I will try to list a few of my favorite astronomy apps for iOS. Caveat: I will have to start with a couple of my own apps, since by definition (being designed by myself, exactly as I wanted them), as far as I am concerned they are the best at what they do, so I cannot objectively rank them among others. Hopefully you will forgive that little bias. In any case, this is not a definitive list, I may add (or remove) apps as new apps or new versions come out.
Polar Scope Align
Current price: FREE (Basic), $1.99 (Pro), $3.99 (Pro Watch)
Thousands of users turn to Polar Scope Align for their polar alignment needs, and with good reason – it supports probably every polar scope used in the field including ones from iOptron, Astro-Physics, Takahashi, Orion, Celestron, Meade, Vixen, Astrotrac, Losmandy, Kenko, Avalon, even early 80’s designs like the Tuthill, or finders like the Telrad. Basically, if you have a polar scope that is not supported, contact me and with some help (e.g. images), I will add it. What’s more, for many of these polar scopes, especially the less precise ones, if you read the in-app instructions you will discover that Polar Scope Align offers you an alternative method of aligning with them to improve their precision.
And all the functionality for polar alignment with all those polar scopes is included in the free version. But there are also the Pro and Pro Watch versions which add tools that I like to use for my astrophotography session: From weather and bubble level to calculators and a Deep Space Object database. The latter is not actually a very well known feature of the app, as it is not very related to polar alignment, but it is probably the feature I use the most in an astrophotography session. Take a look at the video below for a lesser known Deep Space Object database feature that I use often.
Read more about Polar Scope Align.
Pro Watch Version
Current price: FREE (Basic), $14.99 (Plus), $39.99 (Pro)
Most people consider SkySafari to be the best astronomy app overall and, well, they are probably right. It starts out as an excellent planetarium-type program in its Basic (currently free) version, and continues on to be a space-simulator and telescope control software in the Plus/Pro versions. The only gripe I’ve heard about this app is that you have to pay to get a new major version, although it is in any case worth the functionality you pay for at the time, even if it means they might make a better version in the future that you can’t upgrade to.
So, the Basic version does the base planetarium-type stuff that a few other (sometimes free) apps mostly do as well, although it is a solid implementation and easy UI which makes it rank towards the top of this category. If it is free, it is a no brainer. However, the most popular version of SkySafari is probably the “Plus” version. It adds a bigger object database, observation planning tools, object visibility graphs, space-simulator abilities and telescope control. Most amateur astronomers won’t need anything more, but there is also a more expensive “Pro” version which adds the ability to get extensive object databases (like PGC galaxies).