Handleiding Celestron CPC Deluxe 800 HD - 11007 (pagina 24 van 50) (Engels)

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In each hemisphere, there is a point in the sky around which all the

other stars appear to rotate. These points are called the celestial poles

and are named for the hemisphere in which they reside. For example,

in the northern hemisphere all stars move around the north celestial

pole. When the telescope’s polar axis is pointed at the celestial pole, it

is parallel to the Earth’s rotational axis.

Polaris

(North Star)

Fall

Spring

Summer

Winter

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Many methods of polar alignment require that you know how to nd

the celestial pole by identifying stars in the area. For those in the

northern hemisphere, nding the celestial pole is not too dicult.

Fortunately, we have a naked eye star less than a degree away. This

star, Polaris, is the end star in the handle of the Little Dipper. Since the

Little Dipper (technically called Ursa Minor) is not one of the brightest

constellations in the sky, it may be dicult to locate from urban areas.

If this is the case, use the two end stars in the bowl of the Big Dipper

(the pointer stars). Draw an imaginary line through them toward the

Little Dipper. They point to Polaris (see Figure 6-6). The position of the

Big Dipper changes during the year and throughout the course of the

night (see Figure 6-5). When the Big Dipper is low in the sky (i.e., near

the horizon), it may be dicult to locate. During these times, look for

Cassiopeia (see Figure 6-6). Observers in the southern hemisphere are

not as fortunate as those in the northern hemisphere. The stars around

the south celestial pole are not nearly as bright as those around the

north. The closest star that is relatively bright is Sigma Octantis. This

star is just within naked eye limit (magnitude 5.5) and lies about 59 arc

minutes from the pole.

The north celestial pole is the point in the northern hemisphere

around which all stars appear to rotate. The counterpart in the

southern hemisphere is referred to as the south celestial pole.

Declination Drift Method of Polar Alignment

This method of polar alignment allows you to get the most accurate

alignment on the celestial pole and is required if you want to do long

exposure deep-sky astrophotography through the telescope. The

declination drift method requires that you monitor the drift of selected

stars. The drift of each star tells you how far away the polar axis is

pointing from the true celestial pole and in what direction. Although

declination drift is simple and straight-forward, it requires a great deal

of time and patience to complete when rst attempted. The declination

drift method should be done after any one of the previously mentioned

methods has been completed.

To perform the declination drift method you need to choose two

bright stars. One should be near the eastern horizon and one due

south near the meridian. Both stars should be near the celestial

equator (i.e., 0° declination). You will monitor the drift of each star

one at a time and in declination only. While monitoring a star on the

meridian, any misalignment in the east-west direction is revealed.

While monitoring a star near the east/west horizon, any misalignment

in the north-south direction is revealed. It is helpful to have an

illuminated reticle eyepiece to help you recognize any drift. For very

close alignment, a Barlow lens is also recommended since it increases

the magnication and reveals any drift faster. When looking due south,

insert the diagonal so the eyepiece points straight up. Insert the cross

hair eyepiece and align the cross hairs so that one is parallel to the

declination axis and the other is parallel to the right ascension axis.

Move your telescope manually in R.A. and DEC to check parallelism.

First, choose your star near where the celestial equator and the meridian

meet. The star should be approximately within 1/2 an hour of the

meridian and within ve degrees of the celestial equator. Center the star

in the eld of your telescope and monitor the drift in declination.

• If the star drifts south, the polar axis is too far east.

• If the star drifts north, the polar axis is too far west.

Make the appropriate adjustments to the polar axis to eliminate any

drift. Once you have eliminated all the drift, move to the star near the

eastern horizon. The star should be 20 degrees above the horizon and

within ve degrees of the celestial equator.

• If the star drifts south, the polar axis is too low.

• If the star drifts north, the polar axis is too high.

Again, make the appropriate adjustments to the polar axis to eliminate

any drift. Unfortunately, the latter adjustments interact with the prior

adjustments ever so slightly. So, repeat the process again to improve the

accuracy checking both axes for minimal drift. Once the drift has been

eliminated, the telescope is very accurately aligned. You can now do

prime focus deep-sky astrophotography for long periods.

NOTE: If the eastern horizon is blocked, you may choose a star near

the western horizon, but you must reverse the polar high/low error

directions. Also, if using this method in the southern hemisphere,

the direction of drift is reversed for both R.A. and DEC.

Polaris

(North Star)

Big Dipper

Little Dipper

Cassiopeia

N.C.P.

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Celestron CPC Deluxe 800 HD - 11007 Handleiding

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