Estimates the topocentric (observed) pulse frequency
for a selection of strong pulsars, given the observer's
Latitude and Longitude, and UTC time.
See below for instructions. Also be sure
to read the Notes for caveats.
Presets
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| Set Pulsar: |
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| Set City/Town/Observatory: |
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Instructions: Via the drop-down 'Preset'
menus provided, set the target pulsar to one
of the relevant pulsar candidates for amateur
observers. Pulsar candidates are limited to
the pulsar drop-down list entries as they need
to be associated with their individual ephemeris
parameters. (Note: other pulsars can be added
if requested via the email address in 'Contact').
The latitude/longitude for a selection of main
cities/towns/observatories can be selected from
the 'Presets' menu,
or can be
manually entered in free format sexagesimal
- (note: preset public observatory locations
are exact, but the location accuracy of preset
private stations is limited to the closest town
to preserve privacy). Only fields highlighted
in
green
are editable.
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).
Available Calculations:
Two different
calculation modes are provided...
- "Now" shows a single result
for the set UTC date/time.
- "Day:xm" opens a new window
with a table covering 24 hours starting from
hour/minute 00:00 of the set UTC date with an
increment of 'x' minutes (options for 1, 5 and
15 minute increments). The fields are
MJD, UTC Date and Time, Orbital Radial Velocity
(km/s) and Orbital Doppler Shift (ppm).
- "100 Days" opens a new
window with a table covering 100 successive
days starting from the set UTC date at the fixed
time of 00:00 UTC each day. The fields are MJD,
UTC Date and Time, Orbital Radial Velocity (km/s)
and Orbital Doppler Shift (ppm).
The tables can be printed - just
be sure to enable 'background graphics' to print
the highlighting.
Doppler Effect:
For +ve velocities
(moving away) the period Doppler shift is +ve.
Conversely, for -ve velocities (moving towards)
the period Doppler shift is -ve.
Derived from:
Old FORTRAN code
("DOP") to C# to Javascript plus JS code from
many sources. Please report errors or discrepancies
with other sources.
Pulsar Ephemeris Data:
ATNF Pulsar Database
(Manchester, R. N., Hobbs, G.B., Teoh,
A. & Hobbs, M., AJ, 129, 1993-2006 (2005))
- except where later epoch data is available.
J0437-4715 Orbital Data: "Precision
timing of PSR J0437−4715: an accurate pulsar
distance, a high pulsar mass and a limit on
the variation of Newton’s gravitational constant":
J. P. W. Verbiest, M. Bailes, W. van Straten,
G. B. Hobbs, R. T. Edwards, R. N. Manchester,
N. D. R. Bhat, J. M. Sarkissian, B. A. Jacoby
and S. R. Kulkarni.
Notes:
- Pulsar timing is referenced to a barycentric
(centre of gravity of all objects in the solar
system) frame of reference. If use is made of
the pulsar parameters from, say, the ATNF Pulsar
Database, a velocity correction is needed to
convert to a topocentric (earth-based observer)
pulse timing for accurate epoch-folding.
Strictly speaking, the calculation above is
the correction from heliocentric (centre of
the sun), but the difference (< 0.05 ppm)
between barycentric and heliocentric frames
of reference is negligible for amateur purposes
and in any case the difference is less than
the accuracy of the algorithm.
- Binary Pulsars:
some pulsars are
in binary systems (e.g. J0437-4715) which means
that the binary orbital Doppler must be added
to the period calculated here. The orbital Doppler
value for the date/time is calculated and applied
to the period to arrive at the actual topocentric
period. The light
blue highlighted fields are only active
when a binary pulsar has been selected from
the drop-down list.
Note: Currently there
is only one candidate (J0437-4715) - being the
strongest binary pulsar and possessing and almost
perfectly circular orbit making calculation
of orbital a simple solution of a sinusoidal
function. Binary pulsars with eccentric orbits
require a more complex solution.
- The young Vela and Crab pulsars are 'glitchers'.
Vela can suddenly spin-up in frequency by as
much as 3 ppm - although this happens, on average,
only every 2.5 years. In the case of Vela, the
calculation here adds in the average glitch
behaviour to produce a result which, over time,
has minimal excursions from the actual spin
frequency. Consequently, the result will swing
either side (±1.5 ppm) of the actual
spin frequency dependent on when the last glitch
has occurred. Note that the TEMPO Vela result
can be more than 3 ppm out depending on whether
the TEMPO ephemeris data used is updated after
the last glitch(es). For the Crab pulsar
no glitch correction is applied, but the ephemeris
data will be updated periodically (once or twice
a year) to the latest
Jodrell Bank data while
it is available. Note that both pulsars are
slowing down quite rapidly - the Vela pulsar
by about 3.3 ppm/month, and the Crab Pulsar
even more so at about 32 ppm/month.
- Note that the precision in the results exceeds
the accuracy.
This deviation from
mathematical purity is a programming decision
which avoids the need to provide a variable
number of significant digits depending on the
period calculated.
- While the calculation of a pulsar topocentric
correction velocity is exactly the same as the
calculation of radial velocities associated
with VLSR, it excludes the velocity of the sun.
Therefore, this calculator is
not suitable
for determining doppler correction for emission
lines (HI, CH3OH, OH, etc)
- which requires the sun velocity.
