| | 7 | |
| | 8 | I spent most of the week working on the reference photometry database. My main focus was on the MD field database, to confirm the quality of the relative photometry analysis and the zero point measurement. As mentioned last week, I needed to rebuild this database with only a single version of each exposure. I also modified relphot to exclude suspected galaxies from the calibration analysis. Including these fixes, I still found surprisingly high zero point variations: exposures from a single night tended to be in good agreement, but those from nearby nights could be off by as much as 2% in either direction. I would not be surprised at long-term variations, but variations on 2-3 day time scales do not seem reasonable. |
| | 9 | |
| | 10 | Looking at this further, I discovered that the measured zero points trended with the image quality. I believe this is a reflection of a PSF-Aperture correction which uses too small of an aperture: it is still sensitive to the seeing variations. Unfortunately, the dvo database currently does not include the mean FWHM for each chip, so studying this trend was not simple. I created a tool to ingest the fwhm values into DVO, and to define the median exposure FWHM from those values. I was then able to measure the trend of zero point offset vs median FWHM. Fitting a single quadratic curve to all zero point variations from all MD field exposures in a single filter brings the night-to-night scatter into a very respectable range: 1.0-1.5%. The variations that remain (outside of obvious clouds) are somewhat long term (~50 day structures), and seem to agree between the different filters. This is not yet conclusive, but suggests that I now detecting the real system variations (probably atmospheric). Also important: the quadratic fit to all four filters griz agree very well: this makes it reasonable to treat this effect as a color-independent trend. (The MD y-band measurements are somewhat more scattered, and need some further examination). |
| | 11 | |
| | 12 | I am continuing now to work on the assessment of the photometry in the 3pi reference database. |
