The 50cm Telescope

This telescope is constructed for the general citizen, and the main purpose is to share similar interests in astronomy and researchers' excitment. In addition, this telescope is used for full-scale observations for astronomical studies at visible wavelengths with a cooled CCD camera.

50cmTelescope Purposes of use are as follows.

  • Star Party for general public
  • Observation training for students
  • Acquiring pictures for publicity activities
  • Astronomical observations
  • Experimental observations for developments of observational equipment
The 50cm Telescope: Basic Optical Parameters
Type: Cassegrain reflector
Mounting: Fork equatorial mount
Primary mirror diameter:50 cm
Focal length:6030 mm
Angular resolution:0.3 arcsecond (theoretical limit)
Light-gathering power:5000 times of naked-eye
Naked-eye limiting magnitude:16 (under an ideal dark night sky)
Cooled CCD camera limiting magnitude: 21 (Under a clear sky at Mitaka)

Power of Telescopes

A telescope enables us to observe a fine pattern and faint stars thanks to its large aperture. As you know, main roles of telescopes are to collect light and to magnify distant objects.

As for concentration of light, larger mirrors or lenses telescope have, more light they can collect. A human pupils has a diameter of about 7mm, while our telescope has a main mirror 50 cm in diameter. The power of collecting light for our telescope is about 5000 times larger than your eyes.

As for magnification, the lowest and the highest are determined by the aperture. It is not reasonable to raise the power recklessly. The maximum magnification equals to the numerical value of the aperture at the unit of the millimeter as a rough guide. The minimum magnification equales the aperture (mm) divided by 7 mm (the diameter of the human pupils). Thus, in case of the 50 cm aperture, the maximum magnification is 500 times, and the minimum magnification becomes 71 times.

Astronomy Cameras vs. Your Eyes

"Can we watch images of celestial objects as cool as those taken by the Subaru Telescope or the Hubble Space Telescope?"
- Regret to say, no. The reason comes mainly from the following two facts.

First, cameras mounted on such telescopes can store photons from objects for hours, which enables us to obtain diffuse structures of dim objects. On the other hand, human eyes cannot take long-exposure photographs. So we are not good at watching faint objects.

Second, the earth has the atmosphere. The Hubble Space Telescope was launched into outer space to put out the eye over the cumbersome air layers. Atmospheric turbulence makes stellar images lambent, just as stones in the bottom of a river seem wobbling. Even the Subaru Telescope located at the summit of Mauna Kea, one of the best sites for astronomy observation, equips an instrument for compensating the atmospheric turbulence. Such instruments are too expensive to mount on your glasses, so it is difficult to obtain clear images at the Star Party.

However, you should remember photons you see come directly from a celestial object. Your eyes are the terminal station of photons after a long - hours for objects in the solar system, or, even hundreds of years for the Galactic objects - travel through the universe.