photometric system
Definition: Photometric system denotes the physical quantity, process, or instrument component captured by this lecture-specific statement: Narrower filters carry more precise spectral/color information per photon but transmit fewer total photons for a given exposure time, directly reducing signal-to-noise (Lecture 06); filter-system design is therefore a deliberate trade-off between spectral resolution and photometric depth,...
Why it matters here: Lecture 07 established how to convert raw counts into a calibrated magnitude in some band; this lecture asks which band, and why. Every photometric measurement in astronomy is defined relative to a specific filter’s transmission curve, and comparing measurements across observatories or decades requires those filter definitions...
Relationships: It is used with Johnson-Cousins UBVRI to interpret the lecture model. The quantitative connection is the working relation \(\text{color index} = m_{\lambda_1} - m_{\lambda_2}, \qquad \lambda_{\rm eff} = \dfrac{\int \lambda\, T(\lambda)\, S(\lambda)\, d\lambda}{\int T(\lambda)\, S(\lambda)\, d\lambda}\).
Boundary or common confusion: Comparing magnitudes measured in different filter systems (e.g., a Johnson V magnitude and an SDSS g magnitude) as if they were interchangeable. Even similarly named or similarly centered bands generally differ enough in exact bandpass shape that a small but real systematic color term must be applied to convert between systems; treating them as identical...
Johnson-Cousins UBVRI
Definition: Johnson-cousins ubvri denotes the physical quantity, process, or instrument component captured by this lecture-specific statement: Published filter transmission curves for the Johnson-Cousins UBVRI system (Bessell 1990) show each band’s central wavelength and effective bandwidth precisely enough that independent observatories, using physically different glass filters manufactured to the same specification, reproduce...
Why it matters here: Lecture 07 established how to convert raw counts into a calibrated magnitude in some band; this lecture asks which band, and why. Every photometric measurement in astronomy is defined relative to a specific filter’s transmission curve, and comparing measurements across observatories or decades requires those filter definitions...
Relationships: It is used with photometric system, bandpass to interpret the lecture model. The quantitative connection is the working relation \(\text{color index} = m_{\lambda_1} - m_{\lambda_2}, \qquad \lambda_{\rm eff} = \dfrac{\int \lambda\, T(\lambda)\, S(\lambda)\, d\lambda}{\int T(\lambda)\, S(\lambda)\, d\lambda}\).
Boundary or common confusion: Comparing magnitudes measured in different filter systems (e.g., a Johnson V magnitude and an SDSS g magnitude) as if they were interchangeable. Even similarly named or similarly centered bands generally differ enough in exact bandpass shape that a small but real systematic color term must be applied to convert between systems; treating them as identical...
bandpass
Definition: Bandpass denotes the physical quantity, process, or instrument component captured by this lecture-specific statement: A photometric filter is characterized by its transmission curve T(λ), typically summarized by an effective (central) wavelength λ_eff and an effective bandwidth (FWHM) Δλ; only photons within the transmitted bandpass, weighted by T(λ), the atmosphere’s transmission, and the detector’s QE...
Why it matters here: Lecture 07 established how to convert raw counts into a calibrated magnitude in some band; this lecture asks which band, and why. Every photometric measurement in astronomy is defined relative to a specific filter’s transmission curve, and comparing measurements across observatories or decades requires those filter definitions...
Relationships: It is used with Johnson-Cousins UBVRI, effective wavelength to interpret the lecture model. The quantitative connection is the working relation \(\text{color index} = m_{\lambda_1} - m_{\lambda_2}, \qquad \lambda_{\rm eff} = \dfrac{\int \lambda\, T(\lambda)\, S(\lambda)\, d\lambda}{\int T(\lambda)\, S(\lambda)\, d\lambda}\).
Boundary or common confusion: Comparing magnitudes measured in different filter systems (e.g., a Johnson V magnitude and an SDSS g magnitude) as if they were interchangeable. Even similarly named or similarly centered bands generally differ enough in exact bandpass shape that a small but real systematic color term must be applied to convert between systems; treating them as identical...
effective wavelength
Definition: Effective wavelength denotes the physical quantity, process, or instrument component captured by this lecture-specific statement: A photometric filter is characterized by its transmission curve T(λ), typically summarized by an effective (central) wavelength λ_eff and an effective bandwidth (FWHM) Δλ; only photons within the transmitted bandpass, weighted by T(λ), the atmosphere’s transmission, and the detector’s QE...
Why it matters here: Lecture 07 established how to convert raw counts into a calibrated magnitude in some band; this lecture asks which band, and why. Every photometric measurement in astronomy is defined relative to a specific filter’s transmission curve, and comparing measurements across observatories or decades requires those filter definitions...
Relationships: It is used with bandpass, color index to interpret the lecture model. The quantitative connection is the working relation \(\text{color index} = m_{\lambda_1} - m_{\lambda_2}, \qquad \lambda_{\rm eff} = \dfrac{\int \lambda\, T(\lambda)\, S(\lambda)\, d\lambda}{\int T(\lambda)\, S(\lambda)\, d\lambda}\).
Boundary or common confusion: Comparing magnitudes measured in different filter systems (e.g., a Johnson V magnitude and an SDSS g magnitude) as if they were interchangeable. Even similarly named or similarly centered bands generally differ enough in exact bandpass shape that a small but real systematic color term must be applied to convert between systems; treating them as identical...
color index
Definition: Color index denotes the physical quantity, process, or instrument component captured by this lecture-specific statement: A color index, the magnitude difference between two bands (e.g., B-V), is a direct, if crude, proxy for a source’s spectral energy distribution shape: a hotter source is bluer (smaller, often negative, B-V) and a cooler source is redder (larger, positive B-V), because a Planck-like...
Why it matters here: Lecture 07 established how to convert raw counts into a calibrated magnitude in some band; this lecture asks which band, and why. Every photometric measurement in astronomy is defined relative to a specific filter’s transmission curve, and comparing measurements across observatories or decades requires those filter definitions...
Relationships: It is used with effective wavelength, SDSS ugriz to interpret the lecture model. The quantitative connection is the working relation \(\text{color index} = m_{\lambda_1} - m_{\lambda_2}, \qquad \lambda_{\rm eff} = \dfrac{\int \lambda\, T(\lambda)\, S(\lambda)\, d\lambda}{\int T(\lambda)\, S(\lambda)\, d\lambda}\).
Boundary or common confusion: Comparing magnitudes measured in different filter systems (e.g., a Johnson V magnitude and an SDSS g magnitude) as if they were interchangeable. Even similarly named or similarly centered bands generally differ enough in exact bandpass shape that a small but real systematic color term must be applied to convert between systems; treating them as identical...
SDSS ugriz
Definition: Sdss ugriz denotes the physical quantity, process, or instrument component captured by this lecture-specific statement: Modern wide-field surveys (SDSS, Pan-STARRS, LSST/Rubin) adopted a newer ugriz-type filter system, with different central wavelengths and non-overlapping, more rectangular bandpasses than Johnson-Cousins, a deliberate engineering choice driven by the different noise and calibration...
Why it matters here: Lecture 07 established how to convert raw counts into a calibrated magnitude in some band; this lecture asks which band, and why. Every photometric measurement in astronomy is defined relative to a specific filter’s transmission curve, and comparing measurements across observatories or decades requires those filter definitions...
Relationships: It is used with color index, filter transmission curve to interpret the lecture model. The quantitative connection is the working relation \(\text{color index} = m_{\lambda_1} - m_{\lambda_2}, \qquad \lambda_{\rm eff} = \dfrac{\int \lambda\, T(\lambda)\, S(\lambda)\, d\lambda}{\int T(\lambda)\, S(\lambda)\, d\lambda}\).
Boundary or common confusion: Comparing magnitudes measured in different filter systems (e.g., a Johnson V magnitude and an SDSS g magnitude) as if they were interchangeable. Even similarly named or similarly centered bands generally differ enough in exact bandpass shape that a small but real systematic color term must be applied to convert between systems; treating them as identical...
filter transmission curve
Definition: Filter transmission curve denotes the physical quantity, process, or instrument component captured by this lecture-specific statement: A photometric filter is characterized by its transmission curve T(λ), typically summarized by an effective (central) wavelength λ_eff and an effective bandwidth (FWHM) Δλ; only photons within the transmitted bandpass, weighted by T(λ), the atmosphere’s transmission, and the detector’s QE...
Why it matters here: Lecture 07 established how to convert raw counts into a calibrated magnitude in some band; this lecture asks which band, and why. Every photometric measurement in astronomy is defined relative to a specific filter’s transmission curve, and comparing measurements across observatories or decades requires those filter definitions...
Relationships: It is used with SDSS ugriz to interpret the lecture model. The quantitative connection is the working relation \(\text{color index} = m_{\lambda_1} - m_{\lambda_2}, \qquad \lambda_{\rm eff} = \dfrac{\int \lambda\, T(\lambda)\, S(\lambda)\, d\lambda}{\int T(\lambda)\, S(\lambda)\, d\lambda}\).
Boundary or common confusion: Comparing magnitudes measured in different filter systems (e.g., a Johnson V magnitude and an SDSS g magnitude) as if they were interchangeable. Even similarly named or similarly centered bands generally differ enough in exact bandpass shape that a small but real systematic color term must be applied to convert between systems; treating them as identical...