The magnitude scale is based on the relative brightness of stars. The brightest stars are of zero or first magnitude. A few even brighter stars have negative magnitudes. For example, Sirius has a magnitude of approximately -1.5. Vega has a magnitude about 0, while Spica is about magnitude 1. The faintest stars visible to the naked eye in a very dark sky are about magnitude 5 or 6. Note that the higher the magnitude, the fainter object.
Due to the great distance between the stars, they all appear as just tiny points of light. However, the light of Deep Sky Objects (DSOs) is spread out, since they are much larger in extent than stars. Still, the magnitude assigned to represent the brightness of a DSO corresponds to all of that light being concentrated into a single point, as if it were a star. A third magnitude star is easier to see than a third magnitude DSO, since the DSO will show much lower contrast due to its light being spread out.
When filtering the list of Best DSOs, you should enter a Limiting Magnitude that considers both your sky conditions and the size of your optical equipment. The sky conditions are actually more important than the size of your telescope. In a dark rural sky, the DSOs, which are all relativly faint objects, will show good contrast with the background sky and will be easily seen in a telescope or binoculars. However, in an urban or suburban light polluted sky only a few of the brightest DSOs can be seen.
A telescope with a 6 inch or 8 inch diameter objective lens or mirror will show all of the objects on the Best DSOs list, which consists of 56 of the best Galaxies, Nebulas, Planetary Nebulas, Globular Star Clusters and Open Star Clusters. The faintest DSO on the list has a magnitude of about 10. The brightest DSO on the list is just above magnitude 0. The lower you set your Limiting Magnitude in the input form, the fewer DSOs will appear in the filtered list. The Limiting Magnitude entered in the form should be between 1 and 10.
The coordinates of an object in the sky are designated by Right Ascension (RA) and Declination (DECL). Right Ascension is analogous to Longitude on the surface of the earth. Declination is analogous to Latitude. The objects visible in the sky depends on the Latitude of your location on the earth.
Assume that you are at Latitude 40 degrees north. The Declination of the equator is at 0 degrees. Your southern horizon will be at Declination -50 degrees (Your latitude of 40 degrees minus 90 degrees.) Objects in the sky with a Declination less than -50 degrees will not be visible to you regardless of time of night or time of year. Similarly, if you are at Latitude 40 degrees south, your northern horizon will be at Declination 50 degrees (Your latitude 0f -40 degrees plus 90 degrees.) Objects in the sky with a Declination greater than 50 degrees will not be visible to you.
Note that due to the rotatiion of the earth, objects appear to drift accross the sky during the course of the night, just as the sun drifts from east to west during the day. Therefore, new objects appear in the east and set in the west during the course of the night. Also, due to the revolution of the earth around the sun, new objects appear in the night sky in the east during the course of the year, and objects in the west move into the daytime sky.
The Declination of the north pole is at 90 degrees, and the south pole is at -90 degrees. If you are at the equator, the poles will be on your horizon. Your southern horizon will be at Declination -90 degrees and your northern horizon will be at Declination 90 degrees. All objects in the sky will be visible to you at some time of the night, at some time of the year.
If you are at the north or south pole,the equator will be your horizon. Only objects with positive Declinations will be visible from the north pole, while only objects with negative Declinations will be visible from the south pole. However, the objects that are visible to you will be seen all night, all year. They never set below your horizon but just circle around the pole, which is overhead.
For northern hemisphere observers, if you want to filter the Best DSOs list to only show objects visible from your location, you should enter into the form a Minimum Declination corresponding to your southern horizon. You should enter a Maximum Declination of 90 degrees, since that is the highest possible (the pole). The situation for southern hemisphere observers is analogous but reversed.