yep William , Peter is right , but looking at your gear , the 120Ed and HEQ5 should not be to worried with that weight hanging off it , so long as you are careful to balance it right .
I have a Saxon 150mm f/8 refractor ( with Chromocor ) on a HEQ5 in NZ and that set up handles my monster 31mm Celestron Axiom LX @ 700 odd grams and my 17mm T4 nagler easily , so you should be ok .
By the way a 6mm at 500grams ?? is it an Explore Scientific 100 degree ?
Brian.