Every professional research program has its own survey strategy. The biggest observatories, with their large telescopes, usually sample a specific region of the sky, maybe in the middle of a galaxy cluster, and perform deep observations, in order to search for supernovae not discoverable by amateur astronomers.
Amateur astronomers have to work in a different way: they patrol all the sky, not only a limited region, but select a reduced sample of galaxies. Here is the problem: how to select the better galaxies, the ones with good chance to show a supernova explosion.
In the framework of the CROSS program:
1. we discarded the galaxies with distance modulus greater than a certain value, that is all the galaxies in which is difficult to detect type II supernovae;
2. we selected the galaxies with two or more supernova explosions;
3. we discarded the galaxies with declination lower than -15°;
4. we discarded the galaxies with absolute magnitude lower than +18.
In such a way we have a list of 2500 galaxies, of which we downloaded the reference images from the web, mainly from the Palomar Observatory Sky Survey (series II, blue and red images).
Then we selected our better images in order to obtain a list of reference images taken with the same instrument. We performed a datasheet (in Excel), updated daily, in which we can find the galaxies to image every hour of the night, searching in the galaxies not imaged since at least 20 days.