dispersion
Definition: The change in wavelength per detector distance along the spectral direction, often expressed in nm per pixel.
Why it matters here: It converts a line’s pixel location into wavelength and sets part of the available spectral sampling.
Relationships: A wavelength solution models \(\lambda(x)\), whose derivative \(d\lambda/dx\) is the local dispersion.
Boundary / common confusion: Dispersion is not the same as resolving power; closely sampled lines may still be broadened together by the instrument profile.
arc lamp
Definition: A calibration source that produces emission lines at known laboratory wavelengths.
Why it matters here: Its line pattern supplies the pixel-wavelength correspondences used to calibrate a spectrum.
Relationships: Identified arc lines are fitted by a wavelength-solution polynomial and checked through residuals.
Boundary / common confusion: A lamp exposure can drift relative to the science spectrum if instrument conditions change between exposures.
wavelength solution
Definition: A fitted function assigning a wavelength to each spectral pixel coordinate.
Why it matters here: It turns a detector image into a spectrum from which line identities and velocities can be measured.
Relationships: The model \(\lambda(x)\) is constrained by known arc lines; fit residuals become part of the velocity uncertainty.
Boundary / common confusion: A visually good fit can still be wrong if lines are misidentified or the polynomial is extrapolated beyond calibration coverage.
resolving power
Definition: A dimensionless measure \(R=\lambda/\Delta\lambda\) of the smallest wavelength separation an instrument can distinguish near \(\lambda\).
Why it matters here: It determines whether neighboring lines can be measured separately or appear as one blended feature.
Relationships: At fixed \(R\), the resolvable interval is \(\Delta\lambda=\lambda/R\), set by optics, slit, seeing, and sampling.
Boundary / common confusion: High dispersion alone does not guarantee high resolving power if the line-spread function remains broad.
line list
Definition: A table of identified transitions and their laboratory wavelengths used for calibration or interpretation.
Why it matters here: It supplies the trusted wavelength anchors needed to fit and verify a wavelength solution.
Relationships: Observed peaks are matched to list entries, and consistency across many lines tests the identification.
Boundary / common confusion: A line list must match the lamp or species and wavelength convention; forcing an incorrect match can produce a plausible but false solution.
spectral order
Definition: One of the repeated wavelength bands produced by constructive interference in a diffraction grating, especially an echelle.
Why it matters here: Order identification is necessary because different orders can overlap spatially or in wavelength.
Relationships: The grating equation \(m\lambda=d(\sin\alpha+\sin\beta)\) links order number \(m\) to wavelength and geometry.
Boundary / common confusion: An order is not a separate physical source spectrum; it is an instrument mapping that must be extracted and merged carefully.
continuum
Definition: The smoothly varying spectral flux level on which absorption or emission lines are superposed.
Why it matters here: It defines the local reference used to measure line strength, shape, and equivalent width.
Relationships: Continuum normalization divides by a fitted baseline so line departures can be compared across wavelength.
Boundary / common confusion: In crowded or molecular spectra a true line-free continuum may be absent, making placement model-dependent.