| | 1 | = Known Data Quality Issues -- Release 2009.07.07 = |
| | 2 | |
| | 3 | (Up to [wiki:PS1_Science_Processing_Status_20090705 PS1 Science Processing Status (20090705)]) |
| | 4 | (Up to [wiki:IPP_for_PS1 IPP for PS1]) |
| | 5 | |
| | 6 | Here is a list known data quality issues with the Medium Deep field data from April, May, and June 2009, released starting 2009.07.07 (processing tags MD0[678].20090[678].v1) |
| | 7 | |
| | 8 | * Detrending: |
| | 9 | ** Dark Correction : The camera temperatures have been changing more |
| | 10 | than expected this summer. We found in Run 2 that our dark |
| | 11 | corrections were over- or under-correcting the dark current based |
| | 12 | on the reported temperature changes for the devices. We |
| | 13 | attributed this to the detector temperatures, which are measured |
| | 14 | on the package some distance from the actual silicon wafer, |
| | 15 | failing to measure the device silicon temperatures very |
| | 16 | reliably. We decided to limit the dark current model to a |
| | 17 | function of exposure time, and ignore the temperature. However, |
| | 18 | it now appears that the overall camera temperature changes are |
| | 19 | large enough that they cause significant extra dark current. We |
| | 20 | are pursuing a strategy of using a median camera temperature to |
| | 21 | guide the dark correction for all chips. The data in this |
| | 22 | release does not yet reflect that change. |
| | 23 | |
| | 24 | ** y-band fringing. The new y filter appears to produce a much |
| | 25 | stronger y-band fringe pattern. Since we were not expecting |
| | 26 | y-band fringes, they are not measured and corrected. We are now |
| | 27 | generating y-band fringe masters, and will apply the correction |
| | 28 | in future data releases. |
| | 29 | |
| | 30 | * Calibration |
| | 31 | ** Astrometry: Based on reports from Doug Finkbeiner and Eddie |
| | 32 | Schlafly, we have recently realized that the reference astrometry |
| | 33 | and photometry DVO database is in an older format with RA & DEC, |
| | 34 | taken from 2MASS, represented as floats. As a result, there are |
| | 35 | systematic errors on the scale of 0.1 arcsec in localized areas, |
| | 36 | and additional scatter in the range of 50 mas or so. Note that |
| | 37 | the systematic offsets are consistent for large areas, but change |
| | 38 | over the sky. We are updating the reference database to use |
| | 39 | doubles and future releases will use the improved calibration. |
| | 40 | |
| | 41 | ** Photometry: the photometric calibration is based on the synthetic |
| | 42 | grizy generated from a combination of 2MASS, USNO-B, and Tycho |
| | 43 | photometry. In practice, the photometry is dominated by the |
| | 44 | extrapolation from 2MASS. Eric Bell has shown that the |
| | 45 | photometry has systematic trends relative to SDSS. Further |
| | 46 | exploration suggests that the photometry may be most reliable at |
| | 47 | bright magnitudes, but may have systematic biases for fainter |
| | 48 | magnitudes. The effect is most pronounced it the g-band. The |
| | 49 | photometric calibration probably depends on the collection of |
| | 50 | input reference stars used. We are looking into using either an |
| | 51 | empirical correction or simply a brighter subset of the reference |
| | 52 | stars. Our expectation is that the 2MASS-based synthetic |
| | 53 | photometry can potentially yield zero points which are good to 5% |
| | 54 | or so. However, until these biases are understood, the |
| | 55 | calibration should be used with care, and is probably no better |
| | 56 | than several tenths of magnitudes. |
| | 57 | |
| | 58 | * Other |
| | 59 | ** Astrometry keywords. Several issues with the astrometry keywords |
| | 60 | have been reported. We are aware that the headers of the CHIP |
| | 61 | data have both CDi_j style WCS keywords and PC00i00j style |
| | 62 | keywords. Not surprisingly, FITS readers are confused by |
| | 63 | multiple, conflicting representations. We have also had reports |
| | 64 | from Nigel Metcalf that the stack headers are missing both |
| | 65 | EQUINOX and RADECSYS keywords, causing trouble for various |
| | 66 | readers. Finally, the CHIP astrometry is linearized from our |
| | 67 | high-order model. On the scale of a chip, the distortion is |
| | 68 | sufficiently large that the linear version will perform poorly. |
| | 69 | We have not yet chosen one of the non-linear WCS options. |
| | 70 | |
| | 71 | ** FITS Compression, BSCALE & BZERO: We have found a bug in the |
| | 72 | routines that convert from the 32bit float internal |
| | 73 | representation to the 16bit integer output format. The effect of |
| | 74 | this bug is that the data values as written have a constant |
| | 75 | offset of a small number of DN. |
| | 76 | |
| | 77 | ** Missing chips / warps / etc: there is a bug in a portion of the |
| | 78 | photometry code which occasionally results in an unexpected |
| | 79 | failure to detect stars. These images are treated as if they |
| | 80 | were blank, and are assigned a poor data quality. As a result, |
| | 81 | individual chips or skycells may be missing from an exposure. |
