Unihedron SQM-LU-DL User manual
Contents
List of Figures ........................................................ 5
List of Tables ......................................................... 7
1 Introduction ....................................................... 9
1.1 QuickStart ..................................................... 9
1.1.1 Settinguptheinstrument......................................... 9
1.1.2 Savingthedatalog............................................. 10
1.1.3 Getting the data from the data log into MS Excel . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.1.4 Othersoftware ............................................... 10
1.1.5 FITSintegration .............................................. 10
2 Measurements ...................................................... 11
2.1 Gettingaccuratereadings ............................................. 12
2.1.1 Seeingconditions.............................................. 12
2.1.2 Lightpollution ............................................... 12
2.1.3 Otherluminancescales .......................................... 13
3 Theory of operation ................................................... 15
3.1 Lightmeasurement................................................. 15
3.2 CommunicationtothePC............................................. 16
4 Specifications ....................................................... 17
5 Hardware connections .................................................. 20
5.1 Batteryoperation.................................................. 20
5.2 DiagnosticLED................................................... 21
6 Software development .................................................. 23
6.1 Writing your own software interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2 Pascal ........................................................ 24
7 Unihedron Device Manager .............................................. 25
7.1 GettingUDM.................................................... 25
7.2 Installation ..................................................... 25
7.2.1 Systemrequirements............................................ 25
7.2.2 Windows .................................................. 25
7.2.3 Mac ..................................................... 25
7.2.4 Linux .................................................... 25
7.3 Operation...................................................... 26
7.3.1 Startup................................................... 26
7.3.2 Mainscreen................................................. 26
7.3.3 Mainmenu ................................................. 27
7.3.4 Filemenu.................................................. 27
7.3.5 Viewmenu ................................................. 28
3
Contents
7.3.6 Toolsmenu ................................................. 34
7.3.7 Helpmenu ................................................. 42
7.4 Informationtab................................................... 43
7.5 LogContinuous................................................... 44
7.5.1 Import.datintospreadsheet........................................ 49
7.6 Calibrationtab................................................... 50
7.7 ReportIntervaltab................................................. 51
7.7.1 Continuousreports............................................. 51
7.8 Firmwaretab .................................................... 51
7.9 Configurationtab.................................................. 53
7.9.1 Sensorarrangement ............................................ 53
7.10DataLoggingtab.................................................. 54
7.10.1 Vectordatalogger.............................................. 57
7.10.2 Contourplot ................................................ 58
7.10.3 Minimum.datrequirements........................................ 59
7.11Simulationtab ................................................... 59
7.11.1 Simulationfromfile ............................................ 60
7.12Accessoryoptions.................................................. 61
7.12.1 Humidityaccessory............................................. 61
7.12.2 Displayaccessory.............................................. 61
7.12.3 LEDaccessory ............................................... 61
7.12.4 Relayaccessory............................................... 61
7.13Commandlineparameters............................................. 62
8 Commands and responses ............................................... 63
8.1 Commands ..................................................... 63
8.2 Responsedetails .................................................. 65
8.2.1 Readingrequest .............................................. 65
8.2.2 Unaveragedreadingrequest........................................ 65
8.2.3 Linearreadingrequest........................................... 66
8.2.4 Inlinelinearreadingrequest........................................ 67
8.2.5 Unitinformation .............................................. 68
8.3 Calibrationcommands............................................... 68
8.3.1 Calibration information request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
8.3.2 Lightcalibrationcommand ........................................ 69
8.3.3 Darkcalibrationcommand ........................................ 70
8.3.4 Disarmcalibrationcommand ....................................... 70
8.3.5 Manually set light calibration offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
8.3.6 Manually set light calibration temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
8.3.7 Manually set dark calibration time period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
8.3.8 Manually set dark calibration temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
8.4 SnowLEDaccessory................................................ 72
8.5 Dataloggercommands ............................................... 74
8.5.1 FLASHIDreport ............................................. 74
8.5.2 Loggingpointerreport........................................... 74
8.5.3 Loggingcapacityreport .......................................... 75
8.5.4 EraseentireFLASHmemory ....................................... 75
8.5.5 Logonerecord ............................................... 75
8.5.6 Returnoneloggedrecord ......................................... 75
8.5.7 RetrieveallBinary............................................. 77
8.5.8 Getinternalvoltage ............................................ 79
8.5.9 GetEEPROMstatusregister1...................................... 79
8.5.10 Set and read logging trigger mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.5.11 Logging Interval setting response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4Unihedron SQM-LU-DL Operator’s Manual - 20220119
8.5.12 Logging interval reporting period setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
8.5.13 Set and read logging Real Time Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
8.5.14 GetRealTimeClockversion ....................................... 82
8.5.15 Putdataloggingunittosleep ....................................... 82
8.5.16 Gatheralarm0datafromRTC...................................... 82
8.5.17 Mutualaccess................................................ 83
8.6 Continuousreportingcommands ......................................... 84
8.7 Setting Interval reporting parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.7.1 Interval reporting period setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.7.2 Threshold setting for interval reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.7.3 Intervalsettingresponse.......................................... 87
8.8 Simulationcommands ............................................... 88
9 Installation ........................................................ 90
9.1 Mechanicalinstallation............................................... 90
9.1.1 Coverselection ............................................... 90
9.1.2 Covercalibration.............................................. 90
9.1.3 Covermaintenance............................................. 91
10 Default settings ..................................................... 92
11 Firmware upgrade .................................................... 93
11.1Details........................................................ 93
12 Calibration ........................................................ 94
12.1Recalibration .................................................... 94
12.2Lightcalibration .................................................. 94
12.3Darkcalibration .................................................. 94
12.4Confirmation .................................................... 94
13 Troubleshooting ..................................................... 95
14 Glossary .......................................................... 100
15 Bibliography ....................................................... 101
List of Figures
2.1 Mpsasinterpretivescale .............................................. 11
2.2 MpsasvsNELM .................................................. 14
3.1 SQM-LU-DLBlockdiagram............................................ 15
4.1 Frontandbackofofunit ............................................. 19
5.1 Wiredconnection.................................................. 20
5.2 Batteryconnection................................................. 20
5.3 Currentprofile ................................................... 21
5.4 DiagnosticLEDtimingdiagram.......................................... 22
5
7.1 Splashscreen .................................................... 26
7.2 Filemenu ...................................................... 27
7.3 FileOpendialog .................................................. 28
7.4 Viewmenu ..................................................... 28
7.5 View:Log ..................................................... 29
7.6 View:Directories .................................................. 29
7.7 Dataloggingheader ................................................ 30
7.8 SetLocationdialog................................................. 31
7.9 Plotterwindow ................................................... 32
7.10Tool:oldlogtodat ................................................ 34
7.11Tool:dattoMooncsv .............................................. 34
7.12Tool:Commterminal............................................... 35
7.13Tool:.datto.kml ................................................. 35
7.14 Tool : .dat to .kml GoogleEarth result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
7.15Tool:.dattimecorrection............................................. 36
7.16 Tool : .dat local time reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
7.17RSEalgorithm ................................................... 40
7.18Helpmenu...................................................... 42
7.19Help:About .................................................... 42
7.20Founddevice(single)................................................ 42
7.21Founddevice(multiple) .............................................. 43
7.22USBconnectiondetails............................................... 43
7.23Ethernetconnectiondetails ............................................ 43
7.24RS232connectiondetails.............................................. 43
7.25Informationtab................................................... 44
7.26LogContinuouslyscreen.............................................. 44
7.27LogContinuouslyTriggertab........................................... 45
7.28LogContinuouslyThreshold............................................ 45
7.29 Log Continuously Pre-audio alert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
7.30 Log Continuously Alarm for darkness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
7.31LogContinuouslyTransfertab .......................................... 46
7.32LogContinuouslyAnnotationtab......................................... 47
7.33LogContinuouslyGPStab ............................................ 47
7.34Calibrationtab(initial) .............................................. 50
7.35Calibrationtab(populated)............................................ 50
7.36Reportintervaltab................................................. 51
7.37Firmwaretab .................................................... 52
7.38Firmwareselection ................................................. 52
7.39Configurationtab.................................................. 53
7.40Lightcalibrationconfirmation........................................... 53
7.41Darkcalibrationconfirmation........................................... 53
7.42Sensorarrangement ................................................ 54
7.43Dataloggingtab .................................................. 54
7.44RealTimeClocksettingdialog .......................................... 55
7.45Storagenavigation ................................................. 55
7.46DLRetrievedialogwindow ............................................ 56
7.47DLErasedialogwindow.............................................. 57
7.48Dataloggingpageforvectormodel ........................................ 58
7.49Contourplotof336readings ........................................... 59
7.50Fig:Simulationtab ................................................. 60
7.51Accessoriestab ................................................... 61
9.1 Housing ....................................................... 90
9.2 Examplecovercalibration............................................. 91
6
List of Tables
List of Tables
2.1 Apparent Magnitudes of Known Celestial Objects adapted from [2] . . . . . . . . . . . . . . . . . . . . . . 11
7.2 .dat to Sun-Moon-MW-Clouds values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
7.3 Storage navigation button descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
7.4 simin.csvfielddescription ............................................. 60
7.5 Log continuous command line parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
7.6 Select device command line parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
7.7 Select device command line parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
8.1 Summaryofstandardcommands ......................................... 63
8.2 Summaryofdataloggercommands ........................................ 64
8.3 Readingrequestresponse ............................................. 65
8.4 Unaveraged reading request response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
8.5 Linearreadingrequestresponse.......................................... 66
8.6 Inline linear reading request response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
8.7 Unitinformationrequestresponse ........................................ 68
8.8 Calibration information request response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
8.9 Lightcalibrationresponse ............................................. 69
8.10Darkcalibrationresponse ............................................. 70
8.11Disarmcalibrationresponse............................................ 70
8.12 Response for manual setting of light calibration offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
8.13 Response for manually setting of light calibration temperature . . . . . . . . . . . . . . . . . . . . . . . . 71
8.14 Response of manually setting dark calibration time period . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
8.15 Response for manually setting of dark calibration temperature . . . . . . . . . . . . . . . . . . . . . . . . 72
8.16LEDaccessorycommandsummary........................................ 72
8.17LEDaccessoryresponsesummary......................................... 73
8.18 Response of requesting the FLASH chip ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
8.19 Response of requesting the data logging pointer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
8.20 Response of requesting the data logging capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
8.21 Response of requesting to log one record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
8.22 Request to return one logged record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
8.23 Response to return one logged record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
8.24 Response to Binary Retrieve command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
8.25BinaryRetrieverecordcontent .......................................... 77
8.26 Response of requesting internal voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.27 Response of requesting EEPROM Status regoster 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.28 Request to set the logging trigger mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.29 Response of setting or getting the log trigger mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
8.30 Response of viewing or setting logging interval reporting parameters . . . . . . . . . . . . . . . . . . . . . 80
8.31SettingtheRTC .................................................. 81
8.32 Response of getting the RTC values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
8.33 Response of getting the RTC version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
8.34 Response of getting the RTC “Alarm 0” value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
8.35 Response of setting or getting the mutual access mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Unihedron SQM-LU-DL Operator’s Manual - 20220119 7
8.36Summaryofstandardcommands ......................................... 84
8.37 Continuous reporting command summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.38 Response of all continuous reporting requests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.39Intervalreport ................................................... 86
8.40 Response of viewing or setting interval reporting parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8.41 Response of request for internal simulation values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
8.42Requestsimulation(S....x)............................................. 88
8.43 Response of setting simulation values (S...x) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
13.1Readingseemtoobright.............................................. 95
13.2Readingis0.00mpsas................................................ 95
13.3CannotFindUDMsoftware............................................ 95
13.4Drivernotfound .................................................. 95
13.5 USB device not found, or more than one device on the same COM port . . . . . . . . . . . . . . . . . . . 95
13.6USBserialnumbergarbled ............................................ 96
13.7LEDdoesnotlightup............................................... 96
13.8Recordingstops................................................... 96
13.9 Meter cannot be found on battery or by PC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
13.10UDM improperly lists devices under Windows OS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
13.11Meter cannot be found by Windows operating system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
13.12Cannotgetareading................................................ 98
13.13Darkroom-bagreadingsincorrect ......................................... 99
14.1Glossaryofterms.................................................. 100
List of Equations
2.1 MPSAS to cd/m2.................................................. 13
2.2 NELMtoMPSAS ................................................. 14
2.3 MPSAStoNELM ................................................. 14
8.1 RawTemperaturetodegC............................................. 88
8.2 degCtoRawTemperature............................................. 88
8
1 Introduction
Thank you for purchasing the SQM-LU-DL. You will soon be on your way to collecting scientific data.
The SQM series of products have been used in the following applications:
•Quantitatively comparing the sky brightness at different astronomical observing sites.
•Documenting the evolution of light pollution.
•Setting planetarium dome illumination to mimic the skies that people are likely to experience elsewhere in the city.
•Monitoring sky brightness through the night, night-to-night, and year-to-year for astronomical observation records.
•Determining which nights show the greatest promise for finding the ’faintest fuzzies’.
•Calibrating the effect of sky brightness on qualitative measures such as the Bortle Scale or NELM.
•Investigating how sky brightness correlates with the solar cycle and month-to-month sunspot activity.
•Helping to provide local ground truth for future sky brightness prediction with the Clear Sky Clock.
•Helping CCD users make a correlation between the SQM reading and when the background reaches some ADC
level.
•Assisting Sea Turtle researchers in studying the amounts of light in areas where turtle hatchlings are affected by
artificial lights.
•Researching bird-song synchronization with dawn.
•Researching twilight brightness changes with the addition of external Neutral Density filters. Unihedron offers
adapters to attach such filters onto the meter.
•The SQM-LU-DL has the same features as the SQM-LU with the addition of data logging capabilities.
1.1 QuickStart
1.1.1 Setting up the instrument
1. Connect the SQM-LU-DL meter to the computer with the supplied USB cable. Wait for any device drivers to
automatically load (if required). You may have to visit www.FTDIchip.com to get the latest VCP drivers if they
do not get installed automatically.
2. Ensure that your computer date and time is up-to-date.
3. Install and launch UDM (Unihedron Device Manager software supplied on the CD from your File Manager).
4. Click the “Find” button to find attached devices, then click on the SQM-LU-DL that you connected.
5. Click ”Data Logging” tab to go to the setup page.
6. Click the “Set” button in the “Device Clock” area on that page to synchronize the SQM-LU-DL internal time with
your computer time and date.
7. Set the repetition time for the readings in the “Trigger” section on the left side of the window. A good rate is “every
5 minutes”.
8. Keep the SQM-LU-DL connected to the computer for a couple of hours to charge up the internal Supercap; this
allows it to keep the time and date clock running while you are moving from your computer to the measurement
site. A fully charged meter will keep the clock running for a few days unpowered.
9
1 Introduction
9. Disconnect the SQM-LU-DL from the USB cord and plug in the battery pack within eight hours, the LED will come
on for three seconds to indicate that the unit woke up and is working. The LED will come on every minute for a
few seconds, you can use this to determine if the unit is still properly powered. Note: beware that using a “USB
charger” instead of the battery pack may cause problems since some of these chargers make the SQM-LU-DL think
it is connected to a computer instead of a battery, and some other chargers do not have the surge capacity to allow
the SQM-LU-DL to start up properly.
The device is now recording.
1.1.2 Saving the data log
Follow these steps to retrieve the logged data from the SQM-LU-DL:
1. Connect the SQM-LU-DL to the computer with the USB cable.
2. Launch the UDM program and find your device as before.
3. Select the “Data Logging” tab.
4. Click “Retrieve All” and follow the instructions on that page.
5. ONCE YOU’VE CHECKED THAT THE DATA IS SAVED TO DISK PROPERLY, and you want to clear the
data-logger memory in preparation for a new set of records, you can erase the old memory. Warning this step
cannot be undone!
1.1.3 Getting the data from the data log into MS Excel
1. Find the log data. It will be in a *.dat file, where the * will give you the date and time
2. In the Tools pulldown in the UDM program, use the “.dat to Moon .csv” function to convert the log file to a *.csv
file. Do not erase the original log file, because it contains some information (e.g., the type and serial number of your
SQM) that is not converted to the csv file.
3. Open the *.csv file in Microsoft Excel.
4. In Excel, highlight the data, then in the “Data” pulldown, use the “Text to Columns” function to convert the data
into columnar format; the divider is a semicolon.
5. Your data is now in nice columns by date and time, so you can plot it out as you wish.
1.1.4 Other software
•If you are using Windows, you may want to use Knightware SQM-Reader from www.knightware.biz/sqm .
•The CD contains examples of software (Perl, Python, etc.) for various functions that connect to the meter.
1.1.5 FITS integration
Some programs (listed below) gather information from the Unihedron Sky Quality meter products and insert that data
into the Flexible Image Transport System (FITS) header.
•MaxPilote (Freeware) incorporates SQM readings from an SQM-LE/SQM-LU into the FITS header while at the
same time provide constant readings in a live and updated Data Window.
•CCDAutoPilot
•FITS4Win2 uses the MPSAS keyword for sorting and filtering image files.
10 Unihedron SQM-LU-DL Operator’s Manual - 20220119
2 Measurements
The SQM-LU-DL provides readings in Magnitudes per square arcsecond, abbreviated as: mpsas, and written mathemat-
ically as mag
arcsec2.
Mpsas is a logarithmic measurement which means that large changes in sky brightness correspond to relatively small
numerical changes. A difference of 5 magnitudes is a factor of 100 times the intensity. Therefore a sky brightness 5.0mag
arcsec2
darker corresponds to a reduction in photon arrival rate of a factor of 100.
The following schematic gives a rough idea of how to interpret the readings of the SQM:
Figure 2.1: Mpsas interpretive scale
Magnitudes are an astronomical unit of measure for object brightness. Brighter objects have a lower magnitude and
darker objects have a higher magnitude value. For example; a star that is 6th magnitude is brighter than a star that is
11th magnitude.
The star Vega is used a the reference point of ≈0 magnitude. Table 2.1 shows the apparent magnitude of some common
known celestial objects.
Table 2.1: Apparent Magnitudes of Known Celestial Objects adapted from [2]
App. Mag. Celestial Object
−26.73 Sun
−12.6 full Moon
−4.7 Maximum brightness of Venus
+0.03 Vega, the original zero point
+6 Faintest stars observable with naked eye
+27 Faintest objects observable in visible light with 8m ground-based tele-
scopes
+30 Faintest objects observable in visible light with Hubble Space Tele-
scope
Arcsecond is the definition of an arc being divided up into seconds as follows.
1. There are 360 degrees in a circle.
2. There are 60 arcminutes in a degree, and 21600 arcminutes in a circle.
3. There are 60 arcseconds in an arcminute, and 1296000 arcseconds in a circle.
11
2 Measurements
Square arcsecond (arcsec2) is the area covered by a square measuring 1arcsec ×1arcsec .
Magnitude per square arcsecond is the definition of brightness in magnitudes spread out over one square arcsecond
of the sky. For example; if the SQM provides a reading of 20.00 mpsas, that would be like saying that a light of a 20th
magnitude star brightness was spread over one square arcsecond of the sky.
The ”magnitudes per square arcsecond” numbers are commonly used in astronomy to measure sky brightness. More
details can be found at www.stjarnhimlen.se/comp/radfaq.html
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 mags/sq arcsec means the sky is 100x brighter.
The darkest we’ve personally experienced with the SQM in a natural clear sky was 21.80.
2.1 Getting accurate readings
Various factors will cause the night sky brightness to fluctuate. Taking more readings will be useful in ruling out spurious
events. The SQM gathers light for at least a one second period, and the brightness report is based on the light that was
accumulated during that time.
At the darkest sites, natural variations in conditions such as airglow and the brightness of the zodiacal light are limiting
factors.
Prevent artificially high (dark) readings by ensuring that there is nothing blocking the view of the sensor. Avoid taking
readings near trees or buildings that may block the sensor.
A reading of greater than 22.0 is unlikely to be recorded, however there are reasons for extra dark readings:
•Cloud or ash covered sky in a remote area where little natural sky brightness can be recorded.
•Uncorrected readings from covered meter like a weatherproof housing.
•Obstructed view like inside a forest, or where large buildings are located within the field of view, or a person or bird
blocking the view.
When the sky is very dark, the meter takes longer to gather enough light to produce a reading. During this time
of light gathering, the reading will remain constant until the meter can start gathering readings fast enough for new
reading-requests.
The readings are averaged in an 8-cell rolling buffer, so it may take a while for the meter to produce a new value of
darkness when obstructions leave the field of view.
Prevent artificially low (bright) readings by ensuring that there are no lighted objects (street lamps, the moon, etc.)
that shine into the sensor at any angle.
2.1.1 Seeing conditions
Scintillation is due to refractive changes in the atmosphere caused by temperature changes at differing heights.
Stars are too small in comparison to the entire SQM field of view, so scintillation is not expected to alter the SQM
reading significantly.
2.1.2 Light pollution
Undesirable artificial light that reaches you is considered to be light pollution. Much of this light comes from outdoor
illumination of parking lots, street lamps, office buildings, advertising signs, etc..
Other causes of extra light in the night sky are listed below:
12 Unihedron SQM-LU-DL Operator’s Manual - 20220119
2.1 Getting accurate readings
Aurora
Charged particles emitted from the Sun are directed to the poles of the earth by the Earth’s magnetic field. These particles
collide with atoms in the atmosphere and cause light to be emitted. Aiming the meter at the polar regions during Aurora
Borealis (in the North) or Aurora Australis (in the South) will reduce the reading (lighter). Aiming the meter towards
the equator will increase the reading (darker) under these conditions.
Airglow
Airglow is light produced by various phenomenon in the atmosphere which prevent the sky from being totally dark. Effects
of the magnetic poles of the Earth may cause airglow to be brighter near the poles.
The Milky Way
As one goes to sites with darker surface brightnesses, the fraction of the total light received by the SQM-LU-DL which
can be attributed to the Milky Way bulge increases and so the “offset” in mpas will be larger (due to the Milky Way.)
The northern view of the Milky Way contributes about 0.10 mpsas under 21.5 mpsas (moonless) skies.
The southern view of the Milky Way might be as big an effect as 0.56 mpsas in dark skies where it goes near-overhead.
For more information, see Surface Photometries of the Milky Way (Schlosser+ 1997)
vizier.u-strasbg.fr/ftp/cats/VII/199/ReadMe
Moisture
Clouds, fog, and mist will reflect artificial light back down to the Earth causing a brighter (lower) reading. If there is
no artificial light, then clouds may prevent starlight from coming to you and the reading will be darker (higher). This
extra-dark situation can occur in very isolated areas like mountain tops, the ocean, or the desert. You will have to be
aware of this special situation when analyzing readings.
Volcanic eruptions
Dust released into the atmosphere by volcanoes can reflect light from the surface of the earth back down. In a dark
location this dust will prevent the light from stars and Milky Way and produce a darker (higher) reading.
Zodiacal light
The sunlight reflected of off dust particles in the ecliptic plane of our solar system is called zodiacal light.
The amount of light will be different depending on whether the meter is pointed to the poles or plane of the solar
system. It is likely to have less than 2% effect. The primary reason for this small effect is that the brightest and widest
part of the zodiacal light is nearest the horizon where the SQM has almost no sensitivity (due to it being a primarily
zenith-looking device). The portions at higher altitude are the narrowest and faintest and they would barely creep into
the sensitivity cone of the SQM.
2.1.3 Other luminance scales
Candela per square meter (cd/m2) is commonly used by lighting engineers.
To convert the SQM mpsas reading to cd/m2, use Equation (2.1):
[cd/m2] = 10.8×104×10(−0.4×[mag/arcsec2]) (2.1)
Naked eye limiting magnitude (NELM)
Quite often astronomers will refer to a sky by the darkest star they can see, for example a “6th magnitude sky”, in that
case you can see 6th magnitude stars and nothing dimmer like 7th magnitude stars. The term “6th magnitude skies” is
very subjective to a persons ability to see in the night, for example an older person might say “5th magnitude skies” but
a young child with better night vision might say “7th magnitude skies”.
Unihedron SQM-LU-DL Operator’s Manual - 20220119 13
2 Measurements
Nobody has performed the task of defining a relationship between the two methods of sky brightness ( xmagnitude
skies and magnitudes per square arcsecond) -- probably because one is subjective and the other is objective and a wide
variety of people would have to be polled.
An approximation exists for the conversion between NELM and MPSAS. You can use an NELM converter[5] created
by SQM user K. Fisher to do that conversion, or the chart shown in Figure 2.2 and Equations (2.2) and (2.3).
Figure 2.2: Mpsas vs NELM
Convert NELM (V mags) to MPSAS (B) sky brightness [3]
Bmpsas = 21.58 −5×log(10(1.586−N ELM/5) −1) (2.2)
Convert MPSAS (B) sky brightness to NELM (V mags) [4]
NELM = 7.93 −5×log(10(4.316−(Bmpsas/5)) + 1) (2.3)
NSU
Natural Sky Units [1] (NSU) is radiance relative to an assumed natural radiance of 21.6 magSQM/arcsec2.
NSU values indicate how much brighter or darker the sky is compared to a typical historic clear night sky. It is defined
here as NSU = 100.4∆ where ∆ is 21.6 minus the observed value in magSQM/arcsec2.
14 Unihedron SQM-LU-DL Operator’s Manual - 20220119
3 Theory of operation
Figure 3.1: SQM-LU-DL Block diagram
3.1 Light measurement
The SQM-LU-DL measures the darkness of the night sky to provide readings of magnitudes per square arc second through
the USB connection.
A light to frequency sensor (TSL237) provides the micro-controller with a signal whose frequency depends on the
amount of incoming light. And, readings from the temperature sensor are used to compensate the light sensor readings
through the range of operating temperatures.
In bright skies, the sensor produces a high frequency square wave (up to 500kHz). As the sky gets dark, the sensor
produces a low frequency. In absolute darkness, the sensor period is very long, and is defined on the calibration sheet as
“Dark period” with a 300s timeout.
Measurement of this wide range of frequencies is done by timers and counters inside the microprocessor. It is difficult for
a microprocessor to accurately measure the period of high frequency signals, and just as difficult to measure the frequency
of extremely low frequency signals. For this reason the microprocessor has two modes of signal measuring:
Frequency mode: The frequency mode is for bright readings where the frequency is high. The sensor signal is fed
into a counter that is gated at one second intervals to report a frequency measured in Hz.
Period mode: The period mode is for dark readings where the frequency is low. The sensor signal gates a high
speed internal counter which reports the period measured in µs.
Both Frequency and Period results are available when connected directly to the meter and using the “rx” command.
The SQM-LU-DL computes mpsas by deciding if the period or frequency report should be used and then compensating
for temperature. Deciding between period or frequency mode reporting was originally done at a frequency of 354Hz, but
by selecting the “Ideal crossover firmware” setting on the “Report interval” page, the switchover will be 679Hz which
offers the best resolution at the switchover point.
The SQM-LU-DL holds the updated period and frequency recordings in memory while waiting for an asynchronous
user request for a reading. It is possible for the user to see stale readings if the light sensor cannot gather enough light
to produce a reading before the user asks for the next report (i.e. using the “rx” command too frequently). Starting at
firmware version 58, the SQM-LU-DL can provide a status of the reading that the user has requested by using the “r1x”
command which contains a status suffix character of F/P/S whose meaning is:
F: fresh FREQUENCY mode reading.
P: fresh PERIOD mode reading.
S: STALE reading indicates that this reading has not changed since the last reading because there was not enough
light gathered to produce a new reading.
15
3 Theory of operation
3.2 Communication to the PC
Commands sent from a PC through the USB cable to the USB interface are relayed to the micro-controller.
The micro-controller responds to commands by sending data strings to the USB interface which are then relayed to the
PC.
Readings are gathered asynchronously by the micro-controller. Requests from the PC are buffered and dealt with as
time permits.
16 Unihedron SQM-LU-DL Operator’s Manual - 20220119
17
4 Specifications
4 Specifications
USB connection USB B connector.
5m USB A to USB B cable supplied.
USB FTDI VCP driver, serial port emulator at 115200baud.
Physical Size 5.5” x 2.6” x 1.1”
Meter weight 110g
Battery pack with batteries weight 160g
USB cable weight 140g
Meter precision Each SQM-LU-DL is factory-calibrated. The absolute precision of each meter
is believed to be ±10% (±0.10 mag/arcsec2). The difference in zero-point
between each calibrated meter is typically ±10% (±0.10 mag/arcsec2)
Field of View 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 would register 3.0 and 5.0 magnitudes fainter,
respectively.
Power requirement 18mA (from the 5V USB connection). Can be put to sleep and woken up
for battery operated data logging.
3.42V minimum under battery operation.
The included battery adapter can handle up to 30VDC.
Operating temperature range -40°C to 85°C
Temperature sensor Accuracy: ±2°C maximum at 25°C
Location: Internal near light sensor, measures internal meter temperature for
light sensor value compensation.
Temperature update rate 4.3 seconds, 256 samples taken at 60Hz then averaged.
Data logging capacity Latest model (Extended memory version): 1048576 records total.
Initial model (1MB Flash chip, 32 bytes per record): 32768 records total.
Second generation model (16 bytes per record): 65536 records total.
For actual memory capacity; see back label (no entry = 32768 records) or
UDM datalogger page while connected to the meter.
Real Time Clock accuracy +1.73s/day @25°C, -2.27s/day @50°C, per Citizen crystal
CFS206-32.768KDZB-UB
18 Unihedron SQM-LU-DL Operator’s Manual - 20220119
Figure 4.1: Front and back of of unit
Unihedron SQM-LU-DL Operator’s Manual - 20220119 19
5 Hardware connections
The SQM-LU-DL requires one connection to a USB hub or a PC for configuring the device and recovering the readings
as shown in Figure 5.1.
Figure 5.1: Wired connection
The maximum length cable per the USB specification is 15ft (3m). USB extenders exist on the market, some work up
to 198ft (60m).
A battery adaptor with batteries is supplied to allow the SQM-LU-DL to perform data-logging while disconnected from
the computer. This battery connection replaces the USB connection from the computer as shown in Figure 5.2.
Figure 5.2: Battery connection
5.1 Battery operation
The SQM-LU-DL may be operated by an external battery using the supplied “battery to USB adaptor”. The lifetime of
the battery can be determined as shown below. This calculation is also performed and displayed by the UDM software
on the “Data Logging” panel. Figure 5.3 shows experimental results of the current profile using a 10Ω current shunt.
20
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