In February 2001, Tony Tyson and his group showed us some ellipticity maps of their Mosaic images. Ellipticity of each star is measured, as per SExtractor's definitions (see the User Manual), and plotted as vectors showing both amplitude 'e' and angle 'pa'. By definition:

e = 1 - B/A, where B and A are the semi-minor and semi-major axis lengths. So a perfectly round image has an ellipticity of 0 and e=0.1 means the major axis is 10% longer than the minor axis.

Some trends were quite unusual (see example) and obviously contained precious information about image quality. In May, we obtained our first engineering images with specific telescope parameters (like ADC on/off, M1 corrections on/off, etc...) in order to study the effects methodically. We have taken more data since then at various opportunities, sometimes with the complicity of the CTIO Mosaic engineering team. Based on Dave Wittman's (Bell Labs) original programs, Dara Norman (CTIO) has built up additionnal tools to calculate the mean ellipticity vector in the field, substract it from all the stars and do combined plots showing the data before and after substraction. There has been a substantial support from both Dave and Dara to get these tools running and they are now working very efficiently and robustly. Sincere thanks to both of them. The programs and a README file explaining how to use them can be found in /ua76/boccas/4m/ellipticity/.

These ellipticity plots have allowed us to investigate deeper mostly 2 issues: the effects of the ADC in the image quality, and a method to build astigmatism lookup tables for M1. Most images are 10sec unguided R filter exposures (unless written otherwise).



M.B., 30 Sept 01