Coma Repeatibility

As part of the mid-April 1997 engineering run with the 2MASS telescope and 3-channel camera several scans were taken of the Coma galaxy cluster. The scans are all centered on the Coma core, with slight cross-scan position differences (a few arcsec), thus the data set allows a number of crucial repeatibility tests, including photometry, reliability & completeness. The data set consists of eight scans, 3 taken during the night of 970418 and five during the night of 970423. (note: several other scans of Coma were acquired, but had problems associated with variable weather, dome blockage, incorrect pointing positions, and other such problems/conditions that effectively made this data unsuitable for detailed comparison with good scan data.) Both nights have been photometrically calibrated, with the corrections applied to the eight scans. The photometric uncertainties for the data are probably a few percent (based on the calibration data), thus the uncertainty in the galaxy photometry that we measure should be dominated by poisson statistics (and not by problems with the zero point correction, for example).

A great deal of information can be derived from this data set, both a blessing and curse (depending on whether the reader is tolerant of the many gruesome details). I will divide this memo into several sections, covering topics that have thus far been explored in some detail, and topics not yet covered but which require some attention in the near future. Major issues include: how many galaxies are detected and how many times does the detection repeat (repeatibility -- approximately equal to completeness), how many false detections (stars, double stars, pieces of bright stars, artifacts, etc) are found with respect to galaxy detections (reliability), seeing, bright star removal, photometric repeatibility (what is our sensitivity and how does it compare with the level-1 specifications) and repeatibility of other key parameters (e.g., axial ratios), and a slew of bookkeeping issues that are relevant to the extended source archive. The following hyper-links provide quick access to specific topics.

(970522) Note: M. Skrutskie has successfully acquired another set of 10 Coma scans taken under photometric condiditons. This data set will eventually be added to this current set and the results updated as they come rolling in.