Unihedron SQM-LU-DL, Руководство оператора

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2 Measurements
The SQM-LU-DL provides readings in Magnitudes per square arcsecond, abbreviated as: mpsas, and written mathemat-
ically as
mag
arcsec
2
.
Mpsas is a logarithmic measurement which means that large changes in sky brightness correspond to relatively small
numerical changes. A difference of 5 magnitudes is a factor of 100 times the intensity. Therefore a sky brightness 5 . 0
mag
arcsec
2
darker corresponds to a reduction in photon arrival rate of a factor of 100.
The following schematic gives a rough idea of how to interpret the readings of the SQM:
Figure 2.1: Mpsas interpretive scale
Magnitudes are an astronomical unit of measure for object brightness. Brighter objects have a lower magnitude and
darker objects have a higher magnitude value. For example; a star that is 6th magnitude is brighter than a star that is
11th magnitude.
The star Vega is used a the reference point of
≈
0 magnitude. Table 2.1 shows the apparent magnitude of some common
known celestial objects.
Table 2.1: Apparent Magnitudes of Known Celestial Objects adapted from [2]
App. Mag. Celestial Object
−
26.73 Sun
−
12.6 full Moon
−
4.7 Maximum brightness of Venus
+0.03 Vega, the original zero point
+6 Faintest stars observable with naked eye
+27
Faintest objects observable in visible light with 8m ground-based tele-
scopes
+30
Faintest objects observable in visible light with Hubble Space Tele-
scope
Arcsecond is the definition of an arc being divided up into seconds as follows.
1. There are 360 degrees in a circle.
2. There are 60 arcminutes in a degree, and 21600 arcminutes in a circle.
3. There are 60 arcseconds in an arcminute, and 1296000 arcseconds in a circle.
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