ClearSky for Android released

# ClearSky for Android released

After 4 years developing from time to time a planetarium for Android (since the first experiments with this platform back in 2012), I have published the ClearSky planetarium in two versions: a free version quite generous in features and a paid version which is oriented to become useful to amateur astronomers with telescopes. This second page contains a detailed list of features of the commercial version, although the help document included in ClearSky describes everything with even more detail.

The free version contains almost everything a casual observer would need, including the possibility of showing comets and asteroids, which is usually offered only in commercial programs in Android. But the main difference from other tools is the great accuracy in JPARSEC, superior to most free and commercial programs available even on PC platform. In addition, Spanish and English are supported, with absolutely no adds.

## Design and user interface

One of my main concerns when developing ClearSky has been to be as objective as possible, so I haven't taken any other Android planetarium as reference to think about the design or the features. After installing all other free planetariums I think most of them are not really helpful for an amateur astronomer, and even some commercial ones (I admit I haven't paid for any of them, I prefer to enjoy developing my own one) seems to have just 'more options' or objects in the paid version, instead of being focused to 'more activities' like observing with telescopes or planning observations with a list of astronomical events or objects. Most of the development comes from JPARSEC, even in terms of design of the different color squemes, and I have developed this library for years, so this Android planetarium has the benefit of years of experience with many little cosmetic and usability improvements. However, Android development is hard, you have to hit your head against hundreds of walls and overtake all problems until you end up with a finished product.

My point of view for an adequate planetarium for general public is a program that must be extremely easy and confortable to use. I have seen too many planetariums with great graphics, but where it is hard to drag and zoom the sky, and the objects are moving everytime. This seems to be a must for other developers just to aparently mimic the natural sky, but for astronomy you need to take some time to do things and you won't want to have everything moving. Despite this, there are programs reasonably well solved in this sense, for instance the Cosmos Celestron Navigator, but it is still hard to select a body and zoom in/out. In ClearSky the sky is updated in regular intervals, and zoom operations are fast and realiable, a simple click with a finger will identify an object and will show the distance respect the previous body identified, and a double click will center that body without the mess of a menu for just that (which in ClearSky is triggered with a long press offering, among others, the options of details of the object and to track it). An example of less well solved is SkEye, where the zoom and rotation operations will mess everything, making impossible to zoom in a given body. In addition, due to rotation and the use of equatorial positions you don't know where is up, where is the horizon, or the azimuth/elevation direction, and there are too many numbers which are not important for a user. SkEye is surprisingly well valorated despite all this and the fact that I've seen three degrees of error in the position of Jupiter, so I won't call it accurate… ClearSky, although it is not as beautiful as other planetariums (although I think well enough worked in that sense), it has a much more useful user interface, making the program a really useful and confortable tool, not a simple toy to play a few minutes with. For instance, you can directly change from drag to zoom or the opposite all the time (keeping always at least one finger on the screen), something not possible in other planetariums.

Another issue for me is to make the user interface reasonably beautiful and confortable to the eye. This means using the adequate number and distribution of options and adequate icons. In the main window the icons have colors, and there are six for completely different tasks. SkEye shows 9!, which are too many for a phone and some of them are related to the same thing, like changing simulation conditions or the aspect, things that are not oriented to 'activities'. In ClearSky you have a search button which is a must, an option to change the color squeme which is just useful because many users expect and will play with that, and a help/trivia/more options icon (it is configurable) on the right corner of the first row (easier to click with the finger). The trivia is a game to play, just funny if you play it for a few minutes from time to time. I think it is important to have at least one configurable option for the user. The second row shows the view mode (live, text, augmented reality, and chart modes), the list of astronomical events, and the configuration option, again easier to click in that position. Since in the free version there are only two modes (live and chart), the first option will simply swap both of them, in the paid version there is a menu inevitably. The astronomical events is another must in my opinion, since a user will want to know what's going on in the sky, and if the full moon or the astronomical twilight will limit the time interval of really dark skies. All configuration options are provided through the configuration option, although the most important of them are provided in the configurable option, when it is configured to show a menu called 'more options'. The number of configurable options are a lot, almost 100, but they are properly categorized and offered in two levels, showing by default less options. The interfaz section has an option to change for a simple to a complete user interface, showing all posible options.

## Text mode

The text mode is something probably missed in any other Android planetarium. It is only available in the commercial version, since this feature is specific to amateur astronomers, not the general public. It lists all deep sky objects and the main double/variable stars, sorted by name, object type, magnitude, position in the sky, or transit time. Supernovae and novae will also appear if you enable them, but only in case they are visible in the sky also (field of view around 50 degrees or lower). There is an option to set an alarm for the transit time (as in the events obviously), in case you want to observe some bodies in their greatest elevations. You can set an object as reference, so that the azimuth/elevation position (or right ascension/declination in case you prefer equatorial coordinates in the configuration) will appear as offsets respect that reference body. The position column will change to distance, so you can sort objects by their distances to the reference one. Very useful to observe interesting objects close to the one you are currently observing. In case you go to the text mode from the live view mode, the text mode will be also 'live', sorting the objects by default respect their distances to the direction the device points to. If you hold the device adequately on top of the telescope, you can use this feature to convert any telescope into a push-to one. In case the device doesn't point perfectly to the object, there is an align option in this case, replacing the option to set an object as reference. In case you just want a few objects you like you can add objects to a list of favourites and show only them.

## Astronomical events

The list of astronomical events is useful to prepare an observation night. In addition to the main general events offered in the free version, the paid version includes events related to natural satellites (main Jupiter, but also for Saturn and Uranus). The events for natural satellites will show even mutual events of natural satellites (for instance a moon of Jupiter occulting or eclipsing another moon), something very interesting for amateur astronomers and probably only offered in ClearSky because of its great accuracy. The list of events for artificial satellites will show the next transits of the main satellites (ISS, HST, and Tiangong 1), as well as their transits on top of the solar and lunar disks. Iridium flares are also computed and simulated.

## Astronomical equipment

The commercial version of ClearSky can show the field of view of any telescope with horizontal or equatorial mounts. When a telescope is selected, a long click on a star will add another option in the context menu allowing to test the polar alignment of that equipment in that star and in that moment. The program asks for four values (see documentation) and will compute from the deviations of the star (measured in pixels in the camera) where the mount is really pointing to, so that an incremental correction to improve the alignment is possible. The feature is not completely in its final status, but works.

## Other features

The links provided at the top will list most of the features of ClearSky, but I would like to emphasize some of them. First, the catalog of stars, and specially deep sky objects, are very robust. I worked myself on the deep sky catalog for years, fixing and improving things with time and checking coordinates with Simbad (it is based on the revised NGC).

You can reach magnitude 16 in the commercial programs with the only condition of having network connection (previously downloaded fields can be used offline too). Other programs requires 1 GB of star data in your device, which is absurd for a phone. It is something spectacular to resolve globular clusters like M13 in stars, even without deep sky textures, and how star positions and textures match completely.

700 textures of deep sky objects are overlaided on the sky with great accuracy, corrected by precesion and nutation. The commercial version has high resolution textures with the possibility of downloading more.

Visual quality and accuracy are worked to a great level, so that accurate and realistic planetary rendering is possible in ClearSky. It is not as fast as I would like, but good enough (illumination and everything is done pixel by pixel without 3d OpenGL). You can even identify planetary features when clicking with the finger, something in fact used to allow simulating the sky from other bodies.

There are also useful color squemes, like these two screenshots show:

And if you are curious about the trivia, here are two screenshots of it. There are more than 200 different questions, implemented in Spanish and English, although many of them are related to identifying objects or constellations.

## Accuracy

I will not add more words about accuracy, I have already talked about that before, comparing JPARSEC with other PC programs. In ClearSky the accuracy means this program is suitable to studies in the field of ancient arqueoastronomy, since proper motions of stars (the natural change in the shape of constellations) are considered, and planets will appear up to year 3000 B.C. Obviously more accuracy means more complicated and slower algorithms, and this is less compatible with showing the sky in real time (with objects moving), specially in a platform with strong memory and speed limitations. I don't like it, but I admit it can be required to track artificial satellites which moves fast. I currently have a list of a few bugs I have to correct, and some features which will be implemented with time to end up with a product of my (and hopefully also others) like. The hard work is done, but this is just the first release.

## Discussion

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