I have used this App with an iPhone attached to a telescope and captured many objects in the night sky. Numerous star clusters and brighter nebulas have turned out surprisingly well and are easily recognizable. Astro photos are not as simple to take as a snapshot and some post processing is usually needed. NightCap Pro has the ability to average many exposures together, thereby reducing noise in the picture. When numerous pictures have been taken in this way, (NightCap Pro can automate multiple exposures) stacking them together can yield impressive results.
NightCap Pro is a powerful program and is the best Ive found for this type of imaging on an iPhone. It is easy to use and has worked consistently. I have not tried all the modes and features that the program has to offer yet but so far so good. The developer is responsive and helpful.
Nicknametaken111 about NightCap Camera