What are the future works mentioned in the paper "The palomar transient factory photometric catalog 1.0" ?

The authors also plan to include more robust variability information, source morphology and proper-motion measurements of individual sources.

What is the median repeatability of the PTF catalog?

In the magnitude range of 15 to 16, the median repeatability is about 0.01 mag and 95% of the sources have a repeatability better than about 0.03 mag.

Who is the incumbent of the Arye Dissentshik career development chair?

E. O. O. is the incumbent of the Arye Dissentshik career development chair and is grateful for support via a grant from the Israeli Ministry of Science, and the Helen Kimmel Center for Planetary Science.

How many images were selected for each PTFFIELD/ CCDID?

For each set of selected images of a given PTFFIELD/ CCDID, the authors matched the sources in all of the images against a reference image with a matching radius of 1.5″.

(Open Access) The Palomar Transient Factory photometric catalog 1.0 (2012) | Eran O. Ofek

Q: What is the basic principle of the calibration method?

Their photometric calibration method is similar to the classical method of observing standard stars through various air massesand assuming photometric conditions—i.e., the atmospheric transmission properties are constant in time and are a continuous function of air mass.

The Palomar Transient Factory photometric catalog 1.0

Author(s): E. O. Ofek, R. Laher, J. Surace, D. Levitan, B. Sesar, A. Horesh, N. Law, J. C. van

Eyken, S. R. Kulkarni, T. A. Prince, P. Nugent, M. Sullivan, O. Yaron, A. Pickles, M. Agüeros,

I. Arcavi, L. Bildsten, J. Bloom, S. B. Cenko, A. Gal-Yam, C. Grillmair, G. Helou, M. M.

Kasliwal, D. Poznanski and R. Quimby

Source:

Publications of the Astronomical Society of the Pacific,

Vol. 124, No. 918 (August

2012), pp. 854-860

Published by: The University of Chicago Press on behalf of the Astronomical Society of the Pacific

Stable URL: http://www.jstor.org/stable/10.1086/666978 .

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The Palomar Transient Factory photometric catalog 1.0

E. O. O

FEK

R. L

AHER

J. S

URACE

D. L

EVITAN

B. S

ESAR

A. H

ORESH

N. L

J. C.

VAN

YKEN

S. R. K

ULKARNI

T. A. P

RINCE

P. N

UGENT

M. S

ULLIVAN

O. Y

ARON

A. P

ICKLES

M. A

GÜEROS

I. A

RCAVI

L. B

ILDSTEN

10,11

J. B

LOOM

S. B. C

ENKO

A. G

-Y

C. G

RILLMAIR

G. H

ELOU

M. M. K

ASLIWAL

D. P

OZNANSKI

AND

R. Q

UIMBY

Received 2012 February 28; accepted 2012 June 05; published 2012 August 29

ABSTRACT. We constructed a photometrically calibrated catalog of non-variable sources from the Palomar

Transient Factory (PTF) observations. The first version of this catalog presented here, the PTF photometric catalog

1.0, contains calibrated R

PTF

-filter magnitudes for ≈2:1 × 10

sources brighter than magnitude 19, over an area of

≈11; 233 deg

. The magnitudes are provided in the PTF photometric system, and the color of a source is required in

order to convert these magnitudes into other magnitude systems. We estimate that the magnitudes in this catalog

have a typical accuracy of about 0.02 mag with respect to magnitudes from the Sloan Digital Sky Survey. The

median repeatability of our catalog’s magnitudes for stars between 15 and 16 mag, is about 0.01 mag and it is

over 0.03 mag for 95% of the sources in this magnitude range. The main goal of this catalog is to provide reference

magnitudes for photometric calibration of visible light observations. Subsequent versions of this catalog, which will

be published incrementally online, will be extended to cover a larger sky area and will also include g

PTF

-filter

magnitudes, as well as variability and proper-motion information.

Online material: color figures

1. INTRODUCTION

All-sky photometrically calibrated stellar catalogs are being

used to measure the true apparent flux of astrophysical sources.

Other approaches, such as observing standard stars (e.g.,

Landolt 1992), are time-consuming, since they require addition-

al observations, that are not of the source of interest, under pho-

tometric conditions. Therefore, it is desirable to have an all-sky

catalog that contains calibrated stellar magnitudes. To date, the

most widely used catalog for this purpose is probably the

USNO-B1.0 (Monet et al. 2003), which provides the blue,

red, and near-infrared photographic plate magnitudes for about

sources. Unfortunately, the photometric measurements in

the USNO-B1.0 catalog show significant systematic variations

in the magnitude zero point as a function of the position on the

sky (∼0:5 mag), even at small angular scales (Sesar et al. 2006).

The Sloan Digital Sky Survey (SDSS; York et al. 2000) is

calibrated to an accuracy of better than 2% (Adelman-McCarthy

et al. 2008; Padmanabhan et al. 2008). However, SDSS Data

Release 8 only covers about one-third of the celestial sphere.

Another possibility is to use bright Tycho-2 (Høg et al.

2000) stars to photometrically calibrate images (Ofek 2008;

Pickles & Depagne 2010). However, this approach requires that

Tycho stars brighter than magnitude ≈12 are not saturated in the

images.

The Palomar Transient Factory

(Law et al. 2009; Rau et al.

2009) is a synoptic survey designed to explore the transient sky

and to study stellar variability. The project utilizes the 48″

Samuel Oschin Schmidt Telescope at Palomar Observatory.

Benoziyo Center for Astrophysics, Weizmann Institute of Science, 76100

Rehovot, Israel.

Spitzer Science Center, California Institute of Technology, Pasadena, CA

91125.

Division of Physics, Mathematics and Astronomy, California Institute of

Technology, Pasadena, CA 91125.

Dunlap Institute for Astronomy and Astrophysics, University of Toronto,

Toronto, ON M5S 3H4, Canada.

NASA Exoplanet Science Institute, California Institute of Technology,

Pasadena, CA 91125.

Department of Astronomy, University of California, Berkeley, CA 94720-

3411.

Department of Physics, University of Oxford, Oxford OX1 3RH, UK.

Las Cumbres Observatory Global Telescope Network, Santa Barbara, CA

93117.

Department of Astronomy, Columbia University, New York, NY 10027.

Department of Physics, Broida Hall, University of California, Santa

Barbara, CA 93106.

Kavli Institute for Theoretical Physics, Kohn Hall, University of California,

Santa Barbara, CA 93106.

Department of Astronomy, University of California, Berkeley, CA 94720-

3411.

School of Physics and Astronomy, Tel-Aviv University, Israel.

Institute for the Physics and Mathematics of the Universe, University of

Tokyo, Kashiwa-shi, Chiba, 277-8583, Japan.

See http://www.astro.caltech.edu/ptf/.

854

UBLICATIONS OF THE

STRONOMICAL

OCIETY OF THE

ACIFIC

, 124:854–860, 2012 August

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The telescope has a digital camera equipped with 11 active

CCDs,

each 2 K×4 K pixels (Rahmer et al. 2008), and it

has been surveying the northern sky since 2009 March. Each

PTF image covers 7:26 deg

with a scale of 1:01

″

pixel

1

The median point-spread function full width at half-maximum

is ≈2

″

and is uniform over the camera field of view (Law et al.

2010). The PTF main survey is currently performed in the g

band during dark time and in the Mould R band during bright

time, but most of the data taken prior to 2011 January were ob-

tained using the R-band filter. In addition, a few nights around

times of full Moon are used for surveying the sky with narrow-

band Hα filters. An overview of the PTF survey and its first-

year performance is given in Law et al. (2010).

The PTF data are reduced by pipelines running at Californ ia

Institute of Technology’s Infrared Processing and Analysis

Center (IPAC), and the processing includes astrometric and pho-

tometric calibration. Here, we build on the PTF photometric cal-

ibration to construct a catalog of calibrated nonvariable sources.

This catalog can be used to photometrically calibrate other

visible-light observations.

This article is organized as follows: in § 2, we briefly discuss

the PTF photometric calibration; the construction of the photo-

metric catalog is described in § 3; the catalog is presented in § 4;

we discuss accuracy and repeatability in § 5; and finally, we

conclude in § 6.

2. BRIEF DESCRIPTION OF THE PTF

PHOTOMETRIC CALIBRATION

Here, we summarize the photometric calibration of PTF

images, which is fundamental to the construction of the PTF

photometric catalog. A full description can be found in Ofek

et al. (2012).

We used images reduced by the IPAC-PTF pipeline

(Grillmair et al. 2010; Laher et al. 2012, in preparation). The

processing includes splitting the multiextension FITS images,

debiasing, flat-fielding, astrometric calibration, generation of

mask images, source extraction, and photometric calibration.

The astrometric calibration is performed relative to SDSS when

possible and the UCAC-3 catalog (Zacharias et al. 2010) when

SDSS information is not available. If a UCAC-3 solution is not

found, then the astrometry is solved against USNO-B1.0

(Monet et al. 2003). The median astrometric rms in single axis

is 0.11″, 0.13″, and 0.4″ for the SDSS, UCAC-3, and USNO-

B1.0 catalogs, respectively. The masks flag pixels with image

artifacts, including ghosts, halos, aircraft/satellite tracks, satura-

tion, CCD bleeding, and dead/bad pixels. Sources that contain

masked pixels inherit the pixels’ flag and these are stored in the

catalogs associated with the processed images.

Our photome tric calibration method is similar to the classical

method of observing standard stars through various air masses

and assuming photometric conditions—i.e., the atmospheric

transmission properties are constant in time and are a continu-

ous function of air mass. On average, we typi cally observe

∼10

SDSS stars with a high signal-to-noise ratio (S/N) per

CCD per night. Therefore, we usually have a sufficient number

of photometric measurements to robustly constrain all calibra-

tion parameters for a given night.

After selecting high-S/N sources, we fit the difference be-

tween the instrumental magnitude measured in the PTF system

(e.g., R

inst

PTF

) and the standard star magnitudes measured by

SDSS (e.g., R

SDSS

) with a simple model. The free parameters

in this model include the global zero point of the image, color

term, extinction coefficient, color/air-mass term, variation of the

global zero point during the night, exposure time, and illumina-

tion correction. Here, the illumination correction represents var-

iation of the photometric zero point as a function of position on

the CCD. The latter correction is represented by two methods

relative to the center of each CCD, which are described in Ofek

et al. (2012). To generate the first version of the PTF photomet-

ric catalog, we used the model in which the variation of the pho-

tometric zero point with CCD position is a two-dimensional,

low-order polynomial in the position across each CCD (i.e.,

eq. [3] in Ofek et al. (2012). The goodness of the fit is described

by several estimators, including the rms of residuals of bright

stars from the best-fit model (parameter APBSRMS in Table 2

in Ofek et al. (2012).

We note that the magnitudes produced by our calibration pro-

cess are related to the SDSS magnitude system (and other sys-

tems) via the aforementioned color terms. In the case where the

r  i color of an object is known, it is possible to convert the

PTF R-band magnitudes to other systems. These transforma-

tions are described in equations (4)–(7) in Ofek et al. (2012,

while the color terms for the 11 active CCDs are given in Table 3

in Ofek et al. (2012. For reference, we present the PTF R-band

filter transmission in Figure 1 and in Table 1. Subsequent

TABLE 1

PTF R-

BAND

ILTER

RANSMISSION

λ (Å) Filter QE

std

Atm. Sys

std

Sys

5,680.0 ..... 0.00 0.66 0.74 0.84 0.00 0.00

5,685.0 ..... 0.00 0.66 0.74 0.84 0.00 0.00

5,690.0 ..... 0.01 0.66 0.74 0.84 0.01 0.01

5,695.0 ..... 0.01 0.66 0.75 0.84 0.01 0.01

5,700.0 ..... 0.01 0.67 0.75 0.84 0.01 0.01

OTE

.—PTF R-band filter transmission (see also Fig. 1). λ is the

wavelength, Filter is the filter transmission, QE is the CCD efficien-

cy, Atm. is the atmosphere transmission calculated using a standard

smooth atmosphere (Hayes & Latham 1975) for 1.7 km elevation and

air mass 1.3, and Sys is the total efficiency calculated by multiplying

the filter, QE and atmosphere transmissions. Subscript std stands for

standard CCD, and hr stands for high resistivity (see Fig. 1 caption).

Table 1 is shown in its entirety in the electronic edition of PASP.

A portion is shown here regarding its form and content.

The camera has 12 CCDs, one of which is not functional.

THE PALOMAR TRANSIENT FACTORY PHOTOMETRIC CATALOG 855

2012 PASP, 124:854–860

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versions of the PTF photometric catalog will include g

PTF

mag-

nitudes, which will enable straightforward transformation into

other magnitude systems.

3. CATALOG CONSTRUCTION

Version 1.0 of the catalog was constructed from PTF data

taken before 2011 November, using the IPAC-PTF pipeline

software version identifier ðSVIDÞ > 47. For this version of

the catalog, we only used PTF fields that were observed with

the Mould R band on at least three photometric nights under

good conditions. The terms “photometric nights” and “good

conditions” are not well defined and depend on the required

photometric accuracy, and we selected images that had photo-

metric and quality parameters within the ranges specified in

Table 2.

A detailed description of the photometric parameters (i.e.,

APBSRMS, α

c;R

, and α

a;R

) and their distributions is available

later in this article. Here, the MoonESB is the theoretical V -

band excess in sky surface magnitude (negative number) due

to the Moon, and this excess is calculated using the algorithm

of Krisciuna s & Schaefer (1991). APBSRMS is the root mean

square of bright stars of the nightly photometric calibration re-

siduals from the best fit, α

c;R

is the R-band r  i color-term

coefficient of the nightly photometric solution, and α

a;R

the nightly R -band extinction coefficient. The allowed ranges

of these parameters (listed in Table 2) correspond to 3 times

the one standard deviation

from the median value of each pa-

rameter over all data taken with a given CCD.

We choose PTF fields

that have at least three images taken

on three different nights and which meet the criteria listed in

Table 2. The requirement to analyze only images that were ob-

served on three or more photometric nights is important in order

to remove outliers that may be present in the data. For example,

if a night was photometric for 90% of the time (e.g., clouds en-

tered toward the end of the night), then our pipeline might claim

that the night was photometric, but the calibration of some of the

data would be poor. Therefore, in order to obtain the calibrated

source magnitudes, it is important to average the data taken over

several photometric nights. Moreover, observations taken on

multiple nights allow us to calculate variability and proper-

motion indicators.

To expedite processing in cases where we have more than

30 images of the same field, we truncated the number of images

according to the following scheme: if more than 30 images of

the same field taken on less than 30 unique nights were avail-

able, then we selected a single random image from each night. If

more than 30 images taken on more th an 30 unique nights were

available, then we selected the 30 nights with the smallest

APBSRMS parameter and chose one image from each one

of these nights. Histograms of the number of unique nights

and number of unique observations per object in the catalog

are shown in Figure 2.

For each set of selected images of a given PTFFIELD/

CCDID, we matched the sources in all of the images against

a reference image with a matching radius of 1.5″. Here, the ref-

erence image was selected as the image of the field with the

largest number of sources.

We note that in future catalog ver-

sions, we intend to use a deep co-add image for each

PTFFIELD/CCDID as a reference image.

Next, we removed all the measurements that were masked by

one of the following flags: source is deblended by SExtractor

(Bertin & Arnouts 1996), aircraft/satellite track, high dark cur-

rent, noisy/hot pixel, containing possible optical ghost, CCD-

bleed, radiation hit,

saturated pixels, dead pixel, not a number,

halo around bright star, and dirt on opti cs. These flags are de-

scribed in detail in Laher et al. (2012, in preparation). The re-

maining photometric measurements are used to calculate the

TABLE 2

ANGES OF

ARAMETERS OF

OOD

ATA

Parameter CCDID Min Max Units

Seeing .......... All … 4.0 arcsec

MoonESB ...... All −3.0 … mag arcsec

2

APBSRMS ..... All … 0.04 mag

c;R

............ 0 0.190 0.244 mag mag

1

a;R

............ 0 −0.182 −0.044 mag airmass

1

c;R

............ 1 0.175 0.235 mag mag

1

a;R

............ 1 −0.177 −0.039 mag airmass

1

c;R

............ 2 0.178 0.226 mag mag

1

a;R

............ 2 −0.187 −0.037 mag airmass

1

c;R

............ 4 0.189 0.249 mag mag

1

a;R

............ 4 −0.200 −0.038 mag airmass

1

c;R

............ 5 0.200 0.254 mag mag

1

a;R

............ 5 −0.198 −0.036 mag airmass

1

c;R

............ 6 0.201 0.249 mag mag

1

a;R

............ 6 −0.183 −0.027 mag airmass

1

c;R

............ 7 0.172 0.238 mag mag

1

a;R

............ 7 −0.177 −0.033 mag airmass

1

c;R

............ 8 0.181 0.229 mag mag

1

a;R

............ 8 −0.175 −0.049 mag airmass

1

c;R

............ 9 0.165 0.237 mag mag

1

a;R

............ 9 −0.188 −0.038 mag airmass

1

c;R

............ 10 0.176 0.260 mag mag

1

a;R

............ 10 −0.188 −0.038 mag airmass

1

c;R

............ 11 0.188 0.248 mag mag

1

a;R

............ 11 −0.189 −0.039 mag airmass

1

OTE

.—Min and Max specify the range minimum and maximum,

respectively. See text for details.

We used the sixty-eighth percentile divided by 2 as a robust estimator for

one standard deviation.

The CCD number is designated by CCDID, which ranges from 0 to 11

(CCDID ¼ 3 is inoperable).

A PTF field, denoted by PTFFIELD, is uniquely associated with a prede-

fined sky position.

Typically, this is the image with the best limiting magnitude.

The current radiation hit/CCD bleed flag in the version of the PTF IPAC

pipeline used here is not a good indicator for radiation hits.

856 OFEK ET AL.

2012 PASP, 124:854–860

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TABLE 3

PTF P

HOTOMETRIC

ATALOG

1.0

J2000

(deg)

J2000

(deg) N

obs

night

BestRMS

(mag)

PTF

(mag)

ΔR

PTF

(mag)



PTF

(mag)

PTF

(mag)

type

(mag)

Δμ

type

(mag) PTFFIELD CCDID Flag

42.532500 ....... −31.087161 23 12 0.022 16.152 0.021 0.017 0.026 0.058 0.095 100111 09 1

42.786188 ....... −31.086008 21 12 0.022 17.172 0.030 0.022 0.037 0.058 0.060 100111 09 1

42.593730 ....... −31.085419 22 12 0.022 18.507 0.063 0.055 0.071 0.106 0.106 100111 09 1

42.525416 ....... −31.081556 23 12 0.022 14.225 0.015 0.017 0.014 0.046 0.072 100111 09 1

42.953363 ....... −31.080395 23 12 0.022 15.286 0.025 0.029 0.022 0.003 0.063 100111 09 1

OTE

.—Table is sorted by declination. See text for column descriptions. Table 3 is shown in its entirety at http://irsa.ipac.caltech.edu/ and on the Vizier Online Data Catalog