further Information

Check the Unihedron.com website for
updates and additional information.

Mailing List

Join the SQM mailing list for notifications
and to share experiences with other users
by sending an e-mail to:
sqm - [e-mail]

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Contact Information

Unihedron
4 Lawrence Ave
Grimsby, ON L3M 2L9
Canada
Tel: [phone]

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Unihedron is a proud member of the
International Dark-Sky Association
(www.ida.org) and supports its goals.
Please consider joining to help preserve
the beauty of the night sky for future
generations.

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Warranty

Unihedron warrants this product 1 year.

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Last updated: September 9, 2012