Data Fields in the Database

The names for the individual parameters as they appear in the menus of the Download Data pages are the descriptors as they are used by the database. They are therefore not necessarily self-explanatory. The tables below provide brief descriptions of the available parameters for each type of data set. For more details see the References page.

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Weather Station Data

Parameter Description Units Comment
air_temp Air temperature degrees C Taken ~2 m above the ground, averaged over 1 min
humidity Relative humidity % Taken ~2 m above the ground, averaged over 1 min. Sensors have non-linearity around 30%, values larger than 100% (see Known Issues)
wind_speed Wind speed km/h Taken ~2 m above the ground, averaged over 1 min. Mechanical sensors sometimes get stuck (see Known Issues)
wind_direction Wind direction degrees east of north Taken ~2 m above the ground, averaged over 1 min. Mechanical sensors sometimes get stuck, some offsets exist (see Known Issues)
pressure Barometric air pressure mbar (=hPa) Taken ~2 m above the ground, averaged over 1 min
solar_rad Solar irradiation W/m^2 Averaged over 1 min
soil_temp Soil temperature degrees C Taken ~10 cm below the surface, averaged over 1 min
net_rad Net radiation W/m^2 Difference between the downward and upward energy fluxes, measured ~2 m above the ground, averaged over 1 min
heat_flux Ground heat flux W/m^2 Heat flux measured ~10 cm below the surface, averaged over 1 min. Flux sensors installed upside down at some sites (see Known Issues)

DIMM Data

Parameter Description Units Comment
seeing_6pt Seeing (6-pt extrap.) arcsec Seeing corrected to zenith for a wavelength of 500 nm and extrapolated to 0 ms from 6 different exposure times [~6, 12, 18, 24, 30, 36 ms] using an exponential function. This is the preferred seeing measurement of the TMT DIMMs.
seeing_2pt Seeing (2-pt extrap.) arcsec Seeing corrected to zenith for a wavelength of 500 nm and extrapolated to 0 ms from 2 different exposure times [~6, 12 ms] using the extrapolation of Tokovinin (2002).
strehl[12] Strehl ratio none Average Strehl ratios of the two star images on the DIMM CCD. Note that due to a bug in the code, the value of the Strehl ratios in the database is off by a multiplicative factor (see Known Issues).
airmass Air mass none Air mass of the DIMM observation
star Star ID none Star ID in form 'XXX XXX', for example, 'Alp Eri'. This is the same for DIMM and MASS.
exposure Exposure time ms Exposure time of the individual DIMM images. The value for this parameter is 5 ms for all data in the database. This is, however, simply the exposure time as set in the configuration files. The real exposure time is ~6 ms.
points_good Number good data points none Number of valid one-dimensional (1d) DIMM exposures used for a seeing measurement. Exposures are checked for star flux, star image positions etc. before being used in the seeing calculation.
points_all Number all data points none Number of all 1d DIMM exposures taken for a seeing measurement. The value for this parameter is 6000 for all data in the database.
fluxes[12] Measured star flux Counts Integrated fluxes of the two stellar images averaged over all valid 1d DIMM frames
fluxesb Measured background flux Counts/pixel Mean background level averaged over all valid 1d DIMM frames
peak_flux[12] Peak flux CCD counts Flux in the brightest pixels in the two stellar images, averaged over all valid 1d DIMM images
scintillation[12] Scintillation index none Scintillation index of the integrated flux in the valid 1d images
star_pos_[12] Average star position pixel Average position of the stellar image centroids in the 1d DIMM images
star_pos_sep Average star separation pixel Average separation of the two star images
star_pos_s[12][if] Initial/final star position pixel Initial and final positions of the two star images on the CCD. This is used to assess tracking performance of the telescope.

MASS Data

Note: The data in the MASS database have been reprocessed to take into account the effect of strong scintillation and correct analysis parameters. The data file as well as the download button are therefore called 'mass_redo'. This is no cause for alarm.

Parameter Description Units Comment
fixed_cn2_[123456] Layer strength m^1/3 Strengths of the MASS layers (note that this is Cn2*dh, not Cn2) when using a 6-layer profile with fixed layer altitudes.
fixed_seecn Seeing from Cn2 profile arcsec Free atmosphere seeing calculated from the fixed_cn2 values. This is the most reliable seeing measurement from the MASS.
fsee Free atmosphere seeing from turbulence moments arcsec Free atmosphere seeing calculated directly from the turbulence moments. This works even if the Cn2 profile restoration fails (or its error estimate is deemed too high), but it is usually a less reliable value than fixed_seecn.
isopl Isoplanatic angle from turbulence moments arcsec Isoplanatic angle calculated from turbulence moments. It is recommended that this value is not used, but that the isoplanatic angle is calculated directly from the fixed_cn2 values as this results in a more reliable estimate.
tau Coherence time from turbulence moments ms Turbulence coherence time calculated from turbulence moments. This is the best estimate of tau0 available from the MASS, but it requires some postprocessing. If you are not familiar with this, we strongly recommend that you read some of the publications on the topic.
fixed_z_[123456] Layer altitude km Altitudes of the MASS layers when using a 6-layer profile with fixed layer altitudes. For our data, these values are always 0.5, 1, 2, 4, 8 and 16 km.
float_cn2_[123] Floating model layer strength m^1/3 Strengths of the MASS layers (note that this is Cn2*dh, not Cn2) when using a 3-layer profile with variable layer altitudes.
float_z_[123] Floating model layer altitude km Altitudes of the MASS layers when using a 3-layer profile with variable layer altitudes.
Other MASS parameters     For descriptions of other parameters, please download the Turbina User Guide from the Documentation page of the MASS Group. If you need any of those parameters for your analysis, you are probably aware of this anyway (or you should be).

Dust Sensor Data

Parameter Description Units Comment
channel_03[1234] Particle density particles/ft^3 Particle density in the 0.3-μm channel. There are four such measurements per measurement period of ~6 minutes.
channel_05, channel_[125]0 Particle density particles/ft^3 Particle density in the 0.5, 1.0, 2.0, and 5.0-μm channels.

Sonic Anemometer Data (both at 7 m and on 30 m tower)

Parameter Description Units Comment
velocity_[xyz] Wind velocity component m/s x, y and z components of the wind velocity, averaged over 1 min. These are with respect to the internal reference system of the sonic anemometer. Check out the Known Issues page for instructions how to convert these to cardinal directions.

30-m Tower Temperature Data

Parameter Description Units Comment
t_ground Ground (2m) temperature degrees C Temperature measured close to the ground (~2 m elevation), averaged over 2 sec. This sensor is collocated with but physically different from the air_temp sensor in the Weather Station Data section.
t_[123] Tower temperature degrees C Temperature measured at the three elevation on the 30-m tower, averaged over 2 sec, with (t_1,t_2,t_3) = (28,20,11) m at Armazones and (11,20,30) m at Tolonchar