| 48 | | After the databases were created, I ran the full-scale relative astrometry and photometry analysis on them. In the initial step, I performed just the basic calculation of simple averages (see run.addstar). In the next step, I performed the more detailed relative photometry and astrometry analysis. |
| | 48 | After the databases were created, I ran the full-scale relative astrometry and photometry analysis on them. The relative photometry analysis currently runs separately for each target average photcode (grizy). The analysis determines the average magnitudes of the stars in the images, based on the available measurements, then uses these averages to determine the relative offsets of the image zero points. It stabilizes the system of equations by re-setting the photometric clusters of images to 0.0. The program performs a requested number of iterations on the photometry. As the analysis proceeds, the code attempts to identify outlier detections for individual stars, then outlier stars (variable stars) from the sample, and finally outlier images -- those with excessively large scatter (ie, poor photometric conditions). As these outlier / poor entries are discovered, they are excluded from the constraints on the average magnitudes. I ran 20 iterations of relative photometry, using the commands listed below. Note that only the high signal-to-noise measurements were used to determine the relative photometry (SIGMA_LIM), and that the zero points were calculated for the full GPC1 exposures, with the individual chips held fixed relative to one another (-imfreeze -mosaic). The -statmode option specifies how the averaging is performed; in this case, the analysis performs a weighted mean of the inner 50% of the available measurements. |
| | 49 | |
| | 50 | {{{ |
| | 51 | foreach filter (g r i z y) |
| | 52 | relphot $region -v -D GRID_TOOFEW 10 -D MOSAICNAME GPC1 -D SIGMA_LIM 0.025 -D CATDIR $catdir -update -statmode INNER_WTMEAN -nloop 20 -imfreeze -mosaic $filter |
| | 53 | end |
| | 54 | }}} |
| | 55 | |
| | 56 | The relative astrometry analysis currently performs only one stage of the analysis at a time. Either the average positions of objects in the database are calculated based on the current image calibration parameters, or the image calibrations are re-measured based on the average positions of the objects in the database. Like the relative photometry analysis, this process requires a series of iterations to converge on a good solution. In this example, I ran 4 iterations. Currently, relastro does not perform a very robust outlier rejection. The commands used are listed below (repeated 4 times in the actual analysis): |
| | 57 | |
| | 58 | {{{ |
| | 59 | relastro $region -v -D GRID_TOOFEW 10 -D MOSAICNAME GPC1 -D CATDIR $catdir -update-objects -update |
| | 60 | relastro $region -v -D GRID_TOOFEW 10 -D MOSAICNAME GPC1 -D CATDIR $catdir -update-chips -update |
| | 61 | }}} |
