Divide and conquer

A project log for explorad

Work in progress: The explorad combines a microcontroller, a display, and star charts in a head-up display for astronomical telescopes.

christophChristoph 06/14/2014 at 16:170 Comments

Until I have figured out what catalogues I want to include in my device's database, I can work on the representation of the celestial sphere as a number of patches. In a system with very limited memory and low CPU power (even though the mk20dx256 has lots more than the AVRs I worked with earlier), I need to partition the data.

Starting with an icosahedron, I will divide the sphere into a number of patches (triangles), from 20 upwards. Refining it by dividing each patch into 4 (or 9, that works as well) new triangles, I can get a larger number of smaller patches, and so on - resulting in 80, 320, ... - you get the idea.

What level of refinement is "right"? That is subject to the following considerations:

And now it's time for some math. The surface ratio between a spherical cap and the whole sphere is

Putting in the display's diagonal yields

for h, so we get

which means that the device display 1/268th of the sky. This now must be divided by the number of patches we display at a time, which is 13. One patch must therefore cover 1/3484th of the sky - that's a lot of patches! A possible icosahedron refinement that gets close to this is 20x4x4x9 = 2880. I'll go for that.

If you find seríous errors in the above, please tell me!

Each patch will be stored in a file on the SD card. The information stored per patch is: