Radial Velocity/VLSR/Observation Frequency Calculators

Calculates the topocentric radial velocity of an observer in a given direction (equatorial coordinates) and observer's latitude and longitude, and UTC time.

May be useful for determining observational frequencies or correcting observation velocities for cosmic spectral lines - e.g., HI emissions and masers (but not pulsars). 

See instructions and usage below.

Radial Velocity Calculator
   
UTC (DD/MM/YYYY hh:mm:ss):
RA (hh mm ss.s):
DEC (±dd mm ss.s):
Latitude (±dd mm ss.s):
Longitude (dd mm ss.s):   E W
   
   
 Radial Velocity (±km/s):
   


Location Preset
   
Set City/Town/Observatory:
   

Instructions: Enter the target equatorial coordinates (RA/DEC - default W3(CH3OH)) and either use the 'Location Preset' drop-down menu to set the observer's latitude and east longitude, or manually enter in free format sexagisimal.

Time (UTC) can be set by the 'UTC Now' button or entered manually in the format of DD/MM/YYYY hh:mm:ss (observe format exactly).

Use of the Calculated Radial Velocity: The radial velocity calculated here is the observer's topocentric velocity towards the specified direction referenced to the LSR (local standard of rest).  However, in most cases the spectral line source (e.g. W3) has its own velocity w.r.t. the LSR, and so the observed velocity will be different to that calculated above.  As the observed velocity will be different for each observatory and varies over the day and year, in order to compare results between observatories and over time, a common VLSR reference is assigned to the source signal.  Use the calculators below to find a VLSR, or an expected observation frequency.

First, the Spectral Line of interest needs to be selected...

Spectral Line Preset
   
Spectral Line:
Rest Frequency (GHz):
   

then find...
  • VLSR: To correct your observation frequency peak to VLSR (to label a graph and/or to compare against published values).

    Say you observe a peak (Fobs) at 12.177 GHz while observing W3(CH3OH).  Enter this in the 'Fobs' field.  Enter the radial velocity for the source direction.

    Using the  'Radial Velocity Calculator' at the top of the page, set to your latitude and longitude and the time of the observation, and RA and DEC set to the coordinates of W3(CH3OH), to calculate the radial velocity.

    Note: By default the radial velocity calculated by the 'Radial Velocity Calculator' at the top of this page is loaded into this 'VLSR Calculator'.

    VLSR Calculator
    Enter Fobs (GHz):
    Radial Velocity (±km/s):
     
     
    VLSR (±km/s):

     
  • Observation Frequency: To know where to tune your receiver to receive, say, the 12 GHz Methanol emission line from the W3(CH3OH) maser source (e.g., the receiver bandwidth is narrow and if mis-tuned and the signal could be out-of-band).

    You will need to know the VLSR of W3(CH3OH).  The literature gives about VLSR = -44 km/s for this source.

    Using the  'Radial Velocity Calculator' at the top of the page, set to your latitude and longitude and the time of the observation and RA and DEC set to the coordinates of W3(CH3OH) to calculate the radial velocity.

    Note:
    By default the radial velocity calculated by the 'Radial Velocity Calculator' at the top of this page is loaded into this 'Observation Frequency Calculator'.

     
    Observation Frequency Calculator
    Enter Published VLSR (±km/s):
    Radial Velocity (±km/s):
     
     
    Observation Frequency (GHz):

     

Derived from: Old FORTRAN code ("DOP") -> C# -> Javascript plus JS code from many sources. Please report errors or discrepancies with other sources.

Verification: Radial velocity results were checked against Parkes ATNF Radial Velocities and Python code from MPIfR.  The comparison below is for W3(CH3OH) and the Effelsberg Observatory where the results are compared to the average of the ATNF and MPIfR results...

...which, for the dates and times indicated, shows < 0.25 ppm discrepancy.