There is the DewBuster
http://www.dewbuster.com/index.html
I now use a Thousand Oaks - got cheap - which is a slow pwm type and is OK. Previously used lab power supply.
Your scope is the best dew point sensor. The best commercial sensors use a cyclically heated-cooled mirror to detect dew. None will track true conditions better than a big piece of glass exposed to the sky.
A "set X degrees above ambient" type is reliable. For set and forget operation in an observatory, use a plug-in mains timeclock to switch power supply on at night. Avoids cooking scope on hot summer days.