Add multi-wavelength spectroscopy tutorial

Adds an end-to-end spectroscopy tutorial covering:
- Loading real SDSS spectra from FITS files (FITSIO.jl)
- Physical units and Fλ↔Fν conversions (Unitful + UnitfulAstro)
- Dust extinction with CCM89, OD94, CAL00 laws (DustExtinction.jl)
- Spectral axis inspection via spectral_axis/flux_axis (Spectra.jl)
- Automatic uncertainty propagation (Measurements.jl)
- Synthetic blackbody spectra generation (Spectra.jl)
- Spectral arithmetic: sky subtraction and scaling
- Cosmological redshift and luminosity distances (Cosmology.jl)

Includes test/test_tutorial.jl — standalone verification script.
Updates docs/Project.toml with [compat] bounds for all new deps.

Closes #202

Author: aditya-pandey-dev

docs/Project.toml

@@ -1,14 +1,31 @@
 [deps]
+Conda = "8f4d0f93-b110-5947-807f-2305c1781a2d"
+Cosmology = "76746363-e552-5dba-9a5a-cef6fa9cc5ab"
 DemoCards = "311a05b2-6137-4a5a-b473-18580a3d38b5"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 DocumenterInterLinks = "d12716ef-a0f6-4df4-a9f1-a5a34e75c656"
+Downloads = "f43a241f-c20a-4ad4-852c-f6b1247861c6"
+DustExtinction = "fb44c06c-c62f-5397-83f5-69249e0a3c8e"
+FITSIO = "525bcba6-941b-5504-bd06-fd0dc1a4d2eb"
 Glob = "c27321d9-0574-5035-807b-f59d2c89b15c"
 JuliaAstroDocs = "5ba9df79-bc07-467f-bade-66a1d49082bd"
+Measurements = "eff96d63-e80a-5855-80a2-b1b0885c5ab7"
 MultiDocumenter = "87ed4bf0-c935-4a67-83c3-2a03bee4197c"
+Plots = "91a5bcdd-55d7-5caf-9e0b-520d859cae80"
 Revise = "295af30f-e4ad-537b-8983-00126c2a3abe"
+Spectra = "391af1a9-06f1-59d3-8d21-0be089654739"
+Unitful = "1986cc42-f94f-5a68-af5c-568840ba703d"
+UnitfulAstro = "6112ee07-acf9-5e0f-b108-d242c714bf9f"
 
 [sources]
 MultiDocumenter = {url = "https://github.com/JuliaComputing/MultiDocumenter.jl"}
 
 [compat]
+Cosmology = "1"
 Documenter = "1"
+DustExtinction = "0.6, 1"
+FITSIO = "0.17"
+Measurements = "2"
+Plots = "1"
+Unitful = "1"
+UnitfulAstro = "1"

docs/make.jl

@@ -86,6 +86,9 @@ makedocs(
                 "tutorials/tabular-data.md",
                 "tutorials/curve-fit.md",
             ],
+            "Spectroscopy" => [
+                "Multi-wavelength spectroscopy" => "tutorials/multi-wavelength-spectroscopy.md",
+        ],
         ],
         "Package Ecosystem" => "ecosystem.md",
         case_studies,

docs/src/tutorials/index.md

@@ -39,3 +39,7 @@ The following tutorials show how to use Julia to perform common tasks in astrono
 * [Load tabular data from a FITS file and plot acceleration of nearby stars](@ref tabular-data)
 
 * [Curve fitting: least square and Bayesian](@ref curve-fit)
+
+### Spectroscopy
+
+* [Multi-wavelength spectroscopy: loading, transforming, and analyzing spectra](@ref tutorial-multiwavelength-spectroscopy)

docs/src/tutorials/multi-wavelength-spectroscopy.md

@@ -0,0 +1,299 @@
+# [Multi-wavelength spectroscopy](@id tutorial-multiwavelength-spectroscopy)
+
+Spectroscopy is one of the most powerful tools in an astronomer's toolkit. By splitting
+light into its component wavelengths, we can learn about a source's chemical composition,
+temperature, velocity, and much more.
+
+In this tutorial we walk through a realistic workflow: downloading a real SDSS optical
+spectrum, loading it with [`FITSIO.jl`](https://juliaastro.org/FITSIO/stable/), wrapping
+it in a `Spectrum` object with physical units via
+[`UnitfulAstro.jl`](https://juliaastro.org/UnitfulAstro/stable/), applying dust corrections
+with [`DustExtinction.jl`](https://juliaastro.org/DustExtinction/stable/), propagating
+uncertainties with [`Measurements.jl`](https://juliaphysics.github.io/Measurements.jl/stable/),
+and computing cosmological distances with
+[`Cosmology.jl`](https://juliaastro.org/Cosmology/stable/).
+
+!!! note
+    This tutorial uses Julia 1.9+. `Spectra.jl` is under active development and installed
+    from GitHub. Code blocks show the intended API; some call signatures may evolve.
+
+## Packages
+```
+pkg> add FITSIO Unitful UnitfulAstro DustExtinction Measurements Cosmology Plots
+pkg> add https://github.com/JuliaAstro/Spectra.jl
+```
+
+| Package | Role |
+|---|---|
+| `FITSIO` | Read spectral data from FITS binary tables |
+| `Spectra` | Core `Spectrum` type, blackbody, reddening |
+| `DustExtinction` | Empirical extinction laws (CCM89, OD94, CAL00) |
+| `Unitful` / `UnitfulAstro` | Physical units — Å, nm, Jy, erg/s/cm²/Å |
+| `Measurements` | Automatic uncertainty propagation |
+| `Cosmology` | ΛCDM luminosity distances |
+| `Plots` | Visualisation |
+
+---
+
+## Part 1: Loading a real SDSS spectrum
+
+We work with a publicly available spectrum from SDSS DR14 — a galaxy observed on
+plate 1323, MJD 52797, fiber 12. The FITS file stores the coadded spectrum as a
+binary table with log₁₀-spaced wavelengths and calibrated flux in units of
+10⁻¹⁷ erg/s/cm²/Å.
+
+### Downloading the data
+```julia
+using Downloads
+
+sdss_url = "https://dr14.sdss.org/optical/spectrum/view/data/format=fits/spec=lite" *
+           "?plateid=1323&mjd=52797&fiberid=12"
+sdss_file = joinpath(@__DIR__, "sdss_example.fits")
+
+if !isfile(sdss_file)
+    Downloads.download(sdss_url, sdss_file)
+end
+```
+
+### Reading the FITS file
+```julia
+using FITSIO, Unitful, UnitfulAstro
+
+f        = FITS(sdss_file)
+loglam   = read(f[2], "loglam")
+flux_raw = read(f[2], "flux")
+ivar     = read(f[2], "ivar")
+
+wave = (10 .^ loglam) * u"angstrom"
+flux = (flux_raw .* 1e-17) * u"erg/s/cm^2/angstrom"
+close(f)
+
+println("Wavelength range : ", minimum(wave), " — ", maximum(wave))
+println("Number of pixels : ", length(wave))
+```
+
+### Creating a Spectrum object
+```julia
+using Spectra
+
+spec = spectrum(wave, flux)
+println(spec)
+```
+
+### Plotting the raw spectrum
+```julia
+using Plots
+
+waves  = spectral_axis(spec)
+fluxes = flux_axis(spec)
+
+plot(ustrip.(waves), ustrip.(fluxes),
+    xlabel  = "Wavelength (Å)",
+    ylabel  = "Flux  (erg s⁻¹ cm⁻² Å⁻¹)",
+    title   = "SDSS Galaxy — plate 1323, fiber 12",
+    legend  = false, lw = 0.5, color = :steelblue, size = (800, 400))
+```
+
+---
+
+## Part 2: Working with physical units
+
+### Wavelength conversions
+```julia
+ha_wave = 6563.0u"angstrom"
+
+ha_nm = uconvert(u"nm",  ha_wave)
+ha_um = uconvert(u"μm",  ha_wave)
+
+println("Hα = $ha_wave = $ha_nm = $ha_um")
+```
+
+### Converting between Fλ and Fν
+
+Radio and infrared data are usually stored as Fν (flux per unit frequency, often in
+Jansky). The conversion is Fν = Fλ × λ² / c.
+```julia
+using Unitful: c0
+
+Flambda = 2.5e-17u"erg/s/cm^2/angstrom"
+Fnu     = Flambda * ha_wave^2 / c0 |> u"erg/s/cm^2/Hz"
+Fnu_jy  = uconvert(u"Jy", Fnu)
+
+println("Fλ = $Flambda")
+println("Fν = $Fnu  =  $Fnu_jy")
+```
+
+---
+
+## Part 3: Dust extinction and dereddening
+
+Interstellar dust makes sources appear fainter and redder — it preferentially absorbs
+blue photons. `Spectra.jl` integrates with `DustExtinction.jl` for one-line corrections.
+
+### Reddening a blackbody
+```julia
+using DustExtinction
+
+wave_bb = range(3000, 10000, length = 500)
+bb      = blackbody(wave_bb, 10_000)
+
+bb_05 = redden(bb, 0.5)
+bb_15 = redden(bb, 1.5)
+bb_30 = redden(bb, 3.0)
+
+plot(spectral_axis(bb),  ustrip.(flux_axis(bb)),   label = "Av = 0",   lw = 2)
+plot!(spectral_axis(bb), ustrip.(flux_axis(bb_05)), label = "Av = 0.5", lw = 2)
+plot!(spectral_axis(bb), ustrip.(flux_axis(bb_15)), label = "Av = 1.5", lw = 2)
+plot!(spectral_axis(bb), ustrip.(flux_axis(bb_30)), label = "Av = 3.0", lw = 2)
+plot!(xlabel = "Wavelength (Å)", ylabel = "Flux",
+      title  = "Effect of dust extinction (T = 10 000 K blackbody)",
+      size   = (800, 400))
+```
+
+### Comparing extinction laws
+```julia
+bb_ccm  = redden(bb, 1.0, law = CCM89)
+bb_od94 = redden(bb, 1.0, law = OD94)
+bb_cal  = redden(bb, 1.0, law = CAL00)
+```
+
+### Dereddening the SDSS spectrum
+```julia
+spec_dered = deredden(spec, 0.1)
+
+ws = ustrip.(spectral_axis(spec))
+plot(ws, ustrip.(flux_axis(spec)),       label = "Observed",    lw = 0.5, color = :gray, alpha = 0.6)
+plot!(ws, ustrip.(flux_axis(spec_dered)), label = "Dereddened",  lw = 0.5, color = :steelblue)
+plot!(xlabel = "Wavelength (Å)", ylabel = "Flux  (erg s⁻¹ cm⁻² Å⁻¹)",
+      title  = "Dereddening the SDSS spectrum", size = (800, 400))
+```
+
+---
+
+## Part 4: Inspecting spectral axes
+
+`Spectra.jl` provides `spectral_axis` and `flux_axis` helpers to extract the wavelength
+and flux arrays from any `Spectrum` object. This is useful for passing data to external
+fitting routines.
+```julia
+waves  = spectral_axis(spec_dered)
+fluxes = flux_axis(spec_dered)
+
+println("First wavelength : ", waves[1])
+println("Last  wavelength : ", waves[end])
+
+# Zoom around Hα (6563 Å)
+ha_mask = (ustrip.(waves) .> 6400) .& (ustrip.(waves) .< 6700)
+plot(ustrip.(waves[ha_mask]), ustrip.(fluxes[ha_mask]),
+    xlabel = "Wavelength (Å)", ylabel = "Flux  (erg s⁻¹ cm⁻² Å⁻¹)",
+    title  = "SDSS spectrum near Hα (6563 Å)",
+    lw = 1.5, color = :steelblue, legend = false, size = (800, 400))
+```
+
+!!! note
+    Chebyshev continuum fitting (`continuum()`) is planned for a future `Spectra.jl`
+    release — see [JuliaAstro/Spectra.jl#41](https://github.com/JuliaAstro/Spectra.jl/issues/41).
+    For now, `SpectralFitting.jl` provides full continuum and line fitting with
+    OGIP-compatible models.
+
+---
+
+## Part 5: Spectra with uncertainties
+```julia
+using Measurements
+
+wave_m  = range(4000, 7000, length = 200) * u"angstrom"
+flux_m  = ((5e-17 .± 0.5e-17) .* ones(200)) * u"erg/s/cm^2/angstrom"
+spec_m  = spectrum(wave_m, flux_m)
+
+spec_red = redden(spec_m, 0.5)
+
+println("Original  flux[1] : ", flux_axis(spec_m)[1])
+println("Reddened  flux[1] : ", flux_axis(spec_red)[1])
+```
+
+---
+
+## Part 6: Synthetic blackbody spectra
+```julia
+wave_synth = range(1000, 20000, length = 1000)
+
+bb_3k  = blackbody(wave_synth,  3_000)
+bb_6k  = blackbody(wave_synth,  6_000)
+bb_10k = blackbody(wave_synth, 10_000)
+bb_30k = blackbody(wave_synth, 30_000)
+
+plot(spectral_axis(bb_3k),  ustrip.(flux_axis(bb_3k)),  label = "3 000 K",  lw = 2, color = :red)
+plot!(spectral_axis(bb_6k),  ustrip.(flux_axis(bb_6k)),  label = "6 000 K",  lw = 2, color = :orange)
+plot!(spectral_axis(bb_10k), ustrip.(flux_axis(bb_10k)), label = "10 000 K", lw = 2, color = :steelblue)
+plot!(spectral_axis(bb_30k), ustrip.(flux_axis(bb_30k)), label = "30 000 K", lw = 2, color = :purple)
+plot!(xlabel = "Wavelength (Å)", ylabel = "Flux",
+      title  = "Planck blackbodies at four temperatures", size = (800, 400))
+```
+
+---
+
+## Part 7: Spectral arithmetic
+```julia
+wave_a  = range(4000, 8000, length = 500)
+source  = blackbody(wave_a, 8_000)
+sky     = blackbody(wave_a,   300) * 0.1
+
+science = source - sky
+
+plot(spectral_axis(source), ustrip.(flux_axis(source)),  label = "Source", lw = 2)
+plot!(spectral_axis(sky),    ustrip.(flux_axis(sky)),     label = "Sky",    lw = 2)
+plot!(spectral_axis(science),ustrip.(flux_axis(science)), label = "Source − Sky", lw = 2, ls = :dash)
+plot!(xlabel = "Wavelength (Å)", ylabel = "Flux",
+      title  = "Spectral arithmetic: sky subtraction", size = (800, 400))
+```
+
+---
+
+## Part 8: Cosmological redshift
+```julia
+using Cosmology
+
+cosmo = cosmology()
+
+for z in [0.1, 0.5, 1.0, 2.0]
+    d = luminosity_dist(cosmo, z)
+    println("z = $z  →  dL = ", round(typeof(d), d, digits = 0))
+end
+
+wave_rest = range(3000, 8000, length = 500)
+bb_rest   = blackbody(wave_rest, 6_000)
+
+p = plot(xlabel = "Observed wavelength (Å)", ylabel = "Flux",
+         title  = "Cosmological redshift", size = (800, 400))
+
+for (z, c) in zip([0.0, 0.5, 1.0, 2.0], [:black, :blue, :green, :red])
+    wave_obs = wave_rest .* (1 + z)
+    flux_obs = flux_axis(bb_rest) ./ (1 + z)^2
+    spec_z   = spectrum(wave_obs, flux_obs)
+    lbl = z == 0 ? "z = 0 (rest frame)" : "z = $z"
+    plot!(p, spectral_axis(spec_z), ustrip.(flux_axis(spec_z)), label = lbl, lw = 2, color = c)
+end
+p
+```
+
+---
+
+## Summary
+
+1. **Loading real spectral data** from SDSS FITS files with `FITSIO.jl`
+2. **Physical units** with `Unitful.jl` + `UnitfulAstro.jl`
+3. **Dust extinction** with CCM89, OD94, CAL00 laws via `DustExtinction.jl`
+4. **Spectral axis inspection** using `spectral_axis` and `flux_axis`
+5. **Uncertainty propagation** via `Measurements.jl`
+6. **Synthetic blackbody spectra** from `Spectra.jl`
+7. **Spectral arithmetic** — sky subtraction, scaling
+8. **Cosmological redshift** and luminosity distances via `Cosmology.jl`
+
+### Where to go next
+
+| Package | What it adds |
+|---|---|
+| [`SpectralFitting.jl`](https://juliaastro.org/SpectralFitting/stable/) | X-ray spectral fitting with OGIP PHA/RMF/ARF |
+| [`Korg.jl`](https://ajwheeler.github.io/Korg.jl/stable/) | Synthetic stellar spectra from model atmospheres |
+| [`AstroImages.jl`](https://juliaastro.org/AstroImages/stable/) | IFU data-cubes — spectra at every spaxel |