Definition of an Astronomical Source in the Time Estimator

Definition of an astronomical source involves specification of the spatial profile , the Source brightness and units and any emission line .

Spatial profile

The astronomical target may be a point or an extended source. The nominal point source has a Full Width Half Maximum size which is a combination of diffraction, the image quality, and the spatial resolution of the Science Instrument. The adopted extent of the point source (in arcsec) is reported in the time estimator results. The extended source is a source with uniform surface brightness. 


Source brightness and units

The source flux density or, for extended sources - surface brightness, are specified in familiar astronomical units available via the pull-down menus. The source brightness is converted into the internal time estimator units of photons/s/nm/m^2 (per square arcsec for surface brightness) using the relationships described below. 

(a) mag

Zeropoints giving continuum photons/s/nm/m^2 for a zero magnitude source are assumed to be:

Waveband Wavelength (um) Zeropoint
(photons/s/nm/m^2)
J 1.25 1.97 * 10^7
H 1.65 9.6 * 10^6
K 2.20 4.5 * 10^6
Kdark
2.45
3.23*10^6
KL
2.80
1.9*10^6
L' 3.76 9.9 * 10^5
M 4.77 5.1 * 10^5

These values were derived from the CIT system used in the STScI units conversion tool (UBVRI) and Cohen et al. (1992. AJ, 104, 1650; JHKL'M' from Vega and Sirius for 10 and 20um).

(b) Jy

The photon flux density as a function of the source brightness in Jy (i.e. 10^-26 W/m^2/Hz) is given by:

Jy eqn

where lambda is the wavelength in microns.

(c) W/m^2/um

The photon flux density as a function of the source brightness in W/m^2/um is given by:

Watt eqn

where lambda is the wavelength in microns.

(d) ergs/s/cm^2/um

The photon flux density as a function of the source brightness in ergs/s/cm^2/um is given by:

ergs um eqn

where lambda is the wavelength in microns.

(e) ergs/s/cm^2/Hz

The photon flux density as a function of the source brightness in ergs/s/cm^2/Hz is given by:

ergs Hz eqn

where lambda is the wavelength in microns.


Emission Line

Gaussian emission line plus continuum

This includes a Gaussian emission line with the central wavelength, integrated flux (in W/cm^2 or ergs/s/cm^2), and linewidth provided by the user added on top of a flat continuum.