___Пособираю методики определения поверхностной яркости. В интернетах описано хаотично, никого сильно не интересует этот вопрос.
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"Уранометрия". Второй том. М.К., Э.Б..
Surface Brightness (SB) As a broad generalization, surface brightness is calculated by dividing a galaxy`s magnitude by its area. More precisely, once the total V magnitude and dimensions are known, the mean surface brightness is computed as follows:
V`25 = VT + ΔV + 5logD – 2.51log (D/d) – 0.26
where
V
T = total V magnitude
ΔV = 0.25 or 0.33 for cD or E type galaxies (T = -6 to 4),
ΔV = 0.13 or 0.16 for S0-, S0, S0+ type galaxies (T = -3 to -1),
ΔV = 0.11 for all ther galaxy types (T ≥ 0),
D = major axis in arc
minutes, and
d = minor axis in arc
minutes.
The value of ΔV is dependent upon the morphology of the galaxy and the magnitude isophote to which the dimensions are measured. For about 30% of the galaxies in the DSFG (deep sky field guide), our measured dimensions match those of the RC3 (Third Reference Catalogue of Bright Galazies), which indicates an isophote of 25.0 mag/arc
sec2. For those galaxies, the lover
ΔV values (0.25 and 0.13) were used in the surface brightness computation. For the remainder, it was determined that our average isophote was 24.5 for elliptical galaxies, 24.7 for S0 types, and 25.0 for spirals. This corresponds to
ΔV values of 0.33, 0.16 and 0.11, respectively.
The total magnitude in combination with the surface brightness provides a
better indication of the visibility of a galaxy than the magnitude alone. For example, UGC 1378 (Chart 2) has a surface brightness of 14.7, meaning that it will appear as if each square arc
minute of its area is as bright as a 14.7 magnitude star. (Try defocusing a 14th magnitude star until it is an arc
minute across to get an idea of how faint this is!). Therefor, this galaxy will be more difficult to observe than its total magnitude (V = 12.6) might indicate. Experience suggests that objects with surface brightnesses fainter than
14.5 will be
difficult to detect no matter what the total magnitude is. The
"average" galaxy has a surface brightness of about
13.5, while the
highest surface brightness objects have values of
12.5 or brighter.
The examples of the Andromeda Galaxy and its companions (Chart 30) serve to show that surface brightness does not depend on total magnitude. The main galaxy, M31, is a naked-eye object but has a surface brightness of
13.5, typical of mid-stage spirals. The small companion, NGC 221 = M32, is nearly five magnitudes fainter overall yet is visible in handheld binoculars because its surface brightness,
12.5, is so
high. The companion NGC 205, although about the same total magnitude as M32, is much more difficult to see at a mean surface brightness of 14.0. The more distant companions NGC 185 and NGC 147 have still lower surface brightnesses, and despite being among the brightest galaxies in the sky by total magnitude, are often elusive to those seeking them from light-polluted observing sites.
Finally, we recommend that the surface brightness values presented here be regarded as qualitative indicators of relative brightness. The RC3 cites an average error of 0.33 for its its surface brightness values, and we believe that
our average error is somewhat
greater than that. We have of not normalized the density of the DSS images to achieve uniform isophotal dimensions and many of the magnitudes used are of an unknown type. But even assuming perfect data, there can be structural variations within the same morphological type which will affect the visibility of a ticular galaxy.
___Методы
английской Википедии https://en.wikipedia.org/wiki/Surface_brightness Англ. Википедия пишет, что ПЯ обычно измеряют в
секундах, но при этом в статьях
графы "ПЯ" нет.
Русская Википедия статьи не имеет, но зато во многих статьях
есть графа "ПЯ", и соответствует
минутам.
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Р.Кл. "Визуальная астрономия дальнего космоса" The human eye.
Units of brightness. Two factors govern the eye`s detection of light. One is the total brightness of an object, and the other is its surface brightness. Surface brightness is the total amount of light divided by the area over which it spread.
A physicist might describe an object`s total brightness by units of power (that is, energy flow), such as watts or number of photons per second. However, more specialized terms have been introduced for visual perceptions. The term illuminance describes the total light output of an object in the wavelengths seen by a typical human eye. One of the earliest units of illuminance measure was a candle. Requirements ofr precision led to the unit called the candela, the total visual ight emitted in all directions by a standard candle made of a specific material and of a certain size. The lumen (lm) is another common unit of measure, and is equal to a candle divided by 4 π (=12.5664).
Surface brightness, intensity per unit area, is described by another term, luminance. Note the subtle, but important, difference between luminance and illuminance. Common units of luminance are candelas per`square meter, of lumens per square meter.
Astronomers use their brightness unit: the stellar magnitude. And instead of linear measurements of distance on a surface, they use angular measurements of distance on the sky.
The magnitude was invented because the
eye responds to light approximately
logarithmically. One magnitude corresponds to a change in brightness by a factor of 1001/5, which is about 2.51. Five magnitudes is a factor of 100 in brightness and 10 magnitudes is a factor of 100 times 100 or 10 000.
Since astronomical objects cover an area of sky, their surface brightnesses are described in magnitudes per square arc-second. The full Moon, for example, is a half degree (1800 arc-seconds) in diameter, so it covers 2.5 million square arc-seconds of sky. Dividing its brightness by its area gives it a surface brightness of 3,6 magnitudes per square arc-second.
Astronomical objects differ vastly in both total brightness and surface brightness. The full Moon and the planer Mars have nearly the same surface brightness; their total amounts of light are so different only because the Moon covers a mech larger area of sky. The Moon and the Sun, on the other hand, have nearly the same apparent size. Here it`s a difference in surface brightness that causes such dissimilar a mounts of light.
An object of a certain total brightness (such as the Moon) also illuminates the surface of the Earth with a certain number of lumens per square meter. Any astronomical object illuminates the Earth`s surface in such a manner. Examples are in Table 2.1.
The illumination an object causes on the Earth`s surface is directly relevant to astronomy. A telescope objective has a given area on which light from the object falls. The illumination per unit area times the area of the objective determines how many lumens are delivered to the eye.
Conversion between common units of surface brightness is shown in Table 2.3. The surface brightnesses of some familiar astronomical objects are shown in Table 2.3.
A catalog of deep-sky objects.
S.B. The surface brightness in magnitudes per square arc second. This value is only a rough approximation computed from the visual magnitude and the object`s size. The S.B. can be found by the equation:
S.B. = v mag + 2.5 log (2827ab)
where a and b are the object`s major and minor dimensions in arc-minutes (an elliptical shape is assumed). The constant equals π(60 arc-sec/arc-minute)2/4.
