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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Unihedron SQM-LU Users manual
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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Unihedron SQM-LU Users manual
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.
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 indows, 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 indows, Linux, or Mac, you can use the Perl scripts supplied on the CD.
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Illustration 1: SQM-LU block diagram

Unihedron SQM-LU Users manual
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.
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Illustration 2: Front and back of unit

Unihedron SQM-LU Users manual
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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Unihedron SQM-LU Users manual
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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Unihedron SQM-LU Users manual
Command Description
rx Reading request
cx Calibration information request
ix Unit information request (note lower case “i”)
zcalAx Arm Light Calibration command
zcalBx Arm Dark Calibration command
zcalDx Disarm Calibration command
zcal5#####
###x
Manually Set Light Calibration Offset
zcal6#####
###x
Manually Set Light Calibration Temperature
zcal7#####
###x
Manually Set Dark Calibration Time Period
zcal8#####
###x
Manually Set Dark Calibration Temperature
0x19 Reset microcontroller (see bootloader). Hexadecimal value 19.
:Intel Hex firmware upgrade initiation (see bootloader)
P########xSet period (in seconds) for interval reporting to EEPROM and RAM for booting
and immediate use. Firmware feature=13.
p########xSet period (in seconds) for interval reporting to RAM for immediate use.
Firmware feature=13.
T########xSet threshold (in mag/arcsec2) for interval reporting only to EEPROM and RAM
for booting and immediate use. Firmware feature=13.
t########xSet 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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Unihedron SQM-LU Users manual
4.2 Response details
4.2.1 Reading request
The “Reading” request “rx” commands the SQM-LU to provide the current darkness value as well as
all variables used to generate that result.
The format of the response is:
Column Value Description
0rIndicates that a reading is being returned.
2-8 06.70m Reading in magnitudes per square arc second.
Leading space for positive value.
Leading negative sign (-) for negative value.
A reading of 0.00m means that the light at the sensor has reached the
upper brightness limit of the unit.
10-21 0000022921Hz Frequency of sensor in Hz.
23-33 0000000020c Period of sensor in counts, counts occur at a rate of 460.8 kHz
(14.7456MHz/32).
35-46 0000000.000s Period of sensor in seconds with millisecond resolution.
48-54 039.4 Temperature measured at light sensor in degrees C.
Leading space for positive value.
Leading negative sign (-) for negative value.
55-56 Carriage return (0x0d), Line feed (0x0a).
Table 2: Reading request response
An example is:
r, 06.70m,0000022921Hz,0000000020c,0000000.000s, 039.4
01234567891012345678920123456789301234567894012345678950123456
Future revisions of this reading string will only modify reported values beyond position 54. Characters
0 to 54 may be considered stable.
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