Perhaps the lowest price in Europe ...

... for a dielectric zenith mirror with 99% reflection?

99% reflection and long life

Thanks to dielectric (non-metallic) coating, the Castell zenith mirror not only has a high reflectance of 99%, but also a long service life. The long service life is achieved because the non-metallic dielectric surface reacts much less with the acids and bases in the air than an aluminum coating with a quartz protective layer.

Ring clamping instead of fixing screws

The 31.7mm socket has a ring clamp with a large knurled screw.

Test: checking the reflection

Test procedure: Take a test photos with a camera through the diagonal (ISO200, manual mode), then measure the brightness value in Photoshop using a histogram.
This test serves as an estimate of the zenith reflection. The following models were tested:

• LACERTA (nominal value:99%)
  Measured: Luminance = 98.9% Green / Visual = 99.2%
  (Very high reflectivity in the entire RGB range, but also the most expensive zenith mirror in this test series. It is identical to other well-known models)
  
• SkyWatcher (nominal value:99%)
  Measured: Luminance = 96.1% Green / Visual = 95.3%
  (evenly in the entire RGB range, but shows less reflectivity than advertised both in luminance and in green / visual)
  
• Castell (nominal value:99%)
  (Visually, it reaches almost 99%; in the blue area, however, a little less reflectivity and therefore a slightly yellow tinge; but is advantageous for Fraunhofer refractors with blue halos)
  Measured: Luminance = 95.3% Green / Visual = 98.2%
  
• SkyWatcher (nominal value:91%)
  Measured: Luminance = 90.3% Green / Visual = 92.4%
  (As advertised, a good zenith mirror without higher reflectivity criteria. The image is, however, much darker than in the other three zenith mirrors)