iObserve is the astronomical app you were waiting for. It is a Mac app (10.9+) that allow you to push the preparation of your observations to a much higher level. It is a 6-years old app, and after more than 4 years as a paid app in the MacAppStore, it is now free. You can read here why it is so. Read below what others have said about it.

Objects & Plots

Object Information

Finding Charts

Observatories

Converters

Websites & Miscellaneous

What others have said about iObserve (emphasis are mine!):

• I love it (★★★★★) - Version 1.4 (the one currently in the App Store) - Mar 25, 2015: "Cool Airmass plot. Easy access to finder charts from SDSS, DSS, 2MASS, and AAVSO. One click for data from VizieR, SIMBAD, NED and WikiSky. Fancy tools to convert coordinates, time, flux, and distance. All I need for easy peasy observation planing." (a new German user).

• Best in its class (★★★★★) "There is no other app for planing astronomical observations like iObserve. Because of its all-in-one approach, iObserve is also a discovery machine, allowing the user to make connections that were not obvious. And this is perhaps the best thing that can be said about any app. And the author has an amazing roadmap for the app, I can only hope he achieves it. Truly stellar, well done!" (a happy austrian user)

• "Must have for serious astronomers" (★★★★) "Every observer with a mac should have this app, and observers without a mac should buy a mac so they can use this app. It has proven invaluable on multiple observing runs both in the planning phase an while observing." (a happy US user). 

• "Efficient observing " (★★★★) "I witnessed the development of this app from its early beta versions and am pleased that it turned out to be much more stable and clean. I use it everytime I go observing at large observatories [...]." (a canadian "Some Dude") 

• "iObserve" (★★★★) "The Application works extremely well. I needed support and it was very efficient, i had an answer in a few hours." (some enthusiastic italian 'Mr. Fox')

• "A must have app for all serious astronomers" (★★★★) "This little application has now matured very nicely from its origins a few years ago. (...) It will appeal to serious astrophotographers, deep sky enthusiasts and – of course – professionnal astronomers. Yes, you can find the same information if you scout around the web, but iObserve puts it all in once place, and a lot more besides. I couldn't work without it. And the developer, Cédric, provides outstanding support." (a nice follower from Australia)

Image Credits: E.S.O./L. Calçada