An easy way to make a horizon profile
Many planetarium programs allow for the creation of a custom horizon profile. Such profile shows any obstacles on the horizon such as trees, buildings, fences, etc. With that information it is a lot easier to know whether you have clear line of sight to an object, or whether the view is obstructed by a rooftop or tree. One particular use-case in the AstroWorldCreations observatory is the use of MountWizzard mount modelling software. By its nature this software will plot a pattern of observation points across the whole sky to build the model. So almost by definition it will hit some obstructed views. The software makes use of a horizon profile to prevent this to happen.
For a stationary observatory, a horizon profile is a one-time event. But for a mobile setup, the horizon profile will be different each time you setup the telescope in a slightly different spot. So making a horizon profile is a default step in the setup process. Standing outside following the compass, estimating the altitude of objects, noting it down and later programming it in the software can be time-consuming. In this post a faster and easier way is described to do this. All you need is a mobile phone (iPhone in the explanation below) and SkySafari Pro.
The first step is to enable screen recording on the iPhone. This is done under Settings > Control Centre. Make sure that ‘Screen Recording’ is under ‘Included Controls’. If it is not there, it is under ‘More Controls’. Simply click the green ‘+’ sign and the tool will be added to the ‘Included Controls’. In the Control Centre the dot with the concentric circle should be visible. Pressing that button will initiate screen recording. While recording, there will be a red button in the top left part of the screen. Tapping this will stop the recording. Recordings are stored in the photos library.
The next step is to open SkySafari Pro on the iPhone. Tap on the menu option at the bottom called ‘Compass’. The icon should change from a compass needle to a circle with a diagonal line. The sky now follows the movement of the phone. The screen shows the objects in the sky where the phone is pointed at. At the bottom of the screen, there is a little button labelled ‘AR’. This stands for Augmented Reality. Tapping that button will add the real image from the camera, with the night sky as an overlay. The relative brightness of the image and the overlay can be changed by swiping up and down. Swiping up makes the camera image brighter. Swiping down makes the overlay brighter.
The next step is to stand at the point where the telescope is or will be set up. Point the camera to the north and start the screen recording. Stay at the same place, but quietly turn around while holding the phone up, constantly ensuring that all horizon obstacles are properly in the image. Once the whole 360º has been made, screen recording can be stopped. You now have a video on your phone that you can manually scroll through, assessing altitude and azimuth numbers for all horizon obstacles. It is important to do this while there is enough light for the camera to make a reasonable video. If it is too dark it will be hard to recognise the various obstacles.
Now it’s time to put this information into the software where the horizon profile will be used. In the case of MountWizzard this is quite simple. It can be done graphically in the 'Hemisphere’ window. Set the Operation Mode into ‘Edit Horizon Mask’. Click on any coordinate and a profile point will be added. Click an existing point with the right mouse button and the point is deleted. It is not possible to move points around. Alternatively, a small text-file with Altitude and Azimuth values of the horizon points can be edited/created with a standard text editor. The extension of the file needs to be *.hpts. Here is an example of such a file. In MountWizzard, under the menu ‘Model Points’, horizon mask files can be loaded and saved, and settings can be turned on or off to delete points below horizon.
In KStars it is possible to define what is called an ‘Artificial Horizon’. Under the Settings menu, select ‘Artificial Horizon…’ and enter Azimuth-Altitude values. This can be done manually or by clicking points on the map. For both options, the method described above can be used to recreate the actual horizon of the observatory. However, the artificial horizon function in KStars is not very intuitive and seldom used.