Multiobject Spectroscopy:Preparing and performing

Michael BaloghUniversity of Durham

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

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

Basic Principles: What is MOS?

Basic principles

Basic principles

Basic principles

Preimaging

PreimagingNeed an image from which to design mask. Does not usually have to be from same telescope.

1. photometric calibration

2. astrometric calibration

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!

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.

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)

1. First guess at plate solution. Overplot bright stars from USNO catalogue

2. Remove “bad” objects

3. Recentre and recompute mapping. Iterate until achieve ~0.1” accuracy

Astrom output

Astrom output

Mask Design

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

Dispersion direction

Dispersion direction

Differential Refraction

20” Lewis et al. 2002

67°

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

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.

Preparation

Choose galaxy priorities

Pick alignment stars so as to cause minimum disturbance to targets

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Dispersion direction

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

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

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

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?

Mask designNext: convert galaxy coordinates to x,y mask positions.

Observer should not have to worry about this!

LDSS2 example

LDSS2 masks

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.

At the telescope

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

Calibrations

1. Arcs

2. Flats

3. Flux standards

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

HeNeAr arc lamp (LDSSS2)

HeNeAr arc lamp (LDSS2)

Open filter Blue filter

Red Blue

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)

Dome Flats

Irregularslit

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.

Close up and go to bed!