I want to know if there is a tool to know the correlation between BTC and other altcoins. Thank you
See LeGaulois's post above it should give some good insight into this.
But coming from a more experience based and subjective state, I have seen altcoins following bitcoin many times when the bullish trend starts. This follows a plateau and then an altcoin bleeding out as people start selling them. Bitcoin tend to maintain its plateau for longer periods before going bearish again.
Singularly an altcoin rarely do start a bullish trend but it is very short lived, often speculated to be coordinated pump and dumps backed by advisors to scalp off the market and exit the project they were advising. This sector is like that, if you are buying bitcoin, you can remain in peace if you bought at low prices and hoping for the long term profit. If you bought altcoins then it is a never ending cycle of anxiety and depression. Some do make money off it too.