3.9 Flux Data [Flux and Other_Fluxes]

Flux information must be provided for all targets, and there can be more than one entry for a given target. STScI uses flux information to test for over-illumination of sensitive detectors. All entries are values as observed at the Earth, rather than intrinsic values.

The flux information is provided in two separate fields:

• Flux in V Magnitude with an uncertainty, which is required for targets observed by the FGS and ACS/SBC. For all other instrument configurations, it's optional.
• Other Fluxes (separated by commas), which is entered in free text.

In the "Other Fluxes" field, the spectral type and color index could be provided if you think it's important. As many additional flux values as appropriate for the requested exposures should be provided. For example, ultraviolet or emission-line fluxes should be given if the target is to be observed in the ultraviolet or through a narrow-band filter, or several magnitudes might be provided if the target is a variable star to be observed at various brightness levels. In some cases (Targets of Opportunity, variable objects, etc.) the estimated flux data may be very uncertain, but the best available estimates should nevertheless be given, along with appropriate uncertainties and comments.

It may be important to specify the flux of a background source as well as the target flux. For example, a globular cluster in M87 may be seen against the bright background of the galaxy. The keyword -BKG should be appended to a background flux specification in this case (see footnote 2 to Table 3.12). Use a comma to separate entries if more than one flux value is given.

Flux must be given as F(lambda) rather than F(nu). Recall that the conversion is:

F(lambda) = (3 x 1018 F(nu)) / lambda2,
 

where lambda is in Ångstroms and F(nu) is in erg/(cm² sec Hz). For example, if lambda = 1500Å, and F(nu) = 1.0 x 10^-26, then F(lambda) = 1.3 x 10^-14.

The flux data are to be expressed in the format shown in Table 3.12. Do not enter explicit units.

Text Proposal File

If you are using the Text Proposal File, flux items in a list must be separated by commas.

3.9.1 General Guidelines on What Flux Data to Include

The following summary provides general guidelines for what flux information must be included in five general cases. See the Instrument Handbooks for more detailed descriptions of how to make the exposure time calculations.

Point source, non-dispersive instrument

1. Target flux: V magnitude, (B-V), E(B-V), spectral type.
   Flux at specified wavelength may be substituted for V magnitude.
   If no entry for E(B-V) is given, E(B-V) = 0 will be assumed.
2. Background (optional): Broad-band surface brightness or surface brightness at specified wavelength; -BKG must be specified in the name of the flux parameter.
3. Flux in wavelength range of observation.

Extended source, non-dispersive instrument

1. Target flux: V surface brightness, (B-V), E(B-V).
   Flux at specified wavelength may be substituted for V surface brightness.
   If no entry for E(B-V) is given, E(B-V) = 0 will be assumed.
2. Background (optional): Broad-band surface brightness or surface brightness at specified wavelength; -BKG must be specified in the name of the flux parameter.
3. Surface flux at wavelength of observation and size of the region specified.

Point source, dispersive instrument

1. Target flux: V magnitude, (B-V), E(B-V), spectral type.
   Flux at specified wavelength may be substituted for V magnitude.
   If no entry for E(B-V) is given, E(B-V) = 0 will be assumed.
2. Background (optional): Surface brightness of continuum; -BKG must be specified in the name of the flux parameter.
3. Continuum flux in wavelength range of observation.
4. Line flux and line width of brightest emission line in the wavelength range of observation.

Extended source, dispersive instrument

1. Target flux: V surface brightness, (B-V), E(B-V).
   Flux at specified wavelength may be substituted for V surface brightness.
   If no entry for E(B-V) is given, E(B-V) = 0 will be assumed.
2. Background (optional): Surface brightness of continuum; -BKG must be specified in the name of the flux parameter.
3. Surface flux at wavelength of observation and size of the region specified.
4. Line surface flux and line width of brightest emission line in the wavelength range of observation.

Infrared source

1. Target flux: J magnitude, (J-K). Flux at specified wavelength may be substituted for J magnitude.
2. Background (optional): Broad-band surface brightness at specified wavelength; -BKG must be specified in the name of the flux parameter.
3. Note that this refers to the astronomical background and not the thermal background.
4. Flux in wavelength range of observation. Note that this must be in units of erg/(cm² sec Å). The NICMOS units conversion tool on the STScI WWW pages can help you convert your source flux from J magnitude or flux in Janskys into this flux unit.

Note: Details of how the above flux information was derived must be given in the proposal text section in the Observing_Description (see the Call for Proposals). If any of the required flux data cannot be provided or are deemed to be unnecessary, these points must also be explained in that section. Incomplete flux information may delay the implementation of your proposal.

Table 3.12: Formats for Specification of Target Flux Data 

Parameter	Format example	Units
Examples for Stars:
Broad-band magnitude1	V=13.1 +/- 0.5	magnitude
Spectral type	TYPE=G5III
Color Index 1	B-V = 0.86 +/- 0.2	magnitude
Color Excess	E(B-V) = 0.3 +/- 0.2	magnitude
Background Surface Brightness2	SURF-BKG(B) = 20 +/- 0.2	mag/arcsec2
Examples for Galaxies, Nebulae, and other extended sources:
Surface Brightness 1, 2	SURF(V) = 25.0 +/- 1.0	mag/arcsec2
Surface Brightness 1	SURF(B) = 24.5 +/- 0.5	mag/arcsec2
Color Excess	E(B-V) = 2.5 +/- 0.2	mag
Plus whatever other fluxes are relevant to your science program. Some other examples are listed below:
Interstellar Extinction	A(V) = 1.3 +/- 0.1	mag
Flux at a specified wavelength	F(5100) = 51 +/- 3 E-15	erg/(cm2 sec Å)
Continuum Flux3	F-CONT(3500) = 57 +/- 3 E-15	erg/(cm2 sec Å)
Line Flux 3,4,5	F-LINE(3727) = 5 +/- 1 E-14	erg/(cm2 sec Å)
Line Width6	W-LINE(3727) = 2.4 +/- 0.2	Å
Surface Brightness at specified wavelength 2	SURF(5100) = 11 +/- 2 E-15	erg/(cm2 sec Å arcsec2)
Surface Brightness at continuum wavelength 2	SURF-CONT(5000) = 52 +/- 2 E-15	erg/(cm2 sec Å arcsec2)
Surface Brightness of line emission 3, 4, 5	SURF-LINE(5007) = 52 +/- 2 E-15	erg/(cm2 sec arcsec2)
Size (FWHM of circular region)7	SIZE = 25 +/-5	arcsec

1The following broad-band magnitudes may be used: U,B,V,R,I,J,H,K. 2You may append "-BKG" to this reference (just before the wavelength designation) to indicate that it is a background flux value (e.g., SURF-BKG(V) = 18.2 +/- 0.5; SURF-CONT-BKG(5100) = 10 +/- 3 E-15). 3Give wavelength used in keyword in rest frame, but flux in observed frame. 4Line flux should be relative to the continuum, if specified, or relative to zero if not specified. 5Whenever the S/N refers to a spectral line, W-LINE must be given along with F-LINE or SURF-LINE. Values of F-LINE and SURF-LINE outside the Earth's atmosphere are required. 6W-LINE is the full width at half maximum (FWHM). 7SIZE should be included if the exposure time estimate assumed the flux was spread over an extended region; if omitted, the highest spatial resolution of the observing mode will be assumed.