G-band magnitude

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The G-Magnitude (also G-Band-Magnitude or Gaia-Band-Magitude , abbreviation G ) is an apparent brightness of celestial objects as measured by the Gaia space probe .

There are restrictions when measuring the G magnitudes: The sensors are too sensitive to measure stars of magnitude 3 or brighter. In this case the sensors are saturated and cannot output correct values. The sensors begin to saturate below G = 6, which is why values ​​below this threshold are calculated using appropriate correction formulas and are less accurate. The presetting of the probe in nominal operation means that weak objects are only detected up to a G magnitude of approximately 21.

Gaia can not use the pole sequence for calibration , as most of these stars are too bright for Gaia and the stars are not evenly distributed, so the Gaia Spectrophotometric Standard Star Catalog (SPSS) was created with around two hundred stars of different magnitudes and spectra classes for calibration .

The Gaia DR1 , Gaia DR2 and Gaia EDR3 publications each use a different photometric system . The spectral sensitivities of the detectors as well as the transmissivities of the filters and other optical components determined pass bands are each recalibrated. The calibration of the measured values ​​is part of the research and it is to be expected that the following catalogs use modified sensitivity curves.

Unfiltered magnitude G

The G-Magnitude records all radiation in the reflectivity range of the mirrors and in the sensitivity range of the CCD sensors . The sensors can detect electromagnetic radiation with a wavelength of approx. 330 to 1000 nm. This unfiltered “white” frequency band thus includes not only the light visible to the human eye in the range from approx. 400 to 800 nm, but also near ultraviolet radiation (UV-A) in the range from 400 to 330 nm, as well as infrared radiation in the near infrared (IR-A ) from approx. 800 to 1000 nm wavelength. The radiation from these bands is partially absorbed by the atmosphere , so the results of these measurements cannot be directly compared with measurements made by earth-based instruments.

Magnitudes of the two photometers G BP and G RP

In addition to the unfiltered G-magnitude, there are two other magnitudes that are measured by the sensors of the two photometers . The two photometers have upstream prisms and filters or surface coatings that form a band pass so that the blue photometer (BP) records waves between 330 and 680 nm and the red photometer (RP) waves between 640 and 1000 nm. The corresponding brightnesses are G BP and G RP . If possible, all objects are measured in these three frequency bands to characterize the color temperature and the spectral class. If there are usable measurements for all three frequency bands, one speaks of a three-band photometry. According to the definition, G is equal to the sum of the amount of light from the two photometers. In practice there are differences between the real measured values ​​and the mathematical calculation of the value. The difference between G BP and G RP on the one hand and G on the other hand is expressed using the flux excess factor.

Magnitude of the radial velocity spectrometer G RVS

Finally, there is a narrowly defined band of 847–874 nm, which is examined by the radial velocity spectrometer (RVS), but this is only possible if the object is bright enough and has a G magnitude of <17. This range is used to measure Doppler shifts and to calculate the radial velocity from this . The instrument also gives information about metallicity and spectral class and also its own brightness value G RVS .

Measured value abbreviation Frequency range (nm)
G-band magnitude G 350-1000
G-band blue photometer (BP) G BP 330-680
G-band red photometer (RP) G RP 640-1000
G-band radial velocity spectrometer (RVS) G RVS 847-874

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