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This page provides a set of sample Pan-STARRS queries, written for execution with CASJobs.
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This mini-project illustrates how to extract mean objects with predefined quality parameters and then the associated detections. The goal ultimately is to understand the astrometric properties of PanSTARRS Pan-STARRS before the Gaia adjustments, therefore we are interested in the original detections; forced measurements are not useful.
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Note that the mean object astrometric parameters (raMean, decMean) have been post-processed to match with corresponding Gaia sources, and therefore are not indicative of the underlying PanSTARRS Pan-STARRS astrometry.
Step 2: Obtain detections associated with objects selected in Step 1
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Red dots in this plot have ExtendedFlag == true. In the following, we use ExtendedFlag to identify extended objects.
Step 4: compare Gaia and
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Pan-STARRS magnitudes for point sources
The match distance distribution shows clearly that "true" matches likely have match distances << 1". This plot shows that PanSTARRS Pan-STARRS r-band PSF magnitudes and Gaia G band magnitudes match very well for ``true'' point source matches (separation < 0.2"):
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The plot shows two populations of point sources: a narrow band with r-G ~ 0, containing a majority of the matches, and a broader distribution at positive r-G values (brighter in G than in r). Extended sources are mostly brighter in PanSTARRS Pan-STARRS r, as expected since Gaia, with its higher angular resolution, would only measure the core of the object. (We will neglect extended sources henceforth.) It turns out that the second population of point sources, at positive r-G, is constituted of very red stars, for which the extremely broad G passband is dominated by red photons. This becomes clear if the r-G magnitude difference is plotted against PanSTARRS Pan-STARRS colors, e.g., r-i:
where the region of positive r-G (Gaia brighter) corresponds with positive r-i (red stars). The narrowness of the sequence testifies to the quality of Gaia and PanSTARRS Pan-STARRS magnitudes.
We can take one additional step and attempt to define a color transformation that maps Gaia magnitudes (G, B, and R) into PanSTARRS Pan-STARRS r. A degree 4 polynomial results in a very good match, albeit with large scatter at faint magnitudes, where Gaia B and R magnitudes lose precision:
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It would also be possible to define a combination of PanSTARRS Pan-STARRS magnitudes to map into Gaia G, but the above plots clealy show that magnitudes are well matched, so this has not been pursued further.
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Finally, we compare the astrometric match between Gaia and PanSTARRS Pan-STARRS using both mean object quantities and the average of detection positions. Mean object astrometry is obtained from the DR2 table ObjectThin and has been adjusted to Gaia. Detection positions are taken from the DR2 Detection table, and have not been adjusted to Gaia. Accordingly, if we assign to each matched object a ``mean detection'' position corresponding to the average of its detections (as indicated by objID), this position is a true indicator of the underlying quality of PanSTARRS Pan-STARRS astrometry before Gaia matching:
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The black histogram shows the match distance for mean object (typically Gaia-adjusted) positions; it peaks at 5 mas, with a significant tail beyond 20 mas. The red histogram uses the mean position of detections; it peaks at 20 mas, with a significant tail beyond 50 mas. More analysis is in progress to better understand how proper motions might impact the relative astrometry for both matched and unmatched PanSTARRS Pan-STARRS sources. In particular, we plan to use detection positions to obtain proper motion estimates or bounds for sources that do not have Gaia proper motions. Note that separations based on mean detection positions have not been adjusted for the (known) proper motion of the corresponding Gaia source; this is ongoing.
In summary, we show that 1) most Gaia sources within the chosen area have PanSTARRS Pan-STARRS matches; 2) point-source photometry is very consistent between Gaia and PanSTARRSPan-STARRS, with obvious deviations related to both extended sources and point sources of extreme colors; and 3) detection positions can be used to understand the vaidity of PanSTARRS Pan-STARRS astrometry after matching to Gaia, and potentially offer information of source proper motions.