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Low – Resolution Optical Spectroscopy
Tom Matheson
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NOAO Gemini Data Reduction Workshop July 19, 2010
How to getfrom here
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NOAO Gemini Data Reduction Workshop July 19, 2010
To here:
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NOAO Gemini Data Reduction Workshop July 19, 2010
Why Spectroscopy?
•Composition/abundances•Velocity (radial, dispersion, rotation curves)•Temperature•Excitation mechanisms•Density/pressure•Intervening matter
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NOAO Gemini Data Reduction Workshop July 19, 2010
Outline
• Brief overview of the obstacles between a raw frame and the final product• Choosing the right grating in the right spectrograph on the right telescope• The kinds of calibrations you need and how to apply them
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NOAO Gemini Data Reduction Workshop July 19, 2010
Things I Won’t Cover(but are still important)
• Multi-object (see the GMOS presentations)• Nod & Shuffle (see the GMOS presentations)• Extended objects• High-resolution• Infra-red (See NIRI/NIFS presentations)• Software/IRAF parameters (See A User’s Guide to Reducing Slit spectra with IRAF, Massey, Valdes, & Barnes, 1992, available on the NOAO web site)
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NOAO Gemini Data Reduction Workshop July 19, 2010
Problem #1: The CCD
Quantum Efficiency of the Detector
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NOAO Gemini Data Reduction Workshop July 19, 2010
Problem #1: The CCD
Flatfield and Fringing
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NOAO Gemini Data Reduction Workshop July 19, 2010
Problem #2: The Sky
Night-Sky Emission Lines
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NOAO Gemini Data Reduction Workshop July 19, 2010
Problem #2: The Sky
Continuous Absorption
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NOAO Gemini Data Reduction Workshop July 19, 2010
Problem #2: The Sky
Molecular Absorption
Wallace, Hinkle, & Livingston 1993, 1998)
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NOAO Gemini Data Reduction Workshop July 19, 2010
Other Potential Problems• Finding the right grating for your project• Biases, darks, overscan (CCD/electronics effects)• Second-order light• Parallactic angle• Observing standard stars (flux and other)• Getting good wavelength calibration lamps• Extraction of the 1-D spectrum• Wavelength calibration• Flux calibration • Telluric correction
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NOAO Gemini Data Reduction Workshop July 19, 2010
Three Things to Take Away from this Presentation
1. Do no harm
Don’t compromise the dataDo the minimum necessary for removal of instrumental effects and calibration
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NOAO Gemini Data Reduction Workshop July 19, 2010
Three Things to Take Away from this Presentation
2. Look at the data
Don’t expect everything to workA misplaced bias frame or saturated flat field can lead to problems that are difficult to diagnose
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NOAO Gemini Data Reduction Workshop July 19, 2010
Three Things to Take Away from this Presentation
3. Take all the calibration frames you need and then take all the calibration frames you don’t think you need
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NOAO Gemini Data Reduction Workshop July 19, 2010
Planning the Observation: Gratings
There are two basic quantities to consider:
1: Resolving Power = R = = Nm
2: Wavelength Coverage
Resolution a function of dispersion, detectorpixel scale, slit width, and (possibly) seeingEssentially the ability to distinguish nearby features
Limited mainly by size of detector as well as optics,telescope throughput, and detector response
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NOAO Gemini Data Reduction Workshop July 19, 2010
Diffraction Grating Handbook, Palmer 2005Newport/Richardson Gratings
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NOAO Gemini Data Reduction Workshop July 19, 2010
Planning the Observation: Gratings
In practical terms, gratings are described with a few numbers:
1. The number of lines per mm (e.g., R400, B1200) Higher numbers mean better resolution
2. The blaze wavelength, essentially the wavelength with the highest efficiency, but other effects can change this, so you should seek out the efficiency curve
3. Dispersion in Å/pixel4. Resolution, measured with some slit width
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NOAO Gemini Data Reduction Workshop July 19, 2010
Grating Efficiency Curve
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NOAO Gemini Data Reduction Workshop July 19, 2010
Planning the Observation: Gratings
In practical terms, gratings are described with a few numbers:
1. The number of lines per mm (e.g., R400, B1200) Higher numbers mean better resolution
2. The blaze wavelength, essentially the wavelength with the highest efficiency, but other effects can change this, so you should seek out the efficiency curve
3. Dispersion in Å/pixel4. Resolution, measured with some slit width
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NOAO Gemini Data Reduction Workshop July 19, 2010
Second-order Light
Blue light in second order overlaps red light in first order
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NOAO Gemini Data Reduction Workshop July 19, 2010
Second-order Light
Use an order-sorting filter (generally identified with the half-throughput wavelength)
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NOAO Gemini Data Reduction Workshop July 19, 2010
Testing the CCD: BiasesUse biases and flats to determine gain and read noise (for this and a lot more detail about CCDs, see Steve Howell’s talk, or his book, Handbook of CCD Astronomy)
The is your friend. Do enough biases to get above the read noise
For most modern detectors, there isn’t much need to subtract a bias for spectroscopic frames. As long as there isn’t a pattern, any residual pedestal in the bias will be removed by sky subtraction
The real value is as a test of instrument health.
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NOAO Gemini Data Reduction Workshop July 19, 2010
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NOAO Gemini Data Reduction Workshop July 19, 2010
Testing the CCD: Darks
As with biases, dark current in modern optical detectors isn’t usually a serious problem.
It can take a lot of time to get enough darks to be well above the read noise.
Check with the instrument scientist to see if dark current is a concern.
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NOAO Gemini Data Reduction Workshop July 19, 2010
Testing the CCD: Trim & OverscanExamine the CCD, find out about saturation and non-linearity
Determine the useful region of the CCD . If parts of the CCD don’t have counts (or have too many), then that will play havoc with statistics used to scale other calibrations, so make sure you have a well-defined region of the CCD to use.
Look at the overscan in some of your biases and flats. The region defined in the headers often includes portions that aren’t good. Choose a subset of the overscan that gives you an unbiased look
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NOAO Gemini Data Reduction Workshop July 19, 2010
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NOAO Gemini Data Reduction Workshop July 19, 2010
Cross cut of flat
Choose regions with reasonable response
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NOAO Gemini Data Reduction Workshop July 19, 2010
Cross cut of flat
Choose regions with reasonable response
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NOAO Gemini Data Reduction Workshop July 19, 2010
Overscan
Choose regions with reasonable response
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NOAO Gemini Data Reduction Workshop July 19, 2010
Overscan
Choose regions with reasonable response
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NOAO Gemini Data Reduction Workshop July 19, 2010
Overscan
Choose regions with reasonable response
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NOAO Gemini Data Reduction Workshop July 19, 2010
Flat Fields
• Remove pixel-to-pixel variation
• Get enough counts, 10Xobject is a good rule
• You don’t want to imprint the color-temperature of the flat lamp onto your data, so you need to remove the overall trend
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NOAO Gemini Data Reduction Workshop July 19, 2010
Remove shape with fit, typically cubic splineUse lowest order possible to remove signature of the lamp, not the CCD
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NOAO Gemini Data Reduction Workshop July 19, 2010
Normalized flat, fringing still presentUse the same normalization for all flats in one configuration
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NOAO Gemini Data Reduction Workshop July 19, 2010
Flat Fields
Fringing is caused by the light falling on the chip interfering with itself when the chip depth is on the same scale as the light
Depends sensitively on wavelength and chip position, so do red flats at the position of your object
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NOAO Gemini Data Reduction Workshop July 19, 2010
Flat Field Screen
Internal lamps are another common option
Depending on flexure and the optics, this can also be effectivehttp://www.jca.umbc.edu/telescope/UsersGuides/TakingFlats.html
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NOAO Gemini Data Reduction Workshop July 19, 2010
Atmospheric Dispersion
http://www.kenrockwell.com/tech/2008-01-new.htm
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NOAO Gemini Data Reduction Workshop July 19, 2010
Atmospheric Dispersion
http://xkcd.com/766/
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NOAO Gemini Data Reduction Workshop July 19, 2010
Atmospheric Dispersion
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NOAO Gemini Data Reduction Workshop July 19, 2010
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NOAO Gemini Data Reduction Workshop July 19, 2010
Even in the red, dispersion losses can be significant
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NOAO Gemini Data Reduction Workshop July 19, 2010
Lower airmass can help, but still a problem in the blue
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NOAO Gemini Data Reduction Workshop July 19, 2010
Use an ADC
Effective, but still some dispersion at high airmassSlight throughput loss, possible distortions
LRIS ADC design ODI ADC under construction
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NOAO Gemini Data Reduction Workshop July 19, 2010
Use the Parallactic Angle
The parallactic angle is the position angle on the sky at your current azimuth and elevation that orients the slit perpendicular to the horizon, i.e., along the dispersion direction.
See Filippenko 1982, PASP, 94, 715
http://star-www.st-and.ac.uk/~fv/webnotes/chapter7.htm
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NOAO Gemini Data Reduction Workshop July 19, 2010
Extraction from the 2-D Frame
Define a profile, choose a background region to extract
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NOAO Gemini Data Reduction Workshop July 19, 2010
Night Sky Emission Lines
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NOAO Gemini Data Reduction Workshop July 19, 2010
Extraction from the 2-D Frame
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NOAO Gemini Data Reduction Workshop July 19, 2010
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NOAO Gemini Data Reduction Workshop July 19, 2010
Extraction from the 2-D Frame
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NOAO Gemini Data Reduction Workshop July 19, 2010
Make sure you know what you’re extractingMake sure you know what you’re subtracting
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NOAO Gemini Data Reduction Workshop July 19, 2010
Extraction from the 2-D Frame
Trace: Locating the centroid over the dispersion axisUse a low-order fit
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NOAO Gemini Data Reduction Workshop July 19, 2010
Extraction from the 2-D Frame
Standard: Sum of flux in extraction window
Optimal: Each pixel in extraction window weighted by its flux, gives actual variance estimate (Horne 1986, PASP, 98, 609) Cleans cosmic rays too
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NOAO Gemini Data Reduction Workshop July 19, 2010
Wavelength Lamps
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NOAO Gemini Data Reduction Workshop July 19, 2010
Wavelength SolutionIdentify calibration-lamp emission lines
Assign wavelengths to pixels, typically using a polynomial fit
Use as low-order a fit as possible
Telescope/instrument flexure may require lamps at the position of the object depending on the precision you need−check with the instrument scientist
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NOAO Gemini Data Reduction Workshop July 19, 2010
Wavelength Solution
You always have another set of lines with known wavelengthsEven if you use calibration lamps, use sky for zero-point check
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NOAO Gemini Data Reduction Workshop July 19, 2010
Standard Stars
The way to translate counts into flux units
Things change with position and time, so you want a standard star as closely matched to object as possible
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NOAO Gemini Data Reduction Workshop July 19, 2010
Standard Stars
Oke & Gunn (1983) HD19445, HD84937, BD+26 2606, BD+17 4708
Oke (1974)Stone (1977) Feige 34, BD+28 4211
Massey et al. (1988)Oke (1990)Massey & Gronwall (1990)Hamuy et al. (1994)Bessell (1999)
Tables of AB magnitude vs. wavelength, all tied back to Vega
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NOAO Gemini Data Reduction Workshop July 19, 2010
AB magnitude
Oke & Gunn, 1983
Should be a minussign!
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NOAO Gemini Data Reduction Workshop July 19, 2010
Standard Stars: Caveats
Flux is in coarse bins and often a few steps removed from Vega
Relative spectrophotometry is feasible, when all the calibrations are available
Absolute spectrophotometry is difficult, but you can do pretty well with extra effort
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NOAO Gemini Data Reduction Workshop July 19, 2010
Telluric Absorption
Wallace, Hinkle, & Livingston 1993, 1998)
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NOAO Gemini Data Reduction Workshop July 19, 2010
Telluric Absorption
Smooth spectrum star with lots of counts, matched in airmass and resolution
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NOAO Gemini Data Reduction Workshop July 19, 2010
Telluric Absorption
Can scale with airmass0.6 (Wade & Horne 1988), but best to match standard to airmass of object
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NOAO Gemini Data Reduction Workshop July 19, 2010
Telluric Absorption
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NOAO Gemini Data Reduction Workshop July 19, 2010
The Reduced Spectrum
Not the final product. Reduction is a step, not the goal.
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NOAO Gemini Data Reduction Workshop July 19, 2010
Final Reminder
• Do no harm• Look at your data• Make sure you have all the calibrations