J/PASP/119/994 MIPS 24um calibrators (Engelbracht+, 2007)
Absolute calibration and characterization of the multiband imaging photometer
for Spitzer.
I. The stellar calibrator sample and the 24 µm calibration.
Engelbracht C.W., Blaylock M., Su K.Y.L., Rho J., Rieke G.H., Muzerolle J.,
Padgett D.L., Hines D.C., Gordon K.D., Fadda D., Noriega-Crespo A.,
Kelly D.M., Latter W.B., Hinz J.L., Misselt K.A., Morrison J.E.,
Stansberry J.A., Shupe D.L., Stolovy S., Wheaton W.A., Young E.T.,
Neugebauer G., Wachter S., Perez-Gonzalez P.G., Frayer D.T., Marleau F.R.
<Publ. Astron. Soc. Pac., 119, 994-1018 (2007)>
=2007PASP..119..994E 2007PASP..119..994E
ADC_Keywords: Photometry, infrared ; Stars, standard
Keywords: Astronomical Instrumentation
Abstract:
We present the stellar calibrator sample and the conversion from
instrumental to physical units for the 24µm channel of the
Multiband Imaging Photometer for Spitzer (MIPS). The primary
calibrators are A stars, and the calibration factor based on those
stars is 4.54x10-2MJy/sr/(DN/s), with a nominal uncertainty of
2%. We discuss the data reduction procedures required to attain this
accuracy; without these procedures, the calibration factor obtained
using the automated pipeline at the Spitzer Science Center is
1.6%±0.6% lower. We extend this work to predict 24µm flux
densities for a sample of 238 stars that covers a larger range of flux
densities and spectral types. We present a total of 348 measurements
of 141 stars at 24µm. This sample covers a factor of ∼460 in
24µm flux density, from 8.6mJy up to 4.0Jy. We show that the
calibration is linear over that range with respect to target flux and
background level. The calibration is based on observations made using
3s exposures; a preliminary analysis shows that the calibration
factor may be 1% and 2% lower for 10 and 30s exposures, respectively.
We also demonstrate that the calibration is very stable: over the
course of the mission, repeated measurements of our routine
calibrator, HD 159330, show a rms scatter of only 0.4%. Finally, we
show that the point-spread function (PSF) is well measured and allows
us to calibrate extended sources accurately; Infrared Astronomy
Satellite (IRAS) and MIPS measurements of a sample of nearby galaxies
are identical within the uncertainties.
Description:
The data were all obtained using the MIPS small-field photometry
mode astronomical observation template (AOT). For most targets, two
cycles of photometry using 3s exposures were obtained, resulting in
14 individual images at each of two telescope nod positions (excluding
the short exposure that starts the data-taking sequence at each of
the two telescope nod positions). Starting with the raw data
downloaded from the Spitzer Science Center (SSC), these data were
processed using version 3.06 of the MIPS Data Analysis Tool (DAT;
Gordon et al., 2005PASP..117..503G 2005PASP..117..503G), which performs standard
processing of infrared detector array data (slope fitting, dark
subtraction, linearity correction, flat fielding, and mosaicking), as
well as steps specific to the array used in MIPS (droop correction and
dynamic range extension using the first difference frame) -- these
steps are described in more detail in the paper.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 51 347 24um measurements of MIPS flux calibrators
table3.dat 53 22 Data used to compute the 24um calibration factor
table5.dat 112 238 MIPS stellar flux calibrator sample
table6.dat 55 141 Combined measurements and calibration factors
table7.dat 72 21 Stars rejected as 24um calibrators
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 A13 --- Name Name
15- 20 F6.1 d Time [34/11367] Days Since Mission Start
22- 29 I8 --- AOR AOR key (1)
31- 32 I2 s DCET [3/30] Data collection event time
(in this case an individual image)
34- 42 E9.4 s-1 CR Count rate (in DN/s)
44- 51 E8.3 s-1 e_CR rms uncertainty on CR (in DN/s)
--------------------------------------------------------------------------------
Note (1): The "AOR key", or astronomical observation request key, is used by the
Spitzer Science Center to uniquely identify the observation.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Name Name
11- 15 F5.3 mag Ksmag Ks or [24] magnitude (1)
17- 25 E9.4 s-1 CR24 24um count rate (in DN/s)
27- 34 E8.3 s-1 e_CR24 rms uncertainty on 24um count rate (in DN/s)
36- 44 E9.4 MJy/sr.s CF Calibration factor (in MJy/sr/(DN/s)) (2)
46- 53 E8.3 MJy/sr.s e_CF Calibration factor (in MJy/sr/(DN/s))
--------------------------------------------------------------------------------
Note (1): Ksmag=[24] for the stars in this table (see Rieke et al.,
2008AJ....135.2245R 2008AJ....135.2245R, Cat. J/AJ/135/2245), all of which are between
types A0 and A6.
Note (2): The average calibration factor is 4.54x10-2MJy/sr/(DN/s), to which
we have assigned an uncertainty of 2% (see Sect. 3).
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table5.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 A13 --- Name Name
15- 23 A9 --- SpType MK spectral type
25- 30 F6.3 mag SKmag SUPER-Ks magnitude (1)
32- 36 F5.3 mag e_SKmag rms uncertainty on SUPER-Ks magnitude
38- 46 E9.4 Jy F12um ?=- Flux density at 12um (2)
48- 55 E8.3 Jy e_F12um ?=- rms uncertainty on F12um
57- 65 E9.4 Jy F25um ?=- Flux density at 25um (2)
67- 75 E9.4 Jy e_F25um ?=- rms uncertainty on F25um
77- 85 E9.4 Jy F24um Predicted flux density at 24um (3)
87- 94 E8.3 Jy e_F24um rms uncertainty on F14um
96-103 E8.3 MJy/sr bg24 Predicted background level at 24um (sec. 4.3)
105-112 E8.3 MJy/sr e_bg24 rms uncertainty on bg24
--------------------------------------------------------------------------------
Note (1): Super-Ks is the weighted average of J (transformed to Ks) and Ks
(see Sect. 4.1).
Note (2): Taken from the IRAS Faint Source Catalog and modified as discussed
in Sect. 4.1.3.
Note (3): These flux densities apply to the effective wavelength of the 24um
band, 23.675um.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 A13 --- Name Name
15- 17 I3 --- Nm [1/102] Number of measurements (in table2)
19- 27 E9.4 s-1 CR24 24um count rate (in DN/s) (1)
29- 36 E8.3 s-1 e_CR24 rms uncertainty on 24um count rate (in DN/s)
38- 46 E9.4 MJy/sr.s CF Calibration factor (in MJy/sr/(DN/s))
48- 55 E8.3 MJy/sr.s e_CF rms uncertainty on CF (in MJy/sr/(DN/s))
--------------------------------------------------------------------------------
Note (1): The count rates in this table can be converted to janskys by
multiplying by the product of the aperture correction, calibration factor,
and pixel area discussed in the text, or a factor of 6.92x10-6.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table7.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Name
12 A1 --- n_Name [abcd] Note (1)
14- 72 A59 --- Com Reason for rejection
--------------------------------------------------------------------------------
Note (1): Notes as follows:
a = NIR measurements indicate that this star is reddened, so the predicted
flux is likely low.
b = There is a 24um source 14" to the north of this star. SIMBAD does not
indicate that the star is part of a multiple system and the Spitzer
Planning and Observation Tool (SPOT) does not indicate any asteroids in
the field, so the source is likely a background galaxy.
c = Su et al. (2006ApJ...653..675S 2006ApJ...653..675S, Cat. J/ApJ/653/675) find that this
source has a debris disk.
d = This star is not known to be part of a multiple system and no asteroids
were expected in the field, so this star likely has an infrared excess.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
References:
Gordon et al., Paper II 2007PASP..119.1019G 2007PASP..119.1019G
Stansberry et al., Paper III 2007PASP..119.1038S 2007PASP..119.1038S
Lu et al., Paper IV 2008PASP..120..328L 2008PASP..120..328L
(End) Patricia Vannier [CDS] 24-Jul-2014