I concur with the posts indicating your home owners coverage on it own is insufficient, but getting a rider should be no problem. While my HiFi gear fits withing my home owners coverage my astronomy gear does not.
When I did my riders with Safeco I was told to bundle multiple items together to create "systems" and get a rider for each system. This was more cost effective. I was also told to just state what I wanted it insured for as some items cannot be directly replaced and there could be much debate in replacement value. So for my one telescope I simply said $50,000. This included the telescope and several high end eyepieces. Think amp, pre-amp, turntable, speakers. In the end I'm paying to insure an amorphous $50,000 thing.