Saturday, October 20, 2012

more adjustments to the mirrordome

More tweaking after my previous post about mirrordome adjustments in our Planetarium. The distortions of lines which should be vertical becoming slanting on the sides, and of spheres becoming oblate spheroids in projection, were all due to projector and mirror positioning. The default warp settings are for a setup like this, (as shown at Paul's website)

But our mirror and projector were much more angled, like this,

As a result of that, we needed to use keystone correction to make the image on the mirror rectangular in shape (instead of trapezoidal) as seen from the projector as in the somewhat exaggerated drawing below.

The projector and mirror do need to be tilted because ours is a tilted dome, but they should be aligned so that the image on the mirror is rectangular. As Paul pointed out, digital keystone correction reduces the effective number of pixels and so the clarity of the image suffers.

Taking a look at the Bangalore Planetarium as suggested by Paul, found that their Mitsubishi UD 8400 projector with the rated 6500 lumens ($10,000)was giving a much better image, not just in brighness, but also colour accuracy and focus. In comparison, our Optoma EP1080 is 3600 lumens. The pixelation, screen door effect etc was similar, but not so noticeable since their audience sits further away from the dome.

To be done: adjusting projector positioning and colour calibration for more improvements. Also can explore other projectors. Paul also mentioned the Optoma TH7500 (6000 lumens, $9,000) and the Optoma EH2060 (4000 lumens, $1,500) as a cost-effective alternative.

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