Serial devices may be assigned to a group and the user should be a member of the
group.  If that is done, then the permission may be set with 

chmod g+rw ttyS0

However, for universal access use

chmod a+rw ttyS0

since this will always work, unless you need to restrict the telescope control
to specific users.

The rules file goes in /etc/udev/rules.d to automatically assign permission to a
USB to serial converter.  It is set for the specific vendor and model we use,
and should be modified as needed.

We have had one case of a USB to serial converter failing at subfreezing
temperatures, out of its "home use" design range.  This caused a race condition
in the USB subsystem leading to a system crash.  One first use of a USB
converter, check its status with

lsusb : see that it is installed

ls /dev/ttyUSB* : inspect permissions

tail --lines=200 /var/log/messages | less  :  look for errors