The Intes Micro Alter M500.

This is the smaller brother of my STF180.  A Maksutov-Cassegrain of 125 mm diameter f/10.  Actually I was looking for a C-5 OTA when I got an offer for this instrument.  The M500 is heavier than a C-5 but the optical quality is much better.  If you manage to get a Maksutov-Cassegrain to ambient temperature (keep it outside in an observatory), these are truly magnificent instruments.