Evaluation

(a.) The Perth search is finding some intrinsically fainter supernovae in the brighter galaxies which were not seen or were otherwise missed in the visual search. The visual search can miss supernovae within one magnitude of the limit (16.5 - 17.5) through having to observe in moonlight, a factor which does not degrade CCD images to the same degree. In the visual search, objects at the very faint end have been missed on two occasions that are known, and may have occurred on other occasions. NGC 908 was observed by us a few nights after the Perth group had found SN 1994ae, estimated at mag 16.5. While I knew that a supernova was present I did not know its location within the galaxy, and no new star was seen. Also, NGC 1398 was observed by us several days before the Perth group found SN 1996N at mag 16.5. In the first instance, I believe that the supernova was fainter visually than the (red) CCD estimate published. As a result it was not seen by us. In the second instance, the star was seen, but was not recognised as a new object. So, our failure to recognise the new object on this occasion should be put down to human error.

(b.) The Perth search's smaller number of observations per supernova seen (922 : 1090 : 2211) may be due to the particular selection of target galaxies used by them. The visual search covered a much wider range of galaxies, both in type and luminosity. This created a sample of galaxies with a much lower expected supernova rate than the Perth sample.

(c.) Allowance must also be made for the fact that 40 inch and 41 cm figures are only for eighteen months, whereas the Perth figures are for three years. On the average, twice as many supernovae were available to be discovered or seen in a three-year period than in eighteen months.

(d.) A surprising result is that the 40 inch search saw a supernova every 41 hours and the 41 cm search every 38 hours, whereas the Perth computer saw one every 61.5 hours. This difference probably arises from the great difference in the average speed of the Perth search, which, in turn, allowed a much wider range of galaxies to be seen by the two visual searches. We were quite surprised that the 40 inch was able to observe galaxies at least 60% faster than the average speed of the Perth search, despite the fact that the 40 inch does not have the advantage of computerised location of targets. The efficiency of the automatic search could be improved by using improved CCDs and faster computers.

(e.) Large-telescope visual searches can also be improved by the computerised location of target galaxies. In the case of the 40 inch telescope at Siding Spring the use of the Cassegrain focus port provides a much more efficient use of the telescope than would be possible if a standard Newtonian configuration was being used.

(f.) A much longer list of different galaxies needs to be observed regularly with the 40 inch, compared to the smaller telescope, if the number of supernovae to be seen is to be improved and the proper potential of the larger telescope is to be realised. The fainter magnitude limit which is possible with the 40 inch telescope should also be exploited more fully. This, again, emphasises the value of, and need for, a dedicated telescope for this kind of observing.

© Copyright Astronomical Society of Australia 1997