The document describes photometric calibrations performed for the Dark Energy Survey (DES). It discusses: 1) The DES, which will use the Blanco telescope to image 5000 square degrees and use galaxy clusters, weak lensing, and other methods to study dark energy. 2) Calibration procedures including applying zeropoints and extinction corrections on a nightly basis and tweaking results between fields. 3) The author's work developing a program to calibrate standard star field magnitudes in DES filters using instrumental magnitudes, color terms, and iterative statistical analysis. Comparisons showed errors within 0.025 magnitudes.

1. Photometric Calibrations of Star Fields for the Dark Energy
Survey
S.Wyatt1
, J. A. Smith1,2
, D. Tucker2
,
1
Department of Physics and Astronomy, Austin Peay State University, Clarksville TN 37044
2
Department of Experimental Astrophysics, Fermilab, Batavia IL
Abstract
”The Dark Energy Survey (DES) is a 5000 deg2
grizY imaging survey to be conducted using the new 3 deg2
(2. 2-diameter) wide-field mosaic camera (DECam) on the CTIO Blanco 4-m telescope. The
primary scientific goal of the DES is to constrain dark energy cosmological parameters via four complementary methods: galaxy cluster counting, weak lensing, galaxy angular correlations, and Type Ia
supernovae, supported by precision photometric redshifts. We present background information on DES, (the method for the program that performs) and aspects of photometric calibrations of star fields to
be used in the DES nightly calibrations, and the results received from the script.”
The Dark Energy Survey (DES)
• The DES is an international effort which will use the Blanco 4-meter tele-
scope at CTIO [3,4].
• Will catalog a 5,000 degrees2 region of the sky [4].
• Records the data with the DECam
– 570 Megapixel research CCD camera with 74 individual CCDs [4].
– 5 individual transmitting filters: grizY [3]
• The DES will use 4 methods to probe the parameters of Dark Energy [6]:
– Type Ia Supernovae
– Baryon Acoustic Oscillations
– Counting Galaxy Clusters
– Weak Gravitational Lensing
Photometric Calibrations
• Intermediate
– Applying the photometric zeropoints and extinction terms measured in
Nightly Absolute Calibration to all the observations for a given night [5].
• Global Relative
– Tweaks the Intermediate step in two ways
∗ Scattered light effects (Star Flat analysis)
∗ The calculation of the relative field-to-field photometric zeropoint offsets
[5].
• Global Absolute
– Calculates the final overall zeropoints needed, one per filter [5].
• Final
– Applies the final overall zeropoint adjustment to the science field observa-
tions [5].
Star Fields
Figure 1: An illustration of all the star fields highlighted with respect
to the DES footprint. The * designates a non-photometric night
• Throughout nightly observations, standard star fields are shot at different
airmasses.
• This is done to obtain the correct atmospheric extinction coefficients, and
zeropoints for the stars.
My Work in Calibrations
• My objective was to write a program to develop a set of standard magnitudes
for the above star fields.
• The program is written in python, and incorporates AWK and STILTS [7]
commands.
Abbreviated Algorithm
• Inputs an SQL query and separates it based on the DES grizY filters.
• Matches the observations to account for multiple observations of the same
star (based on RA and DEC)
• Applies the following calculation for Instrumental magnitudes:
instru mag = mag psf + 2.5 ∗ log10(exptime) − zeropoint (1)
• Next a function is called on that statistically calibrates each star since there
are multiple matched stars.
• The function outputs the following into a STATS file:
– Mean RA and DEC
– Median Calibrated Instrumental Magnitude
– Mean Calibrated Instrumental Magnitude
– Clipped Mean Calibrated Instrumental Magnitude
– Clipped Mean Error for the Magnitudes
– The number of observations that weren’t clipped
– Clipped Sigma for the Magnitudes
– Residuals for the Magnitudes
• The STATS files are all combined together to form a file for All of the filters
(grizY)
• *Next The outputted All filter file is combined to the original separated files
and matched based on RA and DEC, this is done so that it can use the data
from the original files to perform the following equation for a photometric
solution
photometric solution = instr mag − a − b ∗ (color term) − k ∗ X (2)
• *Then the same function is called again with these new magnitudes, and
they are statistically calibrated.
• Another All filter file is produced, and the * steps are iteratively looped so
that a more precise solution is produced.
Results
• After an iteration of 3, the script outputted an All Filters file that contained
a precise photometric solution for the magnitudes.
• In order to determine whether the code was effective, the solutions for the
Stripe82 star field were compared to the currently accepted values.
Figure 2:
• The outputted magnitudes
of Stripe82 stars in the z-
band minus those of the
Standard star catalog. The
y axis is the standard star
catalog color index i-z.
• Few outliers at the 0.15
magnitude error, but pre-
dominately at the ¡ 0.025
magnitude error.
• I also graphed the internal clipped mean error as a function of RA and DEC.
Clipped Mean Error = Clipped Standard Deviation/ N not clipped (3)
Figure 3: A graph of Stripe82 star field
with the clipped mean error as a function
of color
• I also outputted Color-Color graphs, which can indicate the stellar evolution
of stars in the star field
Figure 4: Color-Color graph of g-r vs r-i
for the Stripe82 star field with the clipped
mean error as a function of color
Conclusions
• The objective of my research was to develop a method to photometrically
calibrate the standard star fields.
• Under the supervision of Douglas Tucker I was able to calibrate a total of
seven star fields with my program.
• The statistical and compared error correlates to ≈ 0.025 magnitude of error
for the star fields.
• The program is designed to be easily adapted to calibrate more star field
queries in the future.
Acknowledgements
• This work was supported by the U.S. Department of Energys Visiting Faculty
Program, run by the Department of Energys Office of Science, in support
of the Fermilab Center for Particle Astrophysics. I would like to thank my
mentor Dr. Douglas L. Tucker for his guidance and assistance. I would also
like to thank Dr. J. Allyn Smith for his assistance.
References
1. Adam Reiss, et al., Observational Evidence From Supernovae for an Accelerating Universe and a Cosmological
Constant, The Astronomical Journal, vol.116, issue 3, pp.1009-1038, May 1998
2. Jeff Filippini, UC Berkeley Cosmology Group, The Standard Cosmology”, August 2005
3. DarkEnergySurvey.org, The Dark Energy Survey - Survey, http://www.darkenergysurvey.org/science/index.shtml
4. Brenna Flaugher, et al., Status of the Dark Energy Survey Camera (DECam) Project, SPIE conference, in
press, March 1, 2012
5. D. Tucker, et al., The Photometric Calibration of the Dark Energy Survey, ASP Conference Series, Vol. 364,
pp. 187-199, 2007.
6. DarkEnergySurvey.org, The Dark Energy Survey - Survey, http://www.darkenergysurvey.org/science/index.shtml
7. STILTS, http://www.star.bris.ac.uk/ mbt/stilts/