STMAG system

The STMAG system of magnitudes sets the zero point for monochromatic magnitudes, specifically setting it to F_λ 3.63×10^-9 erg cm^-2 s^-1 Å^-1. The STMAG system is analogous to the AB system, but adopts a zero point for each wavelength based upon a wavelength-differential rather than a frequency-differential. Though the system is explained as monochromatic, this merely means it is calculated for a band by dividing by the EMR energy received by the wavelength-interval (bandwidth). Such a measure can only be determined from data over a non-zero bandwidth since a zero-width wavelength-interval must have zero energy. The ST magnitude is:

mST = - 5/2 log Fλ - 21.1.

• m_ST - ST magnitude at wavelength λ
• F_λ - monochromatic flux density at wavelength λ.

Magnitudes require some adopted zero point as a consequence of their being logarithmic: "zero flux" cannot be used since zero has no logarithm (or its logarithm might be referred to as "minus infinity"). A brightness must be chosen to be represented by "zero magnitude", part of the definition of the magnitude's scale.

The ST stands space telescope: some Hubble Space Telescope instrument procedures make use of the system.