Rest frequency
Definition: The transition frequency in the emitter's rest frame used as the Doppler reference.
Why it matters here: It is needed to answer At fixed channel width in Hz, why does velocity resolution change with observing frequency? Which band gives finer km/s channels? and to interpret A narrow line can be diluted by channels wider than its intrinsic width.
Relationships: Use it with Bandpass ripple and Radio velocity convention; the lecture model is \(v_{\rm rad}\simeq c\frac{\nu_0-\nu}{\nu_0},\qquad\delta v\simeq c\frac{\delta\nu}{\nu_0}\).
Boundary or common confusion: The linear radio Doppler convention is appropriate for Galactic velocities but not precision high-redshift work; frame and relativistic conventions must be recorded.
Radio velocity convention
Definition: The linear velocity coordinate defined from fractional frequency offset relative to rest frequency.
Why it matters here: It is needed to answer At fixed channel width in Hz, why does velocity resolution change with observing frequency? Which band gives finer km/s channels? and to interpret A narrow line can be diluted by channels wider than its intrinsic width.
Relationships: Use it with Rest frequency and Spectral resolution; the lecture model is \(v_{\rm rad}\simeq c\frac{\nu_0-\nu}{\nu_0},\qquad\delta v\simeq c\frac{\delta\nu}{\nu_0}\).
Boundary or common confusion: The linear radio Doppler convention is appropriate for Galactic velocities but not precision high-redshift work; frame and relativistic conventions must be recorded.
Spectral resolution
Definition: The smallest frequency or velocity separation that the instrumental response can distinguish.
Why it matters here: It is needed to answer At fixed channel width in Hz, why does velocity resolution change with observing frequency? Which band gives finer km/s channels? and to interpret A narrow line can be diluted by channels wider than its intrinsic width.
Relationships: Use it with Radio velocity convention and Velocity frame; the lecture model is \(v_{\rm rad}\simeq c\frac{\nu_0-\nu}{\nu_0},\qquad\delta v\simeq c\frac{\delta\nu}{\nu_0}\).
Boundary or common confusion: The linear radio Doppler convention is appropriate for Galactic velocities but not precision high-redshift work; frame and relativistic conventions must be recorded.
Velocity frame
Definition: The reference motion, such as topocentric, barycentric, or LSR, to which a Doppler coordinate is referred.
Why it matters here: It is needed to answer At fixed channel width in Hz, why does velocity resolution change with observing frequency? Which band gives finer km/s channels? and to interpret A narrow line can be diluted by channels wider than its intrinsic width.
Relationships: Use it with Spectral resolution and Bandpass ripple; the lecture model is \(v_{\rm rad}\simeq c\frac{\nu_0-\nu}{\nu_0},\qquad\delta v\simeq c\frac{\delta\nu}{\nu_0}\).
Boundary or common confusion: The linear radio Doppler convention is appropriate for Galactic velocities but not precision high-redshift work; frame and relativistic conventions must be recorded.
Bandpass ripple
Definition: Frequency-dependent calibration structure that can imitate broad weak spectral features.
Why it matters here: It is needed to answer At fixed channel width in Hz, why does velocity resolution change with observing frequency? Which band gives finer km/s channels? and to interpret A narrow line can be diluted by channels wider than its intrinsic width.
Relationships: Use it with Velocity frame and Rest frequency; the lecture model is \(v_{\rm rad}\simeq c\frac{\nu_0-\nu}{\nu_0},\qquad\delta v\simeq c\frac{\delta\nu}{\nu_0}\).
Boundary or common confusion: The linear radio Doppler convention is appropriate for Galactic velocities but not precision high-redshift work; frame and relativistic conventions must be recorded.