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 recently-adopted 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 (from the maps by Schlegel et al. in ApJ 500, 525, 1998) at your input position. See the note below for more information and cautions on using the extinction values returned by the calculator.
Select the input and output coordinate systems that you want to use. They may be any of the following:
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 legal coordinate input are given below.
The position angle (default is 0 degrees) is optional. Enter it in decimal degrees if you need to precess it.
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.
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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.
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All of the following examples are legal input. In cases of ambiguous input, the output may not be what you intend. If the interpreter returns a warning message instead of doing the calculation you requested, click on your browser's "Back" button to return to the calculator input page and reformat the coordinates in your request. If you use decimal hours or decimal degrees in the RA field, we suggest noting this explicitly as in "14.3256h" or "214.8840d". The examples below show the default behavior of the interpreter.
| 16.3678 | 16 hr 22 min 04.08 sec (if in "longitude" field with equatorial RA set) or |
| 16.3678 degrees (otherwise) | |
| 16 | 16 hr (if in "longitude" field with equatorial RA set) or |
| 16.0 degrees (otherwise) | |
| 6h3m23s | 06 hr 03 min 23 sec |
| 26h7m23s | 02 hr 07 min 23 sec (where 02 hr = 26 - 24) |
| 79d23m90s | +79 deg 24 min 30 sec |
| 20 9 50 | 20 hr 09 min 50 sec (if in "longitude" field with equatorial RA set) or |
| 20.16388 (otherwise) | |
| 20d9'50" | +20 deg 09 min 50 sec |
| 20:9:50 | 20 hr 09 min 50 sec (if in "longitude" field with equatorial RA set) or |
| 20.16388 (otherwise) | |
| 20 9 | 20 hr 09 min of time (if in "longitude" field with equatorial RA set) or |
| 20.15 deg (if in "longitude" field) or | |
| 20 deg 09 min (if in "latitude" field) | |
| 20:9 | Same as "20 9" |
The individual values of the total absorption at each waveband are calculated from the list of A/E(B-V) in Table 6 of Schlegel et al (ApJ 500, 525, 1998). We have adopted the standard Landolt UBVRI filters for the optical total absorptions, and the UKIRT JHKL' filters for the near-infrared total absorptions. Please note that Schlegel et al. calculated the values of A/E(B-V) for these specific bandpasses using a spectral energy distribution for an elliptical galaxy. Therefore, the numbers displayed by NED for a specific object may not be appropriate for other closely related bandpasses or other galaxy types. The total absorptions are nominally consistent with an average R = A/E(B-V) = 3.1, but do not agree numerically with this average. See Appendix B of Schlegel et al (ApJ 500, 525, 1998) and references therein for additional details.
Note, too, the list of cavaets in their Appendix C. In particular, they call attention to the areas within the Holmberg radii of LMC, SMC, and M31 -- total reddenings through these large galaxies are replaced by Galactic reddenings toward them. They also note that no contaminating sources at Galactic latitudes |b| < 5 degrees have been removed from their dust maps, so calculated reddenings at these low latitudes are especially uncertain and untrustworthy. They state that the formal uncertainty in normalizing the dust column density to the reddenings is 10%; this should probably be taken as a lower limit on the formal error of the calculated reddenings at |b| > 5 degrees.
Willick (ApJ 522, 647, 1999) adds the following notes and caution concerning the Galactic extinction calculations:
"Two all-sky Galactic extinction maps are presently available: the older Burstein-Heiles (Burstein and Heiles,ApJ 225, 40, 1978, hereafter BH, and AJ 87, 1165, 1982) maps, which are based on 21-cm column density and faint galaxy counts, and the recently completed Schlegel, Finkbeiner, and Davis (ApJ 500, 525, 1998, hereafter SFD) maps, based on IRAS/DIRBE measurements of diffuse IR emission. The SFD extinctions have been favored in several recent analyses and, indeed, were used in Paper I (Willick,ApJ 516, 47, 1999). Unlike BH, the SFD extinctions are based directly on dust emission and have comparatively high spatial resolution. However, it has not been established beyond doubt that they are free of systematic errors, such as could arise from the presence of cold dust invisible to IRAS. The BH extinctions are also vulnerable to possible systematic effects, such as a variable dust-to-gas ratio and galaxy count fluctuations. Thus, it seems prudent to use both methods, or linear combinations of them, and see what effect this has on the results."
Finally, we note that we have replaced the Burstein-Heiles extinction toward M31 with values that are not affected by the HI emission of M31 itself (see ApJS 54, 33, 1984 for a discussion). This affects not only M31, but several thousand objects within that galaxy. Specifically, we have replaced "data word numbers" 402-405 in "physical record number" 20 in the Burstein-Heiles reddening file "redsouth.dat" with the E(B-V) values 0.068, 0.071, 0.074, and 0.077 (interpolated from surrounding areas), respectively. The original values were 0.043, 0.034, 0.024, and 0.045. (We thank Tod Lauer for alerting us to this problem.)
One additional note: The zero points of these two reddening laws differ by 0.02 magnitudes in E(B-V), with Schlegel et al. adopting a higher zero point than Burstein and Heiles. (We thank David Burstein for this note.)
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