multiobject spectroscopy: preparing and performing michael balogh university of durham
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Multiobject Spectroscopy:Preparing and performing
Michael BaloghUniversity of Durham
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Outline
1. Basic principles: What is MOS?
2. Pre-imaging: photometry and astrometry
3. Mask design
4. Carrying out observations at the telescope
5. Required calibration data
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References
MOS (CFHT)Yee, Carlberg & Ellingson (1996; ApJS 102, 269)
LDSS2 (Magellan)http://www.ociw.edu/magellan_lco/instruments/LDSS2/ldss2_maskgen.html
GMOS (Gemini)http://www.gemini.edu/sciops/instruments/gmos/gmosMOS.html
These lectureshttp://star-www.dur.ac.uk/~balogh/talks/OSIRIS/MOS_prep.html
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Basic Principles: What is MOS?
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Basic principles
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Basic principles
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Basic principles
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Preimaging
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PreimagingNeed an image from which to design mask. Does not usually have to be from same telescope.
1. photometric calibration
2. astrometric calibration
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PhotometryIn principle, only relative photometry is
required.
Exception may be alignment stars – need to ensure they are within required magnitude range
May need to take care that galaxy and stellar photometry are not on the same system!
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AstrometryThis is the crucial step. Need accurate relative
astrometry – take care of image distortions
Starlink astrom software has built-in geometrical corrections for Schmidt, astrographic, and AAT telescopes.
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Astrometry
Transformation from [,] to CCD coordinates:1. Appropriate operations to transform to observed
coordinates at observed epoch.2. Conventional gnomonic projection given chip
centre, to obtain tangential coordinates []3. A cubic distortion correction: scale each of by
(1+q[2+2])q>0 : pincushion distortionq<0 : barrel distortionCan be specified, or fit directly from data (requires at least
10 stars)
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1. First guess at plate solution. Overplot bright stars from USNO catalogue
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2. Remove “bad” objects
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3. Recentre and recompute mapping. Iterate until achieve ~0.1” accuracy
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Astrom output
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Astrom output
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Mask Design
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Mask design
• Choose list of galaxy targets, astrometrically calibrated. Assign weights if desired
• Choose at least 3 guide stars (preferably 4-5) - useful to overlap in dispersion direction
• Specify length in spectral direction, if using blocking filter
• Choose slit width, orientation, and minimum length
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Dispersion direction
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Dispersion direction
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Differential Refraction
20” Lewis et al. 2002
67°
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Differential Refraction50°
Lewis et al. 2002
Point sources are stretched by ~4” at zenith angles of 67 degrees
Minimize losses by putting slits at paralactic angle. Difficult unless you can guarantee the zenith angle!
E-W slits minimize the effect at high airmass
Take alignment image through same filter used for spectroscopy
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Mask design
Allocate objects to masks. Determine conflicts given by (minimum) slit length and wavelength coverage.
Ensure full wavelength coverage obtained: requires slits to be near centre of mask in one dimension
Expand slit lengths to maximum allowed.
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Preparation
Choose galaxy priorities
Pick alignment stars so as to cause minimum disturbance to targets
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Dispersion direction
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1. Strict prioritisation
Assign slits to highest priority objects first
Guarantees best targets will be observed
May not allocate the most slits possible
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Dispersion direction
Allocates 4 galaxies
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Expand slits?
Assign slits to highest priority objects first
Guarantees best targets will be observed
May not allocate the most slits possible
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Dispersion direction
Allocates 4 galaxies
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2. Monte-Carlo Approach
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Dispersion direction
Allocates 7 galaxies
fTreat weights as a probability
Allows chance to increase number of slits
May choose low priority objects in favour of high
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3. Optimize?
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Dispersion direction
Allocates 7 galaxies
Unsolved problem (Donnelly, Allen & Brodie 1992)
Need to assign a score, or cost function, which will depend on your science goals
How do you find the extremum of this function?
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Mask designNext: convert galaxy coordinates to x,y mask positions.
Observer should not have to worry about this!
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LDSS2 example
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LDSS2 masks
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Mask cutting
Laser cutting preferred to machining, as it generally gives smoother slit edges (?)
Can be done in real-time, at the telescope. LAMA at CFHT is able to cut a mask in ~1 hour so that masks can be made as images are obtained.
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At the telescope
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At the telescope
1. Target acquisition
2. Align targets through same filter used for spectroscopy to minimize refraction effects
3. Realign every ~hour to account for flexure/refraction shifts
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Calibrations
1. Arcs
2. Flats
3. Flux standards
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Arcs
Take at same position to avoid flexure distortions (though this can be corrected using night-sky lines)
Ensure good coverage of full wavelength region of interest. May require using a filter with longer exposure
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HeNeAr arc lamp (LDSSS2)
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HeNeAr arc lamp (LDSS2)
Open filter Blue filter
Red Blue
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Flats
Usually necessary for flux calibration. Useful for identifying slits and mapping distortion.
Must have dome flats, as sky flats will show features in the sky spectrum
Generally too noisy to be useful for taking out pixel-to-pixel variations
May be important if slits are not smooth (machine-cut)
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Dome Flats
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Irregularslit
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Flux Standards
Absolute flux calibration is difficult, as it requires knowing how much of galaxy’s light was included in the slit
Spectral shape can be adequately recovered by observing spectrophotometric standards through a single slit
Need to use flats to correct for slit-to-slit variations, or observe star through all slits.
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Close up and go to bed!