Index: trunk/doc/release.2015/ps1.calibration/Makefile =================================================================== --- trunk/doc/release.2015/ps1.calibration/Makefile (revision 40713) +++ trunk/doc/release.2015/ps1.calibration/Makefile (revision 40714) @@ -17,21 +17,36 @@ quick: calibration.quick.pdf +PDFPICS = \ +pics/A1.pdf \ +pics/A3.pdf \ +pics/A4.pdf + FILES = \ ../inputs/astro.sty \ ../inputs/code.sty \ ../inputs/apj.bst \ -pics/photflat.example.sm.png \ -pics/allsky.photom.sigma.sm.png \ pics/rings.v3.example.png \ pics/KHexample.png \ pics/KHmap.png \ pics/dcr.r2.g.png \ -pics/astroflat.gri.sm.png \ -pics/astroflat.zy.sm.png \ pics/allsky.astrom.sigma.png \ pics/gaia.photom.png \ pics/gaia.astrom.png \ +$(PDFPICS) \ calibration.tex +# pics/photflat.example.sm.png \ +# pics/allsky.photom.sigma.sm.png \ +# pics/astroflat.gri.sm.png \ +# pics/astroflat.zy.sm.png \ + +pics/%.pdf : pics/%.ps + echo $^ + echo $< + echo $@ + echo $* + ps2pdf -dEPSCrop $< $@ + +pdfpics: $(PDFPICS) calibration.pdf: $(FILES) calibration.tgz: $(FILES) Index: trunk/doc/release.2015/ps1.calibration/calibration.tex =================================================================== --- trunk/doc/release.2015/ps1.calibration/calibration.tex (revision 40713) +++ trunk/doc/release.2015/ps1.calibration/calibration.tex (revision 40714) @@ -2817,4 +2817,126 @@ proper motions will obviate the need to correct for the Galactic rotation. +\section{Polar Astrometry Issues} + +Internal consistency testing of the PV3 stacks measurements indicated +potential problems with the astrometric registration of the exposures +in small areas near the North Pole. These issues were originally +suggested by a few high-latitude sources with significant differences +in morphology or position across bands, including strong (and +anomalous) apparent color gradients. Direct investigation of a few of +these anomalous sources demonstrated the presence of significant +misalignments between exposures; one of the worst cases is shown in +Figure~\ref{fig:pole.issue.exampe}. While such sources appeared to be +rare, astrometric registration errors have the potential to affect +several different source properties: morphology and photometry in +addition to astrometry. Therefore we carried out an astrometric +regsitration test for all skycells North of $ \delta=+70\deg$. + +\begin{figure*}[htbp] + \begin{center} + \includegraphics[width=\hsize,clip]{{pics/A1}.pdf} + \caption{\label{fig:pole.issue.example} Example of a stack source badly affected by polar astrometry failures. Source from multiple detections from skycell 2643.093.} + \end{center} +\end{figure*} + +This test was based primarily on the ``original detection positions'', +\ie, the positions of sources (detections) found in individual +exposures as measured after each exposure's astrometric calibration, +but before recalibration of the combined values to the Gaia reference +frame (described in Section 7.3). We started by collecting the +original detection positions (as defined above) for each skycell. To +ensure good signal-to-noise ratios and minimize potential spurious +detections, we used only the top quartile (in flux) of detections +within each chip. We grouped these detections on a filter-by-filter +basis within a radius of $ 2\farcs5 $ (10 pixels), ensuring that each +group contained only one source per exposure, and retaining only +groups with at least five detections; we then recorded the 2-D +position dispersion for each group. The mean positions for each group +were cross-correlated against the Gaia DR2 sources, showing that these +were real sources and providing information on their absolute +astrometry. + +\begin{figure*}[htbp] + \begin{center} +% \includegraphics[width=\hsize,clip]{{pics/A2}.pdf} + \caption{\label{fig:pole.issue.example} Example of a stack source badly affected by polar astrometry failures.} + \end{center} +\end{figure*} + +Overall, the vast majority of the detection groups thus defined have +good consistency between source positions, resulting in an astrometric +dispersion of 1 pixel or less. A few ``bad'' groups, defined as +having an internal dispersion $ > 1 $ pixel, can result from spurious +sources or other anomalies, and are generally rare (fewer than a few +percent of al groups). However, some skycells have a significant +fraction ($ > 10\%$) of bad groups. Direct inspection demonstrates +that the incidence of bad groups is related to astrometric +registration failures. Figure~\ref{fig:pole.astrom.failures} shows an +example of a good and of a bad group. + +%% [Note: the rest of this +%% paragraph, and Figure A3, may be too much information for this +%% paper.] It also appears that registration problems, when present, +%% are not uniform within a skycell; Figure (A3) shows the difference +%% between mean group position and the position of individual detections +%% for all G band exposures overlapping skycell 2637.088, which has one +%% of the worst-case mismatches in the g band. + +% caption: Map of astrometric displacement for all g-band exposures +% overlapping skycell 2637.088, with one of the worst astrometric +% registration issues. [Optional] + +\begin{figure*}[htbp] + \begin{center} + \includegraphics[width=\hsize,clip]{{pics/A4}.pdf} + \caption{\label{fig:pole.bad.histogram} Histogram of the fraction of bad groups for each skycell (red line).} + \end{center} +\end{figure*} + +Bad skycells, defined as those with more than 10\% bad groups, are +essentially limited to the North polar cap ($ \delta > +80^{\degree}$). +Of the 2500 skycells in this region, 164, or 6.6\%, have more than 10\% +bad groups; 64 of these have more than 20\% bad groups. By comparison, +essentially no skycells between $ +70^\degree $ and $ +80^\degree $ have +more than 10\% bad groups. Figure~\ref{fig:pole.bad.histogram} shows a histogram +of the fraction of bad groups for each skycell. + +In order to have an independent validation of the impact of this +astrometric alignment issue, we also carried out a photometric test +based on a comparison of stack to mean object photometry. In the +presence of modest registration errors, mean object photometry would +not be affected, as individual detection woulds have the correct +signal, and averaging their flux in catalog space would yield the +correct total magnitude. On the other hand, imperfect stacking would +result in a dilution of the total signal on a pixel-by-pixel basis, +and result in potentially larger estimated sizes and smaller total +flux for stack sources. Indeed, mean magnitudes are brighter than +stack magnitudes for a significant fraction of the sources in the same +skycells that are identified as bad by the relative astrometry test. +Therefore we confirm that the astrometric registration issues result +in poor stack photometry for the affected skycells. + +\note{discuss the cause of the failure due to the duplicates in the reference catalog, and the original polar astrometry failures} + +As a result of these tests, we decided to 1) exclude from the main DR2 +catalogs all sources in the skycells with more than 10\% bad groups, +and 2) to reprocess all such skycells with an improved procedure. The +reprocessing was carried out in late 2018, and the astrometric +registration test was repeated on the reprocessed exposures. The +reprocessing greatly ameliorated the registration issue, as shown +Figure (A4). Here the red line shows the histogram of the fraction of +bad groups for each skycell {\sl before reprocessing}, while the black +line refers to the results {\sl after reprocessing}. The improvement +is apparent. After reprocessing, only 23 cells, instead of the +original 164, exceed 10\% of bad groups, and even for these the +fraction of bad groups is substantially reduced. Sources in the +previously bad, now fixed skycells will be included in an upcoming +partial release. + +\note{the above is not quite accurate -- a test reprocess demonstrated + partial improvement, but did not use a totally repaired ref catalog. + we are running a new analysis based on a DR2-tied catalog with + pristine source set.} + \section{Conclusion}