Setting up your own dome using Stellarium's spheric mirror distortion feature

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http://www.astro-obsessions/small_planetarium
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Revision as of 05:06, 5 September 2006

People interested in building and operating their own planetarium can subscribe to a YahooGroup, and the related Small Planetarium Forums Site.

http://groups.yahoo.com/group/small_planetarium/

and/or

http://www.astro-obsessions.com/small_planetarium


In the following section I want to describe how you can build your own planetarium using stellariums spheric_mirror distortion feature.

What you need:

  1. A dome. The dome should have a spherical inside. I suppose you can have good results with any dome larger than 2m in diameter, perhaps even with smaller ones.
  2. A spherical security mirror. I recomment a 1/4 mirror, thats a quarter of a sphere. Something like 0.4 or 0.5m in diameter should be ok, for small domes you may need smaller mirrors, decide for yourself.
  3. A video projector
  4. A computer running a current verion of stellarium.

In the following example I assume a dome with 5m diameter and a spheric 1/4 mirror with radius 0.25m. I suggest the following config.ini settings:

[spheric_mirror]
projector_gamma = 0.45
projector_position_x = 0
projector_position_y = 1
projector_position_z = -0.2
mirror_position_x = 0
mirror_position_y = 2
mirror_position_z = 0
mirror_radius = 0.25
dome_radius = 2.5
zenith_y = 0.125
scaling_factor = 0.8
...
flag_gravity_labels            = true
...
flag_menu                      = false

You should be familiar with the keys because there will be no menu exept the tui. When you start stellarium press "PgDown" to get maximum FOV and "CursorUp" in order to look at the zenith. Press "z" 2 times in order to display the azimuth grid. Press "1" in order to get the config window. In the video settings choose the spheric_mirror distortion.

Install the spheric mirror inside the dome. Imagine a coordinate system that is centered in the dome center. The z-axis points upward to the zenith and the y-axis points to some point on the horizon. Look into the direction of the y-axis. Now the x-axis points to a point on the horizon which is near your right hand. In the above config.ini example I suggest that the center of the spheric mirror is at (0,2,0) (I use meters, but feel free to use any units you like).

Install the projector approximately at (0,1,-0.2). Notice the z-coordinate -0.2: the projector is lower than the spheric mirror center so that it shall not occult parts of the image. In my implementation I assume that all light comes from one point, which in reality is not true. The projector_position in config.ini refers to this fictional point. It will lie somewhere inside your projector, I do not know, where.

Turn your projector on and project the image onto the mirror. The upper black curved border of the image should correspond to the upper part of the mirror. Use the projector zoom and the scaling_factor in order to match. Also shift and tilt your projector so that the image zenith and horizon is projected onto your dome zenith and horizon. The azimuth grid should be in place, you know what I mean. Play around with the geometry parameters. In order to get equal illumination throughout the dome, you must set the projector_gamma value to whatever your projector needs.

I hope you will be satisfied with the results. Please send pictures of your new planetarium!

Yours, Johannes

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