Use of the R.A. and Dec Slow-Motion Control Cables

Use of the R.A. and Dec.

Slow-Motion Control Cables

The R.A. and Dec. slow-motion control cables allow fine adjustment of the telescope’s position to center objects within the field of view. Before you can use the cables, you must manually “slew” the mount to point the telescope in the vicinity of the desired target. Do this by loosening the R.A. and Dec. lock knobs and moving the telescope about the mount’s R.A. and Dec. axes. Once the telescope is pointed somewhere close to the object to be viewed, retighten the mount’s R.A. and Dec. lock knobs.

The object should now be visible somewhere in the tele- scope’s finder scope. If it isn’t, use the slow-motion controls to scan the surrounding area of sky. When the object is visible in the finder scope, use the slow-motion controls to center it. Now, look in the telescope with a long focal length (low magnification) eyepiece. If the finder scope is properly aligned, the object should be visible somewhere in the field of view.

Once the object is visible in the telescope’s eyepiece, use the slow-motion controls to center it in the field of view. You can now switch to a higher magnification eyepiece, if you wish. After switching eyepieces, you can use the slow-motion control cables to re-center the image, if necessary.

The Dec. slow-motion control cable can move the telescope

amaximum of 25°. This is because the Dec. slow-motion mechanism has a limited range of mechanical travel. (The R.A. slow-motion mechanism has no limit to its amount of travel.) If you can no longer rotate the Dec. control cable in a desired direction, you have reached the end of travel, and the slow-motion mechanism should be reset. This is done by first rotating the control cable several turns in the opposite direction from which it was originally being turned. Then, manually slew the telescope closer to the object you wish to observe (remember to first loosen the Dec. lock knob). You should now be able to use the Dec. slow-motion control cable again to fine adjust the telescope’s position.

Tracking Celestial Objects

When you observe a celestial object through the telescope, you’ll see it drift slowly across the field of view. To keep it in the field, if your equatorial mount is polar aligned, just turn the R.A. slow-motion control cable. The Dec. slow-motion control cable is not needed for tracking. Objects will appear to move faster at higher magnifications, because the field of view is narrower.

Optional Motor Drives for Automatic Tracking

An optional DC motor drive can be mounted on the R.A. axis of the equatorial mount to provide hands-free tracking. Objects will then remain stationary in the field of view with- out any manual adjustment of the R.A. slow-motion control cable.

Understanding the Setting Circles

The setting circles on an equatorial mount enable you to locate celestial objects by their “celestial coordinates”. Every

object resides in a specific location on the “celestial sphere”. That location is denoted by two numbers: its right ascension (R.A.) and declination (Dec.). In the same way, every location on Earth can be described by its longitude and latitude. R.A. is similar to longitude on Earth, and Dec. is similar to latitude. The R.A. and Dec. values for celestial objects can be found in any star atlas or star catalog.

The R.A. setting circle is scaled in hours, from 1 through 24, with small marks in between representing 10-minute incre- ments (there are 60 minutes in 1 hour of R.A.). The lower set of numbers (closest to the plastic R.A. gear cover) apply to viewing in the Northern Hemisphere, while the numbers above them apply to viewing in the Southern Hemisphere.

The Dec. setting circle is scaled in degrees, with each hash mark representing 1° increments. Values of Dec. coordinates range from +90° to -90°. For Northern Hemisphere observ- ers, use the numbers on the setting circle that are closest to the eastern horizon. The 0° mark indicates the celestial equator; values north of the Dec. = 0° mark are positive, while values south of the Dec. = 0° mark are negative.

So, the coordinates for the Orion Nebula listed in a star atlas will look like this:

R.A. 5h 35.4m Dec. -5° 27'

That’s 5 hours and 35.4 minutes in right ascension, and -5 degrees and 27 arc-minutes in declination (there are 60 arc- minutes in 1 degree of declination).

Before you can use the setting circles to locate objects, the mount must be well polar aligned, and the R.A. setting circle must be calibrated. The Dec. setting circle has been perma- nently calibrated at the factory, and should read 90° when- ever the telescope optical tube is parallel with the R.A. axis.

Calibrating the Right Ascension Setting Circle

1.Identify a bright star near the celestial equator (Dec. = 0°) and look up its coordinates in a star atlas.

2.Loosen the R.A. and Dec. lock knobs on the equatorial mount, so the telescope optical tube can move freely.

3.Point the telescope at the bright star near the celestial equator whose coordinates you know. Lock the R.A. and Dec. lock knobs. Center the star in the telescope’s field of view with the slow-motion control cables.

4.Loosen the R.A. setting circle lock thumb screw located just above the R.A. setting circle pointer; this will allow the setting circle to rotate freely. Rotate the setting circle until the pointer indicates the R.A. coordinate listed in the star atlas for the object. Retighten the thumb screw.

Finding Objects With the Setting Circles

Now that both setting circles are calibrated, look up in a star atlas the coordinates of an object you wish to view.

1.Loosen the Dec. lock knob and rotate the telescope until the Dec. value from the star atlas matches the reading on the Dec. setting circle. Remember to use the +90° to -90°