Changeset 41189 for trunk/doc/release.2015/ps1.calibration/calibration.tex
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trunk/doc/release.2015/ps1.calibration/calibration.tex
r41188 r41189 115 115 bright-star systematic error floor for individual astrometric 116 116 measurements is 16 milliarcseconds.} \textmod{The Pan-STARRS Data Release 2 117 (DR2) astrometr yis tied to the Gaia DR1 coordinate frame with a117 (DR2) astrometric system is tied to the Gaia DR1 coordinate frame with a 118 118 systematic uncertainty of $\sim 5$ milliarcseconds.} 119 119 \end{abstract} … … 132 132 Type Ia supernovae to measure the history of the expansion of the 133 133 universe. The majority of the time (56\%) was spent on surveying the 134 $\frac{3}{4}$ of the sky north of $-30 $ Declination with134 $\frac{3}{4}$ of the sky north of $-30\degree$ Declination with 135 135 \grizy\ filters in the so-called $3\pi$ Survey. Another $\sim 25\%$ 136 136 of the time was concentrated on repeated deep observations of 10 … … 571 571 derived from the projection of the reference coordinates. One caveat 572 572 is that the chip reference coordinates are also degenerate with the 573 fitted distortion. In order to avoidbeing sensitive to the exact573 fitted distortion. \textmod{To avoid} being sensitive to the exact 574 574 positions of the chips at this stage, we measure the local gradient 575 575 between the focal plane and tangent plane coordinate systems. We then … … 635 635 \begin{itemize} 636 636 \item \code{ZPT_REF} : the nominal zero point for this filter 637 \item \code{ZPT_OBS} : the measured zero point for this chip / 638 exposure 639 \item \code{ZPT_ERR} : the measured error on \code{ZPT_OBS} 637 \item \code{ZPT_OBS} : the measured zero point for this chip / exposure 638 \item \code{ZPT_ERR} : the standard deviation of \code{ZPT_OBS} 640 639 \item \code{ZPT_NREF} : the number of stars used to measure \code{ZPT_OBS} 641 640 \item \code{ZPT_MIN} : minimum reference magnitude included in analysis … … 663 662 664 663 The master DVO database is used to perform the full photometric and 665 astrometric calibration of the data. During these analysis steps, a666 wide variety of conditions are noted for individual measurements, for 667 the objects (either as a whole or for specific filters) and for the 668 images. A set of bit-valued flags are used in the database to record 669 the se conditions.664 astrometric calibration of the \textmod{PS1} data. During these 665 analysis steps, a wide variety of conditions are noted for individual 666 measurements, for the objects (either as a whole or for specific 667 filters) and for the images. A set of bit-valued flags are used in 668 the database to record these conditions. 670 669 % 671 670 Table~\ref{tab:measure_mask_values} lists the flags specific to … … 868 867 calibrations. 869 868 870 Photometric nights are selected and all other exposures are ignored. 869 In this first stage, the goal is to determine an initial 870 highly-reliable collection of zero points for exposures without any 871 confounding systematic error sources. To this end, only 872 photometric nights are selected and all other exposures are ignored. 871 873 Each night is allowed to have a single fitted zero point 872 874 (corresponding to the sum $zp_{\rm ref} + M_{cal}$ below) and a single … … 882 884 \cite{2012ApJ...756..158S} determined flat-field corrections for 883 885 $2\times 2$ sub-regions of each chip in the camera and four distinct 884 time periods (``seasons''). Later analysis (PV2) used an $8\times8$ 886 time periods (``seasons''), ranging from as short as one month to nearly 887 15 months. Later analysis (PV2) used an $8\times8$ 885 888 grid of flat-field corrections to good effect. 886 889 … … 1016 1019 corrections are applied to each of the types of measurements stored in 1017 1020 the database, PSF, Aperture, Kron. The calibration math remains the 1018 same regardless of the kind of magnitude being measured. Also note 1019 that for the moment, this discussion should only be considered as 1020 relevant to the chip measurements. Below we discuss the implications 1021 for the stack and warp measurements. 1021 same regardless of the kind of magnitude being measured \textmod{(see 1022 however Section~\ref{sec:phot.stack} for the difference in the stack 1023 calibration)}. Also note that for the moment, this discussion 1024 should only be considered as relevant to the chip measurements. Below 1025 we discuss the implications for the stack and warp measurements. 1022 1026 1023 1027 When the ubercal zero points and flat-field data are loaded, … … 1105 1109 1106 1110 Similarly for images, we exclude those with more than 2 magnitudes of 1107 extinction or for which the deviation greater of the zero points per1108 star are than 0.075 mags or 2$\times$ the median value, whichever is 1109 greater. These cuts are somewhat conservative to limit us to only 1110 good measurements. The images and stars rejected above are not used 1111 to calculate the system of zero points and mean magnitudes. These 1112 cutsare updated several times as the iterations proceed. After the1111 extinction or for which the standard deviation of the zero points are 1112 more than 0.075 mags or 2$\times$ the median value, whichever is greater. 1113 These cuts are somewhat conservative to limit us to only good 1114 measurements. The images and stars rejected above are not used to 1115 calculate the system of zero points and mean magnitudes. These cuts 1116 are updated several times as the iterations proceed. After the 1113 1117 iterations have completed, the images which have been reject are 1114 1118 calibrated based on their overlaps with other images. … … 1233 1237 they wait for the data they need to receive from their neighbors. The 1234 1238 management of this stage is performed by communication between the 1235 region host . At the end of the iterations, the regions hosts write out1239 region hosts. At the end of the iterations, the regions hosts write out 1236 1240 their final image calibrations. The master machine then loads the 1237 1241 full set of image calibrations and then applies these calibrations … … 1275 1279 data in DVO after the initial relphot calibration to measure the 1276 1280 flat-field residual with much finer resolution: 124 x 124 flat-field 1277 values for each GPC1 chip (40x40 pixels per point). We then used 1281 values for each GPC1 chip (40x40 pixels per point). \textadd{For this 1282 analysis, we did not use the entire database, but instead extracted 1283 relatively bright, but unsaturated measurements (instrumental 1284 magnitudes between -10.5 and -14.5) for stars with at least 8 1285 measurements, including 3 used to measure the average photometry in 1286 the corresponding filter. These measurements were extracted from a 1287 collection of 10 sky regions in both low and high-stellar density 1288 regions covering a total of $\sim 5800$ square degrees of sky. 1289 Unlike the lower-resolution photometric flat-fields determined in 1290 the ubercal analysis, the photometric flat-fields calculated in this 1291 analysis are static in time; they supplement the flats from the 1292 ubercal analysis. A total of 1.95 billion measurements were 1293 extracted for this analysis. 1294 1295 We then used 1278 1296 \ippprog{setphot} to apply this new flat-field correction, as well as the 1279 1297 ubercal flat-field corrections, to the data in the database. At this … … 1563 1581 the modified weight is less than 30\% of the median weight 1564 1582 ($\omega^\prime < 0.3 <\omega>$) then the point is treated as clipped. 1565 The $\chi^2$ is determined from the {\em unclipped} points using the1583 The $\chi^2$ is determined from the \textadd{remaining} {\em unclipped} points using the 1566 1584 standard Poisson errors. Data points which are so excluded are marked 1567 1585 with bit-flags: \code{ID_MEAS_MASKED_PSF}, … … 1570 1588 1571 1589 Bootstrap-resampling analysis is used to assess the errors on the fit 1572 parameters: A number of measurements equal to the number of {\em1590 parameters: A number of measurements equal to the number of remaining {\em 1573 1591 unclipped} data points are randomly selected from the set of 1574 1592 unclipped data points, with replacement after each selection. These … … 1874 1892 position across the sky. For each pixel in these images, we selected 1875 1893 all objects with (14.5, 14.5, 14.5, 14.0, 13.0) $<$ ($g,r,i,z,y$) $<$ 1876 (17, 17, 17, 16.5, 15.5) , with at least 3 measurements in $i$-band (to1894 (17, 17, 17, 16.5, 15.5) magnitudes, with at least 3 measurements in $i$-band (to 1877 1895 reject artifacts detected in a pair of exposures from the same night), 1878 1896 with \code{PSF_QF} $> 0.85$ (to reject excessively-masked objects), … … 3028 3046 community. 3029 3047 3048 \note{need to add discussion of SDSS, DES, LSST, Gaia} 3049 3030 3050 \acknowledgments 3031 3051
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