The intrinsic brightness of SNIa allow them to be discovered to z > 1.5 with current instrumentation (while a comparably deep search for type II SNe would only reach redshifts of z ~ 0.5). In the 1980s, however, finding, identifying, and studying even the impressively luminous type Ia SNe was a daunting challenge, even towards the lower end of the redshift range shown in Fig. 1. At these redshifts, beyond z ~ 0.25, Fig. 1 shows that relevant cosmological models could be distinguished by differences of order 0.2 mag in their predicted luminosity distances. For SNIa with a dispersion of 0.2 mag, 10 well observed objects should provide a 3 separation between the various cosmological models. It should be noted that the uncertainty described above in measuring H0 is not important in measuring the parameters for different cosmological models. Only the relative brightness of objects near and far is being exploited in Eq. (7) and the absolute value of H0 scales out.
The first distant SN search was started by the Danish team of Nørgaard-Nielsen et al. . With significant effort and large amounts of telescope time spread over more than two years, they discovered a single SNIa in a z = 0.3 cluster of galaxies (and one SNII at z = 0.2) [35, 57]. The SNIa was discovered well after maximum light on an observing night that could not have been predicted, and was only marginally useful for cosmology. However, it showed that such high redshift SNe did exist and could be found, but that they would be very difficult to use as cosmological tools.
Just before this first discovery in 1988, a search for high redshift type Ia SNe using a then novel wide field camera on a much larger (4m) telescope was begun at the Lawrence Berkeley National Laboratory (LBNL) and the Center for Particle Astrophysics, at Berkeley. This search, now known as the Supernova Cosmological Project (SCP), was inspired by the impressive studies of the late 1980s indicating that extremely similar type Ia SN events could be recognized by their spectra and light curves, and by the success of the LBNL fully robotic low-redshift SN search in finding 20 SNe with automatic image analysis [56, 67].
The SCP targeted a much higher redshift range, z > 0.3, in order to measure the (presumed) deceleration of the Universe, so it faced a different challenge than the CTSS search. The high redshift SNe required discovery, spectroscopic confirmation, and photometric follow up on much larger telescopes. This precious telescope time could neither be borrowed from other visiting observers and staff nor applied for in sufficient quantities spread throughout the year to cover all SNe discovered in a given search field, and with observations early enough to establish their peak brightness. Moreover, since the observing time to confirm high redshift SNe was significant on the largest telescopes, there was a clear "chicken and egg" problem: telescope time assignment committees would not award follow-up time for a SN discovery that might, or might not, happen on a given run (and might, or might not, be well past maximum) and, without the follow-up time, it was impossible to demonstrate that high redshift SNe were being discovered by the SCP.
By 1994, the SCP had solved this problem, first by providing convincing evidence that SNe, such as SN1992bi, could be discovered near maximum (and K-corrected) out to z = 0.45 , and then by developing and successfully demonstrating a new observing strategy that could effectively guarantee SN discoveries on a predetermined date, all before or near maximum light [70, 71, 72, 76]. Instead of discovering a single SN at a time on average (with some runs not finding one at all), the new approach aimed to discover an entire "batch" of half-a-dozen or more type Ia SNe at a time by observing a much larger number of galaxies in a single two or three day period a few nights before new Moon. By comparing these observations with the same observations taken towards the end of dark time almost three weeks earlier, it was possible to select just those SNe that were still on the rise or near maximum. The chicken and egg problem was solved, and now the follow-up spectroscopy and photometry could be applied for and scheduled on a pre-specified set of nights. The new strategy worked - the SCP discovered batches of high redshift SNe,and no one would ever again have to hunt for high-redshift SNe without the crucial follow-up scheduled in advance.
The High-Z SN Search (HZSNS) was conceived at the end of 1994, when this group of astronomers became convinced that it was both possible to discover SNIa in large numbers at z > 0.3 by the efforts of Perlmutter et al. [70, 71, 72], and also use them as precision distance indicators as demonstrated by the CTSS group . Since 1995, the SCP and HZSNS have both worked avidly to obtain a significant set of high redshift SNIa.