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.
