1. Spectroscopic Observations (Massey & Hanson 2011, arXiv 1010. 5270v2
Spectroscopic Observations (Massey & Hanson 2011, arXiv v2.pdf)
Examples of Spectrographs
Spectroscopy with CCDs
Data Reduction and Calibration
Software Resources

2. Spectrograph Examples: LHIRES III on HLCO 20-inch
Classical Czerny-Turner made by Shelyak
2400, 600, 150 groves per mm gratings maximum R = 18000
two slit plates
used with SBIG CCD camera for data and WATEC IZON webcam for acquisition/guiding
For details see Jenkins 2011 M.S. thesis:

4. Telescope focus
Micrometer to set the grating angle
Guider
CCD

6. MTT Spectrograph at HLCO: Ebert-Fastie design
Fiber fed slit composed of 9 vertical fibers
Single mirror for collimator/camera
Four gratings: 2160 (Lowell), 1200, 600, 50 for R= 40000, 19000, 9000, and 500 (m=1)
Variety of order sorting filters
Ph.D. dissertations of Don Barry (1996) and Reed Riddle (2000)

7. Barry et al. 2002, PASP, 114, 198

8. Prism spectrogaphs dβ/dλ proportional to 1/λ3
Objective prism of trailed stars, record low resolution
grism = prism with grating on one face
grens = lens plus transmission grating

9. Fourier Spectroscopy
Michelson interferometer (interference from path length difference)
recorded intensity is FT of spectrum
useful in IR; no slit so very efficient

10. Fabry Perot Interferometer
produces interference rings (one per order)
result is convolution of image and rings
very spacing of etalons or refractive index of gas between lenses to scan image in wavelength (ex. velocity mapping of nebula in a single emission line)

11. Multi-slit spectrographs
multiple spectra in a single exposure
OK if FOV is small, number of targets is large
build metallic mask for focal plane with small “slitlets” at target locations and circular apertures at positions of guide stars
central wavelengths shifted by spatial position
ex. GMOS/Gemini, IMACS/Magellan

12. Multi-object fiber-fed spectrographs
again multiple spectra in a single exposure
OK with larger FOV, smaller number of targets
micropositioner movers fiber input positions to positions of targets in focal plane (or used with a plug-board grid)
output end of fibers stacked in a slit at spectrograph entrance
ex. Hectospec/MMT, Hydra/WIYN

13. Integral Field Spectroscopy
Obtain spectra at each spatial position in a grid over a small portion of the sky (few arcsec)
microlens array
fiber bundle
combination of above (FLAMES/VLT, GMOS/Gemini)
image slicers: sky is sampled by a stack of long, thin mirrors, each of which is rotated with respect to its neighbors (NIFS/Gemini)

14. Spectroscopy with CCD Detectors
format, pixel size, spectral DQE
read noise, full well potential
data frames (“object” with overscan)
bias frames (zero exposure)
bad pixel mask
dark frames
flat fields (featureless and twilight)
wavelength comparison spectra
spectrophotometric flux standard stars

15. Reduction and Calibration Steps
fit and subtract overscan region
trim
dark subtract
interpolate over bad pixels
construct a master bias frame
construct a normalized featureless flat field; sometimes flat division done after spectrum extraction in echelle and multi-object spectra
construct illumination correction (long slit)

16. Reduction and Calibration Steps
identification of stellar and sky locations across the image
spectrum extraction by summation over spatial direction, or better use weighted or “optimal” extraction = spatial profile fit (helps with cosmic ray removal)
extract wavelength comparison spectra
apply wavelength calibration (heliocentric or barycentric correction)
apply flux calibration (or rectify spectra)

17. Software Packages
IRAF = Image Reduction and Analysis Facility
IDL = Interactive Data Language IDL Astronomy Users Library
Python – PySpecKit
General tips at AstroBetter
Astrophysics Source Code Library