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For F in Jy, use the following formula:
F= F/2,
where is the wavelength in microns (µm), and is a constant chosen from Table 12.1 and depending on the units of F. (This is simply derived, using the fact that d/d= c/2.)
F measured in |
|
---|---|
Wm-2µm-1
|
3x10-12
|
Wcm-2µm-1
|
3x10-16
|
erg sec-1 cm-2 µm-1
|
3x10-9
|
erg sec-1 cm-2 Å-1
|
3x10-13
|
Remember that 1W=107erg sec-1, and 1µm=104Å.
F= 10-m/2.5Fo
where m is the magnitude and Fo the zero-point flux for the given photometric band. We list the central wavelengths and zero-point fluxes for the more commonly encountered photometric bands below in Table 12.2. The CIT system was originally based on Beckwith et al (1976, Ap.J., 208, 390); the UKIRT system is in fact based on the original CIT system, but with adjustments made largely owing to different filter bandpasses. It should be noted that for a given photometric band there will be small differences in the effective wavelength and zero-point flux from one observatory to another, and for astronomical objects with radically different colors, so these figures can only be treated as approximate.
Band |
[µm] |
Fo[Jy](CIT) |
Fo[Jy](UKIRT) |
---|---|---|---|
V
|
0.56
|
3540
|
3540
|
R
|
0.70
|
2870
|
-
|
I
|
0.90
|
2250
|
-
|
J
|
1.25
|
1670
|
1600
|
H
|
1.65
|
980
|
1020
|
K
|
2.2
|
620
|
657
|
L
|
3.4
|
280
|
290
|
L'
|
3.74
|
-
|
252
|
M
|
4.8
|
150
|
163
|
N
|
10.1
|
37
|
39.8
|
Q
|
20.0
|
10
|
10.4
|
Surface brightnesses are generally measured in Janskys arcsec-2, MJy steradian-1 or magnitudes arcsec-2. If you have a surface brightness S in MJy steradian-1, then you can use:
S[Jy arcsec-2] = S[MJy ster-1] x 0.084616.
If you have S in magnitudes arcsec-2, you can simply use the formula and zero-points as given in the previous section for point sources.