Yet another cloudy night, so I took yet another look into my NGC4565 picture and found out a bunch of faint stars near to the galaxy. These stars looked like they did not want to be anonymous anymore, so I decided to at least found out how faint they are. The task was not so trivial, but after installing Astrometrica software (http://www.astrometrica.at/default.html?/catalogs.html) I managed to find them (not all although) in the database:
It turned out that one of the faintest stars registered on my picture (marked with red arrow) has visual brightness of 20.7 mag. Magnitude scale is non linear – faintest stars visible in fairly good conditions with naked eye are 5 mag. In excellent conditions we can see stars up to 6-6.5 magnitude. Each one magnitude more means stars 2.5 times fainter, so we can calculate that 20.7 magnitude star is about one million times fainter than stars visible with naked eye under good conditions. It is quite amazing how telescope with only 6 inches diameter can chase astronomical targets under suburb sky with magnitude limited to 5 mag only.
As Pulsar from http://astropolis.pl found out there are even more faint objects in this picture. Here is another Astrometrica screen shot presenting star with apparent magnitude 21.3mag.