Celestron CR-150 HD Using the Lens Cap Aperture Stop, Observing the Sun, Solarobservinghints

Models: CR-150 HD

1 59
Download 59 pages 47.13 Kb
Page 37
Image 37
Using the Lens Cap Aperture Stop

Using the Lens Cap Aperture Stop

NOTE:

Observing the Sun

Asmentionedintheintroduction,allrefractiveopticswillexhibitsomeamountof chromatic aberration due to the prism effect of lenses. Chromatic aberration will become more pronounced the farther the incoming light rays are off-axis (i.e. passing through the edge of the objective lens) and virtually unnoticeable on-axis (passing through the center of the objective lens). This type of aberration is only evident when observing very bright sources of light, such as bright planets and very luminous stars (like Sirius). There are several techniques that the observer can employ to suppress visible signs of chromatic aberration, these include; reducing the aperture and using filters.

The objective lens cap covering the objective lens of the telescope has a built-in 112mm aperture stop in the center. By leaving the lens cap on the telescope with the aperture stop removed, you will allow all the incoming light to pass closer to the center of the optical axis. Since, most planets are extremely bright objects (visible to the unaided eye) any loss of light from reducing the aperture will be unnoticeable.

The lens cap should always be completely removed when observing deep-sky objects such as galaxies and nebulae, where aperture (light gathering power) is essential and chromatic aberration is not an issue.

Another useful technique for reducing aberrations and improving planetary detail is the use of colored eyepiece filters. Filters are commonly used to bring out particular planetary detail, such as the polar caps on Mars or the bands and zones around Jupiter. The use of colored filters (especially red and green filters) reduces the effect of chromatic aberration as well as improves contrast and resolution.

Although overlooked by many amateur astronomers, solar observation is both rewarding and fun. However, because the Sun is so bright, special precautions must be taken when observing our star so as not to damage your eyes or your telescope.

WARNING:

Never project an image of the Sun through the telescope. Because of the folded optical design, tremendous heat build-up will result inside the optical tube. This can damage the telescope and/or any accesso- ries attached to the telescope.

For safe solar viewing, use a Celestron solar filter. This filter reduces the intensity of the Sun’s light, making it safe to view. With this filter you can see sunspots as they move across the solar disk and faculae, which are bright patches seen near the Sun’s edge. Be sure to cover the objective lens of the finder or com- pletely remove the finder when observing the Sun. This will ensure that the finder itself is not damaged and that no one looks through it inadvertently.

SOLAROBSERVINGHINTS

The best time to observe the Sun is in the early morning or late afternoon when the air is cooler.

To locate the Sun without a finder, watch the shadow of the telescope tube until it forms a circular shadow.

Celestial Observing • 37
Page 37
Image 37
Celestron CR-150 HD Using the Lens Cap Aperture Stop, Observing the Sun, Solarobservinghints, Celestial Observing