NHSC HIFI DP workshop
Caltech, 7-9 February 2011
Conversions of
Intensity Scales and
Frequency Units
Steve Lord (lord@ipac.caltech.edu)
- page 1
X-Axis Conversions
•
HIFI can change the Spectrum Explorer labeling of
a Spectral Data Set.
– Can add an auxiliary axis (e.g. add LSB or IF or USB)
– Can then alter that axis to show Velocity w.r.t. a reference
frequency
– See Developer's reference manual Chapter 8.1.1
- page 2
X-Axis Conversions
• We can use the command line to change the Xunits of a data set:
– ConvertFrequencyTask – HIFI Timeline Products or
Spectral Data Sets
– Converts between 4 views: USB, LSB, IF and Velocity
– For velocity, user must specify Reference Frequency and
whether that is in the USB or LSB (!)
– see HIFI-um Chapter 8.2
- page 3
X-Axis Conversions
• We can use the command line to change the X-units of a
data set:
– ConvertWaveScale – Converts a Spectral Data Set
– Converts between Frequency, Velocity (w/ref. f),
Wavelength and Wavenumber [GHz, km/s, MHz, mm,
m/s,1/m, MHz)]
– See Developer's reference manual Chapter 2.77
– routine has some issues to be resolved for HCSS 6.0
- page 4
Changing Flux Units
• ConvertK2Jy has been implemented in 12/2010
• Operates on Herschel Timeline Products and Spectral Data
Sets
• Is entirely a HIFI routine
• Uses the fact that K "brightness temperature" converts to
Jy/sr in the radio astronomy of using the Rayleigh Jeans
approximation strictly, and the user can provide steradians.
• Uses the fact that the effective aperture is "known" for all
HIFI frequencies, as well as beam efficiencies from using
the HIFI cal tree. (This is accessed either locally or remotely
– time consuming).
• Requires use input of a source size (diameter~HPBW
• ConvertK2Jy – has been implemented 2010
- page 5
ConvertK2Jy (con't)
• Basic Algorithm – 3 cases:
• The size in arc second of the source observed.
• 1) If the size = 0 the pointing like source formula is used:
S/Ta (Jy/K) = 2k / (Ageom * eta_A), where:
S is the Energy flux expressed in Jansky
Ta is the Antenna Temperature measured in Kelvin
k is the Boltzman constant,
Ageom is the effective area of the telescope, which we have assumed to be 3.28m, so
diameter. So, Ageom = 8.45m2 eta_A is the aperture efficiency, to be extracted from the
calibration tree for the frequency of interest
• 2) If the size <= HPBW:
S = Ta * 2k / (eta_B* size^2 /HPBW^2) , where:
eta_B is the Main beam efficiency
HPBW is the Half Power Beam Width (HPBW) of the telescope expressed in square arc second
• 3) In case of size >= HPBW
S ~= Ta * 2k / (eta_B* f_BEAM * lambda^2)
f_Beam is the beam filling factor
- page 6
NHSC HIFI DP workshop
Caltech, 7-9 February 2011
Unit Conversion Tool for Herschel
Runs alternatively:
• within HIPE – can take array
• as external JAVA tool
• as a website (server)
Converts:
• Flux
• Flux Density
• Intensity
• Integrated Intensity
Includes Help Files
- page 7