It's highly recommended to take guide for Jantar mantar, as you won't be able to understand the use of all the yantras otherwise or be sure to google and go before taking a self tour of the place.
How to reach:
Nearest metro station is Patel chowk (yellow line metro) and Rajiv chowk (blue line metro). It's walking distance from both metro stations.
Entry ticket is ₹15 per person
And there is no ticket for children upto 15 yrs
Sunrise to Sunset
When to visit:
Jantar Mantar is open all days of the week ad you can plan a visit to this place even on weekends. Eatables are however not allowed in Jantar Mantar because of the clean initiative by the government