Unihedron SQM-LU User Manual
Sky Quality Meter – Lens USB
SQM-LU
User manual
Unihedron SQM-LU Users manual
Table of Contents
1 Theory of operation................................................................................................................................3
1.1 Quick start.......................................................................................................................................3
2 Specifications.........................................................................................................................................4
3 Hardware connections............................................................................................................................5
2 Software development............................................................................................................................5
4 Commands and responses......................................................................................................................6
4.1 Commands......................................................................................................................................6
4.2 Response details.............................................................................................................................7
4.2.1 Reading request.......................................................................................................................7
4.2.2 Calibration information request..............................................................................................8
4.2.3 Light calibration command.....................................................................................................9
4.2.4 Dark calibration command....................................................................................................10
4.2.5 Disarm calibration command................................................................................................11
4.2.6 Unit information....................................................................................................................12
4.2.7 Manually set light calibration offset..................................................................................... 13
4.2.8 Manually set light calibration temperature........................................................................... 14
4.2.9 Manually set dark calibration time period............................................................................15
4.2.10 Manually set dark calibration temperature..........................................................................16
4.2.11 Setting interval reporting parameters.................................................................................. 17
4.2.11.1 Interval reporting period setting....................................................................................... 18
4.2.11.2 Threshold setting for interval reporting............................................................................18
4.2.11.3 Interval setting response...................................................................................................19
5 Electrical connection............................................................................................................................20
6 Mechanical installation........................................................................................................................ 20
6.1 Cover selection..............................................................................................................................20
6.2 Cover calibration.......................................................................................................................... 20
6.3 Cover maintenance........................................................................................................................21
7 Default settings.....................................................................................................................................22
8 Troubleshooting....................................................................................................................................23
9 Company contact information..............................................................................................................24
10 Glossary..............................................................................................................................................25
11 Appendix A – revision history........................................................................................................... 26
11.1 Manual revision history...............................................................................................................26
11.2 Protocol revision history.............................................................................................................26
11.3 Feature revision history...............................................................................................................26
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Illustration Index
Illustration 1: SQM-LU block diagram......................................................................................................3
Illustration 2: Front and back of unit..........................................................................................................4
Illustration 3: Housing..............................................................................................................................20
Illustration 4: Example cover calibration................................................................................................. 21
Index of Tables
Table 1: Command summary.....................................................................................................................6
Table 2: Reading request response.............................................................................................................7
Table 3: Calibration information request response.....................................................................................8
Table 4: Light calibration response............................................................................................................9
Table 5: Dark calibration response...........................................................................................................10
Table 6: Disarm calibration response.......................................................................................................11
Table 7: Unit information request response..............................................................................................12
Table 8: Response for manual setting of light calibration offset..............................................................13
Table 9: Response for manually setting of light calibration temperature.................................................14
Table 10: Response of manually setting dark calibration time period......................................................15
Table 11: Response for manually setting of light calibration temperature...............................................16
Table 12: Interval report........................................................................................................................... 17
Table 13: Response of viewing or setting interval reporting parameters................................................. 19
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1 Theory of operation
The SQM-LU measures the darkness of the night sky to provide readings of magnitudes per square arc
second through a USB connection.
A light sensor (TSL237) provides the microcontroller with a light level, and readings from the
temperature sensor are used to compensate the light sensor readings for various operating temperatures.
Commands sent from a PC through the USB cable to the USB interface are relayed to the
microcontroller.
The microcontroller responds to commands by sending data strings to the USB interface which are then
relayed to the PC.
Illustration 1: SQM-LU block diagram
1.1 Quick start
1. Your computer must have the FTDI VCP driver installed. You can get the most updated driver
for your operating system from here: http://ftdichip.com/Drivers/VCP.htm
2. If you are using Windows, then download and install the latest Knightware SQM-Reader from
www.knightware.biz/sqm . Follow the instructions for installing and using that software.
3. If you are using Windows, Linux, or Mac, you can use the Perl scripts supplied on the CD.
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2 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 3.6” x 2.6” x 1.1"
Meter precision Each SQM-L is factory-calibrated. The absolute precision of
each meter is believed to be ±10% (±0.10 mag/arcsec²).
The difference in zeropoint between each calibrated SQM-L is
typically ±10% (±0.10 mag/sq arcsec)
Power requirement 18mA (from the 5V USB connection)
Operating temperature range -40°C to 85°C
Temperature Accuracy ± 2°C maximum at 25°C
Temperature update rate 4.3 seconds, 256 samples taken at 60Hz then averaged.
Illustration 2: Front and back of unit
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3 Hardware connections
The SQM-LU requires one connection to a USB hub or a PC.
The maximum length cable per the USB specification is 15ft (3 meters).
2 Software development
The SQM-LU communicates as a standard serial port device using the FTDI software drivers which are
available for all major operating system platforms. http://ftdichip.com/ . The SQM-LU uses the FTDI
FT232R chip.
Once the driver is installed, commands can be sent to the SQM-LU using a serial terminal emulator to
the serial communications port that the device routes to.
When connecting the SQM-LU to a PC where the FTDI device driver is loaded, the serial port label
will be determined at connection time.
Each SQM-LU has a unique serial number usually with a prefix of “FT........”. This serial number can
be used to identify the exact SQM-LU device from other USB devices.
Interface Program overview:
– Data commands are sent to the SQM-LU, and it responds with a string of characters.
– A connection must be made to the serial port assigned to the SQM-LU.
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4 Commands and responses
The SQM-LU accepts a sequence of characters as a command, then executes those commands and
usually provides a response of a sequence of characters.
4.1 Commands
Commands consist of a string of characters.
The first character is the command type
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Command Description
rx
cx
ix
zcalAx
zcalBx
zcalDx
zcal5#####
###x
zcal6#####
###x
zcal7#####
###x
zcal8#####
###x
0x19
:
Reading request
Calibration information request
Unit information request (note lower case “i”)
Arm Light Calibration command
Arm Dark Calibration command
Disarm Calibration command
Manually Set Light Calibration Offset
Manually Set Light Calibration Temperature
Manually Set Dark Calibration Time Period
Manually Set Dark Calibration Temperature
Reset microcontroller (see bootloader). Hexadecimal value 19.
Intel Hex firmware upgrade initiation (see bootloader)
P########x
Set period (in seconds) for interval reporting to EEPROM and RAM for booting
and immediate use. Firmware feature=13.
p########x Set period (in seconds) for interval reporting to RAM for immediate use.
Firmware feature=13.
T########x
Set threshold (in mag/arcsec2) for interval reporting only to EEPROM and RAM
for booting and immediate use. Firmware feature=13.
t########x Set threshold (in mag/arcsec2) for interval reporting only to RAM for immediate
use. Firmware feature=13.
Ix
Request interval settings (note upper case “I”). Firmware feature=13.
Table 1: Command summary
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