5.1. Discovering SNIa
The two high redshift teams both used this pre-scheduled discovery and follow-up batch strategy. They each aimed to use the observing resources they had available to best scientific advantage, choosing, for example, somewhat different exposure times or filters.
Quantitatively, type Ia SNe are rare events on an astronomer's time scale - they occur in a galaxy like the Milky Way a few times per millennium (see, e.g., [12, 60, 61] and the chapter by Cappellaro in this volume). With modern instruments on 4 meter-class telescopes, which observe 1/3 of a square degree to R = 24 mag in less than 10 minutes, it is possible to search a million galaxies to z < 0.5 for SNIa in a single night.
Since SNIa take approximately 20 days to rise from undetectable to maximum light , the three-week separation between observing periods (which equates to 14 rest frame days at z = 0.5) is a good filter to catch the SNe on the rise. The SNe are not always easily identified as new stars on the bright background of their host galaxies, so a relatively sophisticated process must be used to identify them. The process, which involves 20 Gigabytes of imaging data per night, consists of aligning a previous epoch, matching the image star profiles (through convolution), and scaling the two epochs to make the two images as identical as possible. The difference between these two images is then searched for new objects which stand out against the static sources that have been largely removed in the differencing process [73, 74, 76, 87]. The dramatic increase in computing power in the 1980s was an important element in the development of this search technique, as was the construction of wide-field cameras with ever larger CCD detectors or mosaics of such detectors .
This technique is very efficient at producing large numbers of objects that are, on average, at or near maximum light, and does not require unrealistic amounts of large telescope time. It does, however, place the burden of work on follow-up observations, usually with different instruments on different telescopes. With the large number of objects discovered (50 in two nights being typical), a new strategy is being adopted by both the SCP and HZSNS teams, as well as additional teams like the Canada France Hawaii Telescope (CFHT) legacy survey, where the same fields are repeatedly scanned several times per month, in multiple colors, for several consecutive months. This type of observing program provides both discovery of new objects and their follow up, all integrated into one efficient program. It does require a large block of time on a single telescope - a requirement which was not politically feasible in years past, but is now possible.