Four foci
The Subaru Telescope has four focal points: the primary focus, the Cassegrain focus, and two Nasmyth foci. At the prime focus, light collected by the primary mirror is brought to a focus directly. Its key characteristic is that it captures a wide field of view.
The Cassegrain focus forms an image by reflecting light off the secondary mirror and directing it through a hole in the center of the primary mirror. Compared to the prime focus, it is easier to switch instruments at the Cassegrain focus, making it suitable for a variety of observational purposes.
The Nasmyth foci direct the light, after reflection by the secondary mirror, sideways via a tertiary mirror. Since the orientation of the instruments does not change as the telescope moves, these foci are well suited for mounting large or heavy instruments.
Instruments Mounted at Each Focus
1. Instruments at the primary focus
- Instruments that use the wide field of view of the primary focus
- Maximum diameter of field of view*: 1.5 degrees of arc
- Focal ratio: 2.0 Including corrective optical systems
2. Instruments at the Nasmyth foci (visible-light)
- High-resolution spectroscope and other heavy instruments
- Focal ratio 12.6
3. Instruments at the Nasmyth focus (infrared)
- Instruments compatible with adaptive optics are installed
- Focal ratio 13.6
4. Instruments at the Cassegrain focus
- Maximum diameter of field of view: 0.1 degrees of arc
- Focal ratio 12.2
*Varies by type of observation instrument.
