Manual Bresser AR-90 (page 26 of 44) (English)

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Looking at or near the Sun will cause instant and irreversible damage to your eye!

ap p e n d I x a: ce l e s t I a l c o o r d I n a t e s

Every celestial object can be exactly determined by these coordinates.

Using setting circles prerequisites an advanced observing technique. If

you use them for the first time, first point a bright star (the guide star)

with known coordinates and adjust the setting circles to them. Now

you can do a “star hop” to the next star with known coorditates and

compare the setting circles with them. By this way, you will learn which

precise handling is necessary for a successful pointing.

Locating the Celestial Pole

To get basic bearings at an observing location, take note of where the

Sun rises (East) and sets (West) each day. After the site is dark, face

North by pointing your left shoulder toward where the Sun set. To pre-

cisely point at the pole, find the North Star (Polaris) by using the Big

Dipper as a guide (Fig. 35).

Note:

For nearly every purpose (except long-term astrophotography)

average settings of the mount’s azimuth and latitue are sufficient.

Therefore it is not necessary to spend too much time on perfekt

aligning the celestial pole!

Setting Circles

Setting circles included with the Messier-Series models permit the

location of faint celestial objects not easily found by direct visual

observation. With the telescope pointed at the North Celestial Pole,

the Dec. circle (19, Fig. 1d) should read 90° (understood to mean

+90°). Each division of the Dec. circle represents a 1° increment. The

R.A. circle (31, Fig. 1d) runs from 0hr to (but not including) 24hr, and

reads in increments of 5min.

Using setting circles requires a developed technique. When using

the circles for the first time, try hopping from one bright star (the

calibration star) to another bright star of known coordinates. Practice

moving the telescope from one easy-to-find object to another. In this

way, the precision required for accurate object location becomes

evident.

To use the setting circles to locate an object not easily found by direct

visual observation:

Insert a low-power eyepiece, such as a 25mm, into the focuser assem-

bly. Pick out a bright star with which you are familiar (or is easily locat-

ed) that is in the area of the sky in which your target object is located.

Look up the R.A. coordinate of the bright star, and also of the object

you wish to locate, in a star atlas. Point the object at the bright star.

Then loosen the R.A. setting circle lock knob (32, Fig. 1d) and turn the

R.A. setting circle to read the correct R.A. coordinate of the bright star;

lock the R.A. setting circle lock knob onto the object. Next, loosen the

R.A. lock (33, Fig. 1d) and turn the telescope in R.A. to read the cor-

rect R.A. coordinate of the object. Tighten the R.A. lock (33, Fig. 1d).

If the procedure has been followed carefully, the desired object should

now be in the telescopic field of a low-power eyepiece.

If you do not immediately see the object you are seeking, try searching

the adjacent sky area. Keep in mind that, with the 25mm eyepiece, the

field of view of the Messier series is about 0.5°. Because of its much

wider field, the viewfinder may be of significant assistance in locat-

ing and centering objects, after the setting circles have been used to

locate the approximate position of the object.

Fig. 34: Locating Polaris

Polaris

Ursa Minor

Ursa Maior

Cassiopeia

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Bresser AR-90 Manual

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