ASTRA Spectrophotometer:
Reduction and Flux Calibrations
Barry Smalley
Astrophysics Group
Keele University
Staffordshire
United Kingdom
[email protected]
http://www.astro.keele.ac.uk/~bs/
Saul Adelman (The Citadel),
Austin Gulliver (Brandon University),
Barry Smalley (Keele University),
John Pazder (Dominion Astrophysical Observatory), The Future of Photometric, Spectrophotometric
Frank Younger (Aurora Astronomical Services),
and Polarimetric Standardization,
Louis
Boyd & Don Epand
(Fairborn
Observatory)
B. Smalley
The Future
of Photometric,
Spectrophotometric
and Polarimetric Standardization,
Belgium
8-11 May 2006
Blankenberge,
BelgiumBlankenberge,
8-11 May
2006
1
Outline
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From 2-d CCD images to flux-calibrated spectra.
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Spectral Extraction
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Atmospheric Extinction Determination
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Flux Calibration
ASTRA will produce a vast quantity of high quality
data
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procedures automated as far as possible
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sentinels to watch for unusual events
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
2
2-d Images
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Two orders on CCD image
Optimal reductions using CCDSPEC developed by
Austin Gulliver & Graham Hill
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
3
Spectral Extraction
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Standard procedures common to normal spectra
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Correction for CCD properties: dead rows, hot pixels.
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Addition of many Bias and Flat Field frames
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Order tracing and PSF fitting using optimal extraction
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Scattered light removal
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Removal of cosmic rays
Issues to investigate
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Characterize fringing in red
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Shutter timing corrections
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Flexure of spectrograph
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
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Issues for Spectrophotometry
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Spectrophotometry is precise relative photometry
at many wavelengths.
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Extraction must use same relative amount of flux from
all spectra
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extraction of identical fraction of spectral enclosed energy
Spectral width depends on seeing and trailing
Important to preserve the observed count level and
not bin onto a linear wavelength scale.
Output instrumental wavelengths and counts/s per
wavelength bin.
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
5
Simulated Extracted Spectra
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
6
Earth's Atmosphere
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Rayleigh Scattering
ARAY ∝−4
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Aerosol Scattering
AAERO ∝−
 is variable0.5~1.5
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Molecular Absorption
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Ozone bands
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Huggins & Chappuis
Water and Oxygen (Telluric) lines
Night Sky Emission lines
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
7
Atmospheric Extinction
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
8
Extinction Law
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Extinction given by Bouguer's Law:
m=m 0k  X
where
m=observed stellar magnitude
m 0=stellar magnitude above Earth Atmosphere
k =extinction coefficient
X =airmass
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Extinction coefficient is sum of the various
individual contributions
–
B. Smalley
Rayleigh, Aerosol, Ozone, Oxygen, Water
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
9
Airmass
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Airmass determined using simple formula of Young
& Irvine (1967)
X z =sec z [ 1−0.0012sec 2 z−1 ]
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good to better than 1% up to X ≈ 4
Airmass can change significantly during a long
exposure
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can use effective airmass (Stetson 1989)
1
X eff = [ X START 4 X MIDDLE X END ]
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B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
10
Seeing
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In slitless spectroscopy resolution depends on
seeing:

2
2
 ∝  seeing
inst


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seeing is variable:
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temporal t 
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airmass ~X 0.6
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wavelength ~−0.2
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B. Smalley
fixed but resolution different to instrumental
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
11
Spectral Resolution Issues
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Important where
spectrum is varying
most rapidly
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e.g. Balmer lines
λ
4867
4894
4940
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B. Smalley
k(fit)
0.157
0.178
0.174
k(true)
0.184
0.180
0.176
Important on nights of
poor seeing
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
12
Scintillation
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Normal minimum exposure time to 10 seconds to
minimize scintillation noise.
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Dravins et al. (1998) formula:
=0.09 D−2/3 X 1.75 exp
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For ASTRA 10-second exposure gives
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B. Smalley
 
h
2T −1/ 2
H
σ = 0.001 mag. at zenith
σ = 0.007 mag. for X = 3.
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
13
Extinction Determination
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Extinction determination without the use of
assumed stellar true fluxes
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extinction stars are tested to be constant
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Initially will perform single night reductions
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variability << 0.01 mag.
Assess reliability.
investigate multi-night reductions.
The brightest stars will be observed through
neutral density filter
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B. Smalley
determine and monitor throughput
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
14
Extinction variability
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Initially assume perfect night
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time-independent multi-star Bouguer Law fitting
(Sterken & Manfroid 1992)
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assessment of residuals
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looking for correlations with time and/or airmass
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χ² and R-statistics (Baptista & Steiner 1993)
If variable, use kλ(t) as polynomial, adding terms
and use F-test for significance of improvement.
Can also split night into smaller blocks
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B. Smalley
avoid parts of night with poor conditions
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
15
Visual Plotting: Good Night
O2
B. Smalley
H2O
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
16
Visual Plotting: Variability
O3
H2O O2 H2O
H2O
B. Smalley
Grey Cloud
Grey Cloud
O3
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
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Visual Plotting: Poor Seeing
Hγ Hβ
B. Smalley
Hα
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
18
Fitting Telluric Regions
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Non-linear extinction violates Bouguer's Law
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several methods have been used in the literature
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we will use synthetic telluric spectra
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least squares fitting to obtain oxygen and water
columns and their variability
Beware of stellar lines (dis)appearing in and out of
telluric lines due to radial velocity variations
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Earth's orbital velocity (30 km/s): 0.5Å shift at 5000Å
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will assess the level of this uncertainty using model
spectra.
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
19
Telluric Spectra
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Generate synthetic water and oxygen absorption
spectra
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HITRAN modelling (Rothman L.S. et al. 2005)
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simplified 6-layer model of Earth's atmosphere
(Nicholls 1988)
Calculate with varying water and oxygen columns.
Tabulate at high resolution (0.5Å steps) for rapid
interpolation.
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
20
Flux Calibration
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Flux calibration against Vega standard
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requires neutral density filter
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throughput measured and periodically checked
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Ensure Flux consistency within Standards
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Quality Control
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stellar (micro) variability.
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long term variability of extinction stars to ensure that
there are constant.
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instrumental/telescope throughput variations.
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
21
Flux Calibration (cont)
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Re-calibration of whole data set as necessary.
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Ensure spectral resolution effects are allowed for
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For standards use only the best observations at the
highest resolution.
Non-standards will be co-added if non-varying
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variable stars will not be co-added
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B. Smalley
except if variation insignificant between observations.
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
22
Final Stellar Fluxes
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Reduction procedures will store intermediate
results
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extracted spectra as counts
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instrumental counts/second above Earth's Atmosphere
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nightly extinction models
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individual flux calibrated spectra
Results as FITS files including headers and
processing history
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B. Smalley
wavelength,bin size,instrumental counts/second,flux
relative to Vega,absolute fluxes (photons/s/nm),internal
errors,external errors, quality flags
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
23
Summary
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Stellar flux measurements with internal (star-tostar) precision better than 1%.
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down to around 10th mag.
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Faint spectrophotometric standards.
full error propagation to include both internal and
external errors.
Absolute flux scale uncertainties will dominate.
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revision of absolute fluxes required.
http://www.citadel.edu/physics/astra/
B. Smalley
The Future of Photometric, Spectrophotometric and Polarimetric Standardization, Blankenberge, Belgium 8-11 May 2006
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