Take a look at
this 500m lens, for example.
It is very light (700g), and with
suitable adapter you can fit eyepieces on it and use it as telescope/spotting scope.
According to Bob Atkins,
here and
here it is not brilliant, but I found
my specimen is much better than his. His results are worse probably because he noted that lens does not have focus backlash... but I found it has, so focusing is a bit delicate operation.