What hyperparameter values does the LDA mallet model use by default? Is it true that the formula to calculate alpha = 5.0/n(topics)?

I am trying to figure out the default $\alpha$ $\eta$ values used by mallet LDA, but there is not a lot of information on this. I did find a couple of answers, with no proper references, saying that symmetric $\alpha$ can be calculated with 5.0/num_topics? Why is that? Why can't I use 1.0/num_topics to calculate the symmetric $\alpha$, just like in standard LDA? Can someone please help me understand and link me to references?

Thanks in advance.