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This page is still being populated. Please be patient. We will provide more answers as time allowsin includes answers to some of the more common questions about PS1 data.  If you have suggestions for additional topics you would like to see addressed, please send email to archive@stsci.edu.

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The Pan-STARRS project

What is Pan-STARRS?

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The new MAST interface to the Pan-STARRS catalog supports queries to both the DR1 and DR2 PS1 catalogs. It also has an associated API, which is used in the script described here.

PS1 astrometry

How can I search for moving solar system targets in the PS1 images and catalogs?

See the How to search for moving targets in PS1 images and catalogs page for a Python Jupyter notebook that demonstrates one approach to searching the PS1 archive for either images or catalogs that have observations of solar system targets.  A MAST interface is currently under development to make this kind of search easier.

PS1 astrometry

How good is PS1 astrometryHow good is PS1 astrometry?

The positions of DR1 sources were originally determined from the astrometric calibration of PanSTARRS using sources in common with the 2MASS catalog.  The systematic uncertainty in this calibration is not precisely known, but is likely to be close to 0.1".

However, the positions of the mean objects in the DR1 release catalog were recalibrated using Gaia DR1 positions as additional constrains in the astrometric solution.  The Gaia measurements were given very high weight, as detailed in Magnier et al. (2016).  A comparison of Gaia and PanSTARRS positions for the objects in common yields a typical residual of 5 mas (1-sigma, 2-d), with a slightly higher component in the direction of right ascension.  This comparison may not fully reflect PanSTARRS position errors, because of the correlation in the quantities that were compared (Gaia measurements are included in the determination of individual PanSTARRS positions).

An independent test of sources with Gaia data that were not used in the recalibration was carried out by Makarov and collaborators (V. Makarov, C. T. Berghea, & J. H. Frouard, Technical Memorandum, AADD USNO, 2017).  They consider the over 19 million stars for which the duplicated_source flag is set in the Gaia DR1 catalog for which a Gaia-corrected position is available in PanSTARRS DR1.  Their results, summarized in the first plot below, suggests that the residual systematic uncertainties for recalibrated PanSTARRS positions is closer to 20 mas (1-sigma, 2-d) for sources with Gaia magnitudes between G=15 and G=18, increasing for fainter magnitudes to about 35 mas at the Gaia magnitude limit (G=21.7).  Errors are larger towards brighter magnitudes as well, possibly because saturation and proper motion effects become significant in the PanSTARRS measurements.

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PanSTARRS-Gaia astrometric comparison for sources not used in the PanSTARRS calibration.  Courtesy Makarov et al (2017).

The majority of the sources in this category have a value of the astrometric_excess_noise parameter in Gaia DR1 below 1 mas (median value 0.61 mas).  The astrometric_excess_noise parameter estimates the magnitude of residuals per observation beyond the nominal measurement errors after the single-object astrometric fit (see Lindegren et al 2016 for more details).

Makarov et al (2017) obtain similar results from a comparison with quasars with high-quality radio positions but without a Gaia counterpart, although the number of sources in this case is much smaller.

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PanSTARRS-Quasar astrometric comparison for radio sources without a Gaia counterpart.  Courtesy Makarov et al (2017).

A side effect of the inclusion of Gaia DR1 positional data is that the epochMean column in the ObjectThin table, which gives the mean epoch for the RA and Dec measurements, is also affected for objects that included Gaia DR1 data in their position calculations.  The result is that the epochMean date is often later than any of the PS1 measurements given in the Detection table.  Typically the value for objects that include Gaia DR1 data is close to the Gaia DR1 epoch of 2015.0 = MJD 15023.  Note that in these cases, the epochMean value is completely unrelated to the mean date for the PS1 photometric measurements.

An independent test of sources with Gaia data that were not used in the recalibration was carried out by Makarov and collaborators (V. Makarov, C. T. Berghea, & J. H. Frouard, Technical Memorandum, AADD USNO, 2017).  They consider the over 19 million stars for which the duplicated_source flag is set in the Gaia DR1 catalog for which a Gaia-corrected position is available in PanSTARRS DR1.  Their results, summarized in the first plot below, suggests that the residual systematic uncertainties for recalibrated PanSTARRS positions is closer to 20 mas (1-sigma, 2-d) for sources with Gaia magnitudes between G=15 and G=18, increasing for fainter magnitudes to about 35 mas at the Gaia magnitude limit (G=21.7).  Errors are larger towards brighter magnitudes as well, possibly because saturation and proper motion effects become significant in the PanSTARRS measurements.

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PanSTARRS-Gaia astrometric comparison for sources not used in the PanSTARRS calibration.  Courtesy Makarov et al (2017).

The majority of the sources in this category have a value of the astrometric_excess_noise parameter in Gaia DR1 below 1 mas (median value 0.61 mas).  The astrometric_excess_noise parameter estimates the magnitude of residuals per observation beyond the nominal measurement errors after the single-object astrometric fit (see Lindegren et al 2016 for more details).

Makarov et al (2017) obtain similar results from a comparison with quasars with high-quality radio positions but without a Gaia counterpart, although the number of sources in this case is much smaller.

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PanSTARRS-Quasar astrometric comparison for radio sources without a Gaia counterpart.  Courtesy Makarov et al (2017).

It is thus likely that systematic uncertainties of 20 mas (1-sigma, It is thus likely that systematic uncertainties of 20 mas (1-sigma, 2-d) apply to all mean objects in PanSTARRS DR1, with possibly larger uncertainties in regions with a scarcity of sources in common between Gaia and PanSTARRS. 

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How do I convert pixel values into magnitudes?

This is explained in PS1 Stack images, the "Photometric Calibration" section.

Why are the coordinates incorrect when I display a skycell image in DS9?

If the coordinates in your image appear to be far off (by arcminutes or more) from what you expect, you probably have encountered the RADESYS problem using recent versions of DS9.  The full skycell FITS images do not have a RADESYS keyword.  That leads some software to incorrectly interpret the PS1 image coordinates as being equinox 1950 rather than equinox 2000.  At the moment the only known software with this issue is DS9 v8, but it could happen with other software as well.  (Older versions of DS9 correctly interpret the PS1 FITS headers as J2000 coordinates.) 

The fix is to insert the keyword RADESYS = 'FK5' in the header.  PS1 FITS cutout images have a correct RADESYS keyword (as of 2019 March 13), but full skycell FITS images do not.

Stack images, the "Photometric Calibration" section.

Why are the coordinates incorrect when I display a skycell image in DS9?

If the coordinates in your image appear to be far off (by arcminutes or more) from what you expect, you probably have encountered the RADESYS problem using recent versions of DS9.  The full skycell FITS images do not have a RADESYS keyword.  That leads some software to incorrectly interpret the PS1 image coordinates as being equinox 1950 rather than equinox 2000.  At the moment the only known software with this issue is DS9 v8, but it could happen with other software as well.  (Older versions of DS9 correctly interpret the PS1 FITS headers as J2000 coordinates.) 

The fix is to insert the keyword RADESYS = 'FK5' in the header.  PS1 FITS cutout images have a correct RADESYS keyword (as of 2019 March 13), but full skycell FITS images do not.

We hope eventually to fix this issue (and some other problems described in the "FITS image format quirks" section of the PS1 DR2 caveats page), but since the PS1 archive includes 1.5 petabytes of PS1 images, the task of updating all the image files is not simple.

PS1 Timing

What is the time scale used for the PS1 observation times?  Why do the observation times in the image header and catalog disagree?

The times in the warp image headers and in the catalog (e.g., the obsTime column in the Detection table) are defined using international atomic time (TAI) rather than UTC time.  Those times differ by the addition of leap seconds, which leads to header times that differ by 34 or 35 seconds from the UTC times. (See Rots et al. 2015 for more details.) If you are concerned with timing at this level, you may need to convert the times to UTC.  For the warp images, the fix for this is to insert the keyword TIMESYS = 'TAI' in the header.  FITS cutout images have a correct TIMESYS keyword (as of 2022 January 20), but full skycell FITS images do not have a TIMESYS keyword. For the database times, the fix is to subtract 34 or 35 seconds (depending on the date) from the TAI time to get the UTC time.  The astropy.Time module includes convenient functions for converting between TAI and UTC times.

Also note that the epoch given in the FITS header for the warp images is the start time of the observation, while the epoch in the database for Detection entries is the mean time of the observation, which is the start time plus 15 seconds (since the exposure time for PS1 images is 30 seconds).   So you should add 15 seconds to the warp header's MJD-OBS keyword value to get the equivalent value from the Detection.obsTime column in the PS1 database.

Thanks to Peter Van Wylen for discovering the TIMESYS issue and helping identify the fix. And thanks to Jules Halpern for pointing out an error in the original description of the fixWe hope eventually to fix this issue (and some other problems described in the "FITS image format quirks" section of the PS1 DR2 caveats page), but since the PS1 archive includes 1.5 petabytes of PS1 images, the task of updating all the image files is not simple.