Unihedron SQM 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 te perature on the
“dark frequency" of the sensor are
re oved.
The effects of te perature on the
icrocontroller oscillator are
re oved.
It is protected against accidental
reversal of battery polarity.
Each SQM is calibrated using a NIST-
traceable light eter. The absolute
precision of each eter is believed to
be ±10% (±0.10 ag/arcsec²). The
difference in zeropoint between each
calibrated SQM is typically ±10%
(±0.10 ag/sq arcsec)
The brightness of the nu eric LED
display has two (auto atic) 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 easure ent is in progress.
Any kind of 9V battery is usable. The
SQM contains a voltage regulator to
power the sensor, icrocontroller and
other co ponents.
After reading is taken and displayed,
the eter auto atically turns itself off.
The Half Width Half Maxi u
(HWHM) of the angular sensitivity is
~42°.
The unit operates fro a 9V battery.
Size: 3.8 x 2.4 x 1 in. (97 x 61 x 25 )
Maxi u light sa pling ti e: 80
seconds.
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Quick Start
The SQM is very si ple 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 al ost
i ediately. Under the very darkest
conditions (no oon in the sky, far fro
civilization) the SQM ay take up to a
inute to co plete its easure ent.
Please ensure that you aintain 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 ust be no direct
illu ination of the faceplate sensor by a
terrestrial light source if the reading is to
be eaningful.
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Typical Readings
Magnitudes per square arcsecond is a
logarith ic easure ent. Therefore large
changes in sky brightness correspond to
relatively s all nu erical changes. A
difference of 1 agnitude is defined to be
a factor of (100)(1/5) in received photons.
Therefore a sky brightness 5.0
ag/arcsec² fainter corresponds to a
reduction in photon arrival rate of a factor
of 100.
The following sche atic 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
li iting factors.
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Temperature reading
The te perature in °C then °F are
displayed when you press and hold the
button a second ti e. Also, the odel and
serial nu ber are displayed after the
te perature.
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Care of your SQM
The SQM is a fairly si ple and robust
device. Avoid dropping, i ersing, and
co pressing 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 ti e (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 nor al operation
EXCEPT for the reduction in received
light by the sensor. Make sure that the
sensor faceplate has been wiped before
aking easure ents.
During storage, ake sure that the push-
button is not being continuously pressed
since the eter will draw current fro the
battery and drain it in that situation.
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Trou leshooting
After I push the button, no reading is
displayed.
Are you in a very dark location?
Yes → The Sky Quality Meter ay
take up to a inute to acquire a
reading when the sky is very dark. If
your eter is operating properly, you
will here a soft beeping sound while
the easure ent is in progress.
When co plete, the sky brightness
will be displayed for a fixed nu ber
of seconds.
No → Your 9V battery ay need to be
replaced.
OR
The connector to your 9V battery ay
be loose.
If, after you have checked for both of
these possibilities and your SQM still
won't display a reading under nor al
operating conditions, contact
Unihedron for further infor ation
and a possible replace ent.
I don't know how to make sure the SQM
is off.
The SQM functions in such a way that
it is only te porarily on and turns
itself off auto atically. This is a
design feature to axi ize battery
life.
The readings don't repeat exa tly.
Are you pointing the SQM in the sa e
direction each ti e? Under dark
conditions, you ust keep the SQM
pointed in the sa e direction until the
reading appears on the LED display.
Your SQM ust be pointed at an
altitude greater than 40 degrees above
the horizon or you ay detect light
directly fro surface light sources
(cars, buildings, streetlights). It is
nor ally the zenith sky brightness
which is easured.
The readings do not hange when
pointing to various parts of the night sky.
Each SQM reading ust 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
expe ted.
Make sure that no stray light fro
street lights or other sources is getting
into the sensor, the sensor has a full-
angle response of 120°.
Also, check that the IR filter is in place.
This light blue glass can be seen fro
the display side. A issing 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 dayti e sky surface
brightnesses. Nor al lux eters can
be used in such circu stances once the
effective solid angle for the lux eter'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 roo or
if the sensor is faulty.
Sometimes the first reading is different.
As the te perature of the unit changes
slightly due to being powered up, the
very first reading ay be slightly
higher than the following readings.
Ignore this first reading and average
the following ones for the ost
accurate value.
Other scales
To convert the SQM ag/arcsec² reading
to cd/ ², use the following for ula:
[cd/ 2] = 10.8×104 × 10(-0.4*[ ag/arcsec2])
Unanswered Questions
Help us to infor you and other
custo ers better by forwarding
unanswered questions about the SQM and
easuring light pollution to:
[e-mail]
Further Information
Check the Unihedron.co website for
updates and additional infor ation.
Mailing List
Join the SQM ailing list for notifications
and to share experiences with other users
by sending an e- ail to:
sq - [e-mail]
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Contact Information
Unihedron
4 Lawrence Ave
Gri sby, ON L3M 2L9
Canada
Tel: [phone]
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Unihedron is a proud e ber 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: Septe ber 9, 2012