I measured the sizes of some stars on the Young Harris College planetarium's 12.2-meter (40-foot) dome. I also enlisted the help of colleagues at Victor Valley Community College (9.1-meter = 30-foot dome) and Cleveland Museum of Natural History (12.2 dome) to measure the sizes of the same stars made by their Zeiss ZKP-3 and ZKP-3S machines, respectively. The star sizes are given in the table below along with the sizes for my old Spitz A3P (7.3 meter = 24-foot dome). Because all four facilities have different size domes, I have scaled the star sizes to what they would be on a 12.2-meter (40-foot) dome using the formula: scaled size = measured size * (20/facility’s dome radius in feet). For example, Vega on my old 24-foot dome projected by the Spitz A3P was 5 mm, so the scaled size = 5 mm * (20/12) = 8.33 mm.
| Star | Chronos (mm) |
Zeiss ZKP-3S (mm) |
Zeiss ZKP-3 (scaled up; mm) |
Spitz A3P (scaled up; mm) |
| Vega | 10 |
20.64 |
14.67 |
8.33 |
| Rigel | 9 |
25.4 |
14.67 |
11.67 |
| 5 Ursa Minoris | 4 |
3.18 |
2.27 |
11.67 |
| Cursa (Beta Eridanus) | 5 |
6.35 |
4.13 |
16.67 |
I estimate the uncertainties in the measurements as follows: my Chronos + A3P measurements to 0.5 mm, values for Zeiss ZKP-3 given to me to 0.1 mm, values for Zeiss ZKP-3S quoted to 1/32-inch (=0.8 mm). A mix of bright stars and dim stars were measured to account for the fact that star projectors use lenses for their bright star images. The Zeiss ZKP-3(S), Goto Chronos, and Spitz A3P do fine on the bright stars but the A3P has bigger faint stars.