Tip for creating horizon limits

Dale Ghent

I'm experimenting with defining horizon limits in APCC in conjunction with another project, and I was trying to figure a good way to get an accurate representation of my site's horizon lines into APCC without using the drawing tool. I suck at drawing, especially with a mouse, and the flattened, linear projection of the horizon tool in APCC adds additional challenge in terms of transcribing what you see.

You can also use the alt+az tools in the horizon tracking limits editor to "trace" the horizon using your telescope and camera/eyepiece, manually dropping points to form your horizon, but this seemed a little time consuming to me and I didn't quite want to set up my gear in the daytime to do this when there was also no nights in the forecast that had weather that was conducive to imaging (I've reached the stage of things where if I set up, I'm setting up to image. Damnit. :D )

So this led me to look into how APCC stores its horizon limit table and alternate ways of creating horizon lines with accuracy.  This led me to 2 findings:

1. APCC stores horizon limits in a flat text file with a simple format: 
<azimuth> <altitude>

A pair of space-separated numbers, one pair per line. Azimuth is a whole number from 0 to 359. Altitude is a floating point number, 0 to 90, and represents the lowest altitude that has clear sky at the corresponding azimuth. Obviously, there are 360 lines of this data in the file. The file is saved with the .hrz file extension, but it is otherwise a plain text file that can be edited in any text editor. 

2. Smart phones and tablets are brimming with inclinometers, and there are a few apps for both iOS and Android which can read these and, ultimately, log their values. Since I have an iPhone, I used an app called Theodolite which, if you're familiar with the instrument, puts a crosshair on the middle of your phone's video feed along with angle and rotation information. A similar app on Android is Dioptra.

In these apps, one turns on logging and traces the horizon with the phone as the app logs altitude and azimuth it is pointing at, usually in 1 degree increments. Once you trace a full 360 degrees, you can turn off logging and save the data. In the case of Theodolite on my iPhone, it saved it to my iCloud drive as a CSV file. I then opened it up on my Mac, extracted the Azimuth and Altitude columns, edited them to remove any duplicate azimuth entries or gaps (sometimes if you turn too fast, there is a gap in azimuth numbers). In the end I had 360 lines of accurate horizon altitude limits. I saved the file as a space-separated file with a .hrz extension and loaded it into APCC's horizon limits editor. The result is in the attached screenshot.

This took me about 10 minutes to do and required no setup of equipment or drawing with a mouse.

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