NED's Calculators
Velocity Conversion Calculator
Introduction
Most redshifts for extragalactic objects are published as "heliocentric" redshifts -- that is, the earth's rotational and orbital motions have been removed from the measured velocity. It is often necessary to further "correct" the redshifts for other motions: Galactic rotation, peculiar motion of the Galaxy within the Local Group, "infall" of the Local Group toward the center of the Local Supercluster, and motion within the reference frame defined by the 3K microwave background radiation. This calculator enables you to make these corrections as needed. NED has several pre-defined apex vectors that you may use, or you may input your own apex vector.
NOTICE: Conversions between redshift and apparent velocity, and decomposition of an observed redshift into its physical components, are currently computed using traditional approximations
(see Huchra 2008) applicable for low redshifts.
Results accurate at all redshifts, accounting for cosmological effects (see Carr & Davis 2021), are planned for a future NED update.
Data Entry
Enter the input coordinates in decimal degrees or in sexagesimal units.
Input is checked by a software interpreter,
and re-formatted for display. The best way to gauge these
interpreters is to test them; they will post an error message if the input
is not interpretable.
Examples of supported coordinate formats are
- 09h55m52.7s +69d40m46s
- 09:55:52.7 +69:40:46
- 09 55 52.7 +69 40 46
- 148.969687d 69.679383d
Select the input coordinate systems that you want to use.
It may be one of the following:
- Equatorial (default)
- Galactic
- Supergalactic
The equinox may be any date between AD 1500.0 and AD 2500.0.
If you do not prepend a "B" (Besselian dates) or "J" (Julian dates) to
the equinox, the calculator will use Besselian dates for equinoxes before
1990.0, and Julian dates for equinoxes after 1990.0.
Enter the redshift you wish to correct in velocity units of km
sec-1.
If you wish to use one of NED's
pre-defined apex vectors,
choose the one defining the reference frame to which the redshift is
currently referred by clicking on the "From" button and choosing one
of the reference frames named there. Do the same for the "To" button,
choosing the reference frame to which you wish to correct the redshift.
If your redshift is currently referred to a velocity frame for which
NED does not provide an apex vector, click the radio button and
enter your apex vector (coordinates and coordinate system, and velocity)
in the boxes provided.
Results
Click on "Calculate" to run the calculator. It will replace the input
page with a new page showing your input parameters, the type of correction
you requested, and the apex vector used, as well as the output velocity.
If the calculator does not recognize some aspect of your input, it will
display an error message. In this case, use your browser's "Back" button
to return to the input page to correct the input.
NED converts velocities from one reference frame to another using the
standard equation
Vcon = V + Vapex [sin(b) sin(bapex) +
cos(b) cos(bapex) cos(l - lapex)]
where l and b are the object's longitude and latitude, V is its unconverted
velocity, and the apices (with Galactic coordinates) of the various motions
are given as
| Conversion |
lapex |
bapex |
Vapex |
Source |
| Heliocentric to Galactocentric (GSR) |
87.8 deg |
+1.7 deg |
232.3 km/sec |
RC3 |
| Heliocentric to Local Group |
93 deg |
-4 deg |
316 km/sec |
AJ 111, 794, 1996 |
| Heliocentric to 3K Background |
264.14 deg |
+48.26 deg |
371.0 km/sec |
ApJ 473, 576, 1996 |
Courteau and van den Bergh
AJ 118, 337, 1999
have a recent review of the solar motion with respect to the Local
Group.
Back to the Velocity
Conversion Input Page.
Return to beginning of this help page.
Return to NED's Home Page.
Coordinate and Galactic Extinction
Calculator
Introduction
This is an easy-to-use coordinate transformation, precession, and position
angle calculator provided for your convenience. It is flexible enough
to convert accurately between Besselian and Julian equinoxes, taking the
epoch of observation into account when needed. It assumes that
Besselian dates refer to the FK4 system, that Julian dates refer to the
FK5 system, and makes the appropriate transformations. Though the
International Celestial Reference Frame (ICRF) is not explicitly supported,
the FK5 optical system is consistent with ICRF to within the known
errors of the FK5 system (see e.g. Ma et al.
AJ 116, 516, 1998).
The coordinate calculator also returns the foreground
Galactic extinction at your input position using the
Schlafly & Finkbeiner 2011 (ApJ 737, 103, 2011) recalibration
of the
Schlegel, Finkbeiner & Davis 1998 (ApJ 500, 525, 1998; SFD98)
extinction map.
The original SFD98 extinction values are also returned for comparison
purposes.
See this FAQ page
for more information and cautions on using the extinction values
returned by the calculator.
Data Entry
Select the input and output coordinate systems that you want to use.
They may be any of the following:
- Equatorial (default)
- Ecliptic
- Galactic
- Supergalactic
The input and output equinoxes may be any date between AD 1500.0 and
AD 2500.0.
If you do not prepend a "B" (Besselian dates) or "J" (Julian dates) to
the equinox, the calculator will use Besselian dates for equinoxes before
1990.0, and Julian dates for equinoxes after 1990.0.
If you know the epoch of the observation (between AD 1500.0 and AD 2500.0),
enter that in decimal years. The default epoch, 1950.0, is adequate for
all but high-precision conversion between the FK4 and FK5 systems.
Enter the input coordinates in decimal degrees or in sexagesimal units.
Input is checked by a software
interpreter, and re-formatted for
display. The best way to gauge these
interpreters is to test them; they will post an error message if the input
is not interpretable.
Examples of supported coordinate formats are
- 09h55m52.7s +69d40m46s
- 09:55:52.7 +69:40:46
- 09 55 52.7 +69 40 46
- 148.969687d 69.679383d
The position angle (default is 0 degrees) is optional. Enter it in
decimal degrees if you need to precess it.
Results
Click on "Calculate" to run the calculator. It will replace the input
page with a new page showing your input parameters as well as the output.
If the calculator does not recognize some aspect of your input, it will
display an error message. In this case, use your browser's "Back" button
to return to the input page to correct the input.
Notes
Different systems may be used for input and output; for example,
equatorial B1950.0 coordinates on the FK4 system may be precessed and
transformed to ecliptic J2000.0 coordinates on the FK5 system.
Position angles are in degrees, measured from north through east.
The precession and coordinate conversion routines were written by J.
Bennett who has also provided extensive
documentation on them.
Back to the Calculator Input
Page.
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Return to NED's Home Page.
