“Zero Point (photometry)”的意思、由来-开放百科全书

In astronomy, the Zero Point in a photometric system is defined to be the detector count when the apparent magnitude is 0. The zero point is used to calibrate a system to the standard magnitude system, as the flux detected from stars will vary from detector to detector.^[1] Traditionally, Vega is used as the calibration star for the zero point magnitude in specific pass bands (U, B, and V), although often, an average of multiple stars is used for higher accuracy.^[2] It is not often practical to find Vega in the sky to calibrate the detector, so for general purposes, any star may be used in the sky that has a known apparent magnitude.^[3]

General formula

Where M is the magnitude of an object, F is the flux at a specific wavelength, and S is the sensitivity function of a given instrument. Under ideal conditions, the sensitivity is 1 inside a pass band and 0 outside a pass band.^[2] The constant C is determined from the 0 point magnitude using the above equation, by setting the magnitude equal to 0.^[3]

Vega as calibration

Under most circumstances, Vega is used as the zero point, but in reality, an elaborate "bootstrap" system is used to calibrate a detector.^[2]^[4] The calibration typically takes place through extensive observational photometry as well as the use of theoretical atmospheric models.^[4]

Bolometric Magnitude Zero Point

While the zero point is defined to be that of Vega for passband filters, there is no defined zero point for bolometric magnitude, and traditionally, the calibrating star has been the sun.^[5] However, recently, the IAU has recently defined the absolute bolometric magnitude and apparent bolometric magnitude zero points to be 3.0128×10²⁸ W and 2.51802×10^-8 W/m² respectively.^[6]

See also

References

1. ^{{Cite web|url=https://www.eso.org/sci/facilities/lasilla/instruments/wfi/inst/zeropoints.html|title=Zeropoints|last=|first=|date=|website=European Southern Observatory|archive-url=|archive-date=|dead-url=|access-date=}}
2. ^¹²{{Cite book |title=Introduction to Modern Astrophysics |last=Carroll |first=Bradley W. |last2=Ostlie |first2=Dale A. |publisher=Cambridge University Press |year=2017 |pages=77 }}
3. ^¹{{Cite web|url=http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/instruments/phy217_inst_photcal.html|title=Calibrating Photometric Data|last=|first=|date=|website=Sheffield University|archive-url=|archive-date=|dead-url=|access-date=}}
4. ^¹{{Cite web|url=http://web.ipac.caltech.edu/staff/fmasci/home/astro_refs/magsystems.pdf|title=Magnitude and Color Systems|last=O'Connell|first=|date=|website=Caltech|archive-url=|archivedate=|dead-url=|accessdate=17 March 2019}}
5. ^{{Citation|last=Casagrande|first=Luca|title=compute bolometric corrections and synthetic colours for input values of stellar parameters: casaluca/bolometric-corrections|date=2018-10-23|url=https://github.com/casaluca/bolometric-corrections|access-date=2018-12-27}}
6. ^{{cite arxiv|last=Mamajek|first=E. E.|last2=Torres|first2=G.|last3=Prsa|first3=A.|last4=Harmanec|first4=P.|last5=Asplund|first5=M.|last6=Bennett|first6=P. D.|last7=Capitaine|first7=N.|last8=Christensen-Dalsgaard|first8=J.|last9=Depagne|first9=E.|date=2015-10-21|title=IAU 2015 Resolution B2 on Recommended Zero Points for the Absolute and Apparent Bolometric Magnitude Scales|eprint=1510.06262|class=astro-ph.SR}}