Remember, accurate polar alignment is not needed for casu- al visual observing. Most of the time, approximate polar alignment, as outlined previously, will suffice.
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. The Dec. slow-motion control is not needed for tracking. Objects will appear to move faster at higher magnifications, because the field of view is narrower.
Optional Motor Drive
An optional DC motor drive system (EQ-3M, #7828) can be mounted on the AstroView’s equatorial mount to provide hands- free tracking. Objects will then remain stationary in the field of view without any manual adjustment of the R.A. slow-motion con- trol. The motor drive system is necessary for astrophotography.
Understanding the Setting Circles
The setting circles on an equatorial mount enable you to locate celestial objects by their “celestial coordinates.” Every astronomical 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.
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 (the negative sign denotes south of the celestial equator). There are 60 minutes in 1 hour of R.A and there are 60 arc-minutes in 1 degree of declination.
The telescope’s R.A. setting circle is scaled in hours, from 1 through 24, with small lines in between representing 10- minute increments. The upper set of numbers apply to viewing in the Northern Hemisphere, while the numbers below them apply to viewing in the Southern Hemisphere. The Dec. setting circle is scaled in degrees.
Before you can use the setting circles to locate objects, the mount must be accurately polar aligned, and the setting cir- cles must be calibrated. The declination setting circle was calibrated at the factory, and should read 90° when the tele- scope optical tube is pointing exactly along the polar axis.
Calibrating the Right Ascension Setting Circle
1.Identify a bright star near the celestial equator and look up its coordinates in a star atlas.
2.Loosen the R.A. and Dec. lock levers on the equatorial mount, so the telescope optical tube can move freely.
3Point the telescope at the bright star whose coordinates you know. Center the star in the telescope’s eyepiece. Lock the R.A. and Dec. lock levers.
4.Loosen the large setscrew just above the R.A. setting circle and rotate the R.A. setting circle so the pointer indicates the R.A. coordinate listed for the bright star in the star atlas. Do not retighten the setscrew when using the R.A. setting cir- cles for finding objects; the set screw is only needed for polar alignment using the polar axis finder scope.
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. Retighten the lock knob. Note: If the telescope is being aimed toward the south and the Dec. setting circle pointer passes the 0° indicator, the value on the Dec. setting circle becomes a negative number.
2.Loosen the R.A. lock lever and rotate the telescope until the R.A. value from the star atlas matches the reading on the R.A. setting circle. Retighten the lock lever.
Most setting circles are not accurate enough to put an object dead-center in your finder scope’s field of view, but they’ll get you close, assuming the equatorial mount is accurately polar- aligned. The R.A. setting circle should be recalibrated every time you wish to locate a new object. Do so by calibrating the setting circle for the centered object before moving on to the next one.
Confused About Pointing the Telescope?
Beginners occasionally experience some confusion about how to point the telescope overhead or in other directions. In Figure 1 the telescope is pointed north as it would be during polar alignment. The counterweight shaft is oriented down- ward. But it will not look like that when the telescope is pointed in other directions. Let’s say you want to view an object that is directly overhead, at the zenith. How do you do it?
One thing you DO NOT do is make any adjustment to the lat- itude adjusting T-bolts. That will spoil the mount’s polar alignment. Remember, once the mount is polar-aligned, the telescope should be moved only on the R.A. and Dec. axes. To point the scope overhead, first loosen the R.A. lock lever and rotate the telescope on the R.A. axis until the counter- weight shaft is horizontal (parallel to the ground). Then loosen the Dec. lock lever and rotate the telescope until it is pointing straight overhead. The counterweight shaft is still horizontal. Then retighten both lock levers.
What if you need to aim the telescope directly north, but at an object that is nearer to the horizon than Polaris? You can’t do it with the counterweight down as pictured in Figure 1. Again, you have to rotate the scope in R.A. so that the counterweight shaft is positioned horizontally. Then rotate the scope in Dec. so it points to where you want it near the horizon.
To point the telescope directly south, the counterweight shaft should again be horizontal. Then you simply rotate the scope on the Dec. axis until it points in the south direction.
To point the telescope to the east or west, or in other directions, you rotate the telescope on its R.A. and Dec. axes. Depending on the altitude of the object you want to observe, the counterweight shaft will be oriented somewhere between vertical and horizontal.
