Wavelength (nm)

Figure 8.2. Transmission curves for the standard U,B, and V filters used for the measurement of flux densities of celestial objects. The taking of such measurements is called photometry. The ordinate is efficiency of transmission (1.0 is perfect transmission) and the abscissa the incident wavelength.

band is defined by a standard astronomical filter that can be placed in the beam of starlight near the focus of a telescope. Thus, when the flux is measured, only the desired portion of the spectrum reaches the detector (often a photomultiplier tube or CCD detector). The response functions (Fig. 2) do not have sharp frequency cutoffs because they are the measured responses of real, not ideal, filters and detectors.

Define the transmission efficiency of the blue filter combined with the detector efficiency as a function of frequency to be eB. The blue-band flux density is thus,

The ratio of two such quantities, for two different stars, gives, through (12), the difference in their blue magnitudes, mB 2 — mB, 1. Observers can thus obtain accurate blue magnitudes of an uncataloged star or object by comparing it to a star with a well calibrated star blue magnitude.

The magnitudes in the several frequency bands may be denoted in two ways: for the ultraviolet band, both mu and U are used (Table 1). The bolometric magnitude mbol includes the flux over all relevant frequencies in the IR, optical, and UV regions (see below). Values of these apparent magnitudes are given for the sun in Table 1.

Spectral bands to lower frequencies than the V band are now in wide use (Table 2), thanks to detectors and films with extended responses, e.g., R (red), and I (infrared). These bands are also normalized so zero magnitude approximates the fluxes from a star of type A0V adjusted to the distance of Vega. Vega (a Lyrae) was traditionally used as this reference, but it is not easily accessible to southern hemisphere observers and it has dust clouds which give anomalous IR fluxes. The effective (average)

B -band photon flux)

Table 8.1. Solar apparent magnitudes

mU or U

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook

Post a comment