SED Building

J-C. Malapert

SED Prototype

Purpose

SedUI is an interface converting magnitude, from VEGA or AB system, into flux. Its main goal is to compute the AB correction for one given filter in order to get the flux in physical units from a magnitude. Currently, this transformation is available for ESO instruments. It is based on the filter database embedded into the ESO Exposure Time Calculators.

Magnitude Systems

The apparent magnitude in the x band can be defined as

magx = -2.5 log(fx) + const

• Definition of AB System
• Definition of Vega System

Vega <--> AB conversion

Conversion

From the definition of AB magnitudes:

magAB(Obj) = -2.5 x log(∫{fν(Obj) x Rν x dν} / ∫{Rν x dν}) - 48.574 
magAB(Obj) = -2.5 x log(∫{fν(Obj) x Rν x dν} x ∫{fν(Vega) x Rν x dν} / ∫{Rν x dν} x ∫{fν(Vega) x Rν x dν} ) - 48.574
magAB(Obj) = magVega(Obj) + magAB(Vega)

and the conversion between systems is simply given by the AB magnitude of Vega:

conv = magAB(Vega) = -2.5 x log(∫{fν(Vega) x Rν x dν} / ∫{Rν x dν}) - 48.574

Data used for AB conversion

• System transmission curves
• Vega Spectrum

Numerical interpolation and integration

As above, the AB correction values are computed via the use of different filter interpolation methods on Vega spectrum as well as different numerical integration methods:

Instrument / Filter	Cubic Spline Interpolation + five-point Newton-Cotes integration	Linear Interpolation + five-point Newton-Cotes integration	Linear Interpolation + trapeze integration
FORS1 / noFilter	0.19609	0.19597	0.19618
FORS2 / noFilter	0.25614	0.25603	0.25636
ISAAC / L	2.95327	2.95328	2.95326
VIMOS / U	0.58758	0.58729	0.58759

As the results form a complete set, a linear interpolation is carried out using the trapozoidal technique in order that the conversion between AB and VEGA is computed in the most efficient manner i.e. as quickly as possible.

Flux conversion

When magnitudes are expressed in the AB system, the conversion to physical units can be obtained at all wavelengths with the single formule

fν = fAB,ν0 x 10-0.4 * magAB

with:

   fAB,ν0 = 3.718 x 10-20 erg s-1 cm-2 Hz-1 = 3718 Jy

Photometric System

Definition

From wikipedia : "A Photometric system is a set of discrete passbands (of filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided. The first known standardized photometric system is the Johnson-Morgan or UBV photometric system (1953). Nowadays there are more than 200 photometric systems.

Photometric systems are usually characterized according to the widths of their passbands:

• broadband (passbands wider than 30 nm (the most widely used is Johnson-Morgan UBV system)),
• intermediate band (passbands widths between 10 and 30 nm),
• narrow band (passbands widths less than 10 nm).

Photometric standard stars are a series of stars that have had their light output in various passbands of photometric system, measured very carefully"

Terminology and definitions associated to the filters

On the following link, terminology and definitions of filters are given.

Database

Collection provider	covers	URL	category
Asiago Database on Photometric Systems	Asiago Database on Photometric Systems	Asiago	Photometry Systems & Filters
Isaac Newton Group of Telescopes	ING instruments	ING	Photometry Systems & Filters
Anglo-Australian Observatory	AAT instruments	AAO	Photometry Systems & Filters
Advanced Camera for Surveys	HST	ACS	Photometry Systems & Filters
ESO	ESO	ESO	Filters
National Optical Astronomy Observatory	KPNAO CTIO	NOAO	Filters
Telescopio Nazionale Galileo	OIG - DOLORES	TNG	Filters
Nordic Optical telescope	ALFOSC-FIES-MOSCA-NOTCam-SOFIN-StanCam-TURPOL	NOT NOTCam	Filters
National Astronomical Observatory of Japan	SUBARU	FOCAS CIAO IRCS MOIRCS OHS Scam	Filters
Gemimi	gmos-niri-michelle-nici-miri	GMOS NIRI MICHELLE NICI MIRI	Filters
ESO HST	ESO HST	ESO/HST ETC	Filters
CFHT	CFHT	CFHT	Filters
ESO	ESO	ESO	Total Transmission Curves
VIMOS	VIMOS	VIMOS	Total Transmission Curves
HST	HST	HST	Total Transmission Curves
Astro-Wise		Astro-Wise	Standard stars
Johns Hopkins University		voservices	Filters
SDSS	SDSS	SDSS	Filters
Hyper Z		FILTER.RES	Filters

Photometric systems used in VizieR

Photometric systems used in VizieR

-- JeanMalapert - 17 Aug 2006

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