Meade RCX400TM instruction manual Precise Polar Alignment, RCX400 mounted on an equatorial wedge

Fig. 54: RCX400 mounted on an equatorial wedge.

ting circle and slightly loosen the two bolts located under the knob. Now turn the circle unit until it reads 89.2°, the Declination of Polaris. Then tighten down the two bolts and replace the knurled knob.

Should you wish to use the manual setting circles, the R.A. setting circle (Fig. 51) must be calibrated manually on the Right Ascension of a star every time the telescope is set up. (The R.A. setting circle has two sets of numbers, the inner set is for Southern hemisphere use, while the other is for Northern hemisphere use.) Locate a star with which you're familiar. Look up the R.A. for the star in a star chart or other aid. With the star centered in the telescope's eyepiece, move the R.A. setting circle, using one of knobs (C, Fig. 53), so that the R.A. of the star lines up with the tick mark on the base of the telescope (B, Fig. 53).

Precise Polar Alignment

It should be emphasized that while doing casual observing, precise alignment of the telescope’s polar axis to the celestial pole is not necessary. Don’t allow a time-con- suming effort at lining up with the pole to interfere with your basic enjoyment of the tel- escope. For long-exposure photography, however, the ground rules are quite different, and precise polar alignment is not only advisable, but almost essential.

Even though the RCX400 telescopes offers a very precise and sophisticated drive sys- tem, the fewer tracking corrections required during the course of a long-exposure pho- tograph, the better. (“Long-exposure” means any photograph of a celestial object that last for 10 minutes or longer). In particular, the number of Declination corrections required is a direct function of the precision of polar alignment.

Precise polar alignment requires the use of a crosshair eyepiece. The Meade Illuminated Reticle Eyepiece (see OPTIONAL ACCESSORIES, pages 48 - 50) is well- suited in this application, but it is also preferable to increase the effective magnifica- tion through the use of a 2X or 3X Barlow lens as well. Follow this procedure (partic- ularly if the pole star is not visible), sometimes better known as the “Drift” method:

1.Obtain a rough polar alignment as described earlier. Place the illuminated reticle eyepiece (or eyepiece/Barlow combination) into the eyepiece holder of the tele- scope.

2.Point the telescope, with the motor drive running, at a moderately bright star near where the meridian (the North-South line passing through your local zenith) and the celestial equator intersect. For best results, the star should be located within ±30 minutes in R.A. of the meridian and within ±5° of the celestial equator (see CELESTIAL COORDINATES, page 57). Pointing the telescope at a star that is straight up, with the Declination set to 0°, will point the telescope in the right direc- tion.

3.Note the extent of the star’s drift in Declination (disregard drift in Right Ascension):

a.If the star drifts South (or down), the telescope’s polar axis is pointing too far East.

b.If the star drifts North (or up), the telescope’s polar axis is pointing too far West.

4.Move the wedge in azimuth (horizontally) to effect the appropriate change in polar alignment. Reposition the telescope’s East-West polar axis orientation until there is no further North-South drift by the star. Track the star for a period of time to be certain that its Declination drift has ceased.

5.Next, point the telescope at another moderately bright star near the Eastern hori- zon, but still near the celestial equator. For best results, the star should be about 20° or 30° above the Eastern horizon and within ± 5° of the celestial equator.

6.Again note the extent of the star’s drift in Declination:

a.If the star drifts South, (or down) the telescope’s polar axis is pointing too low.

b.If the star drifts North, (or up) the telescope’s polar axis is pointing too high.

7.Use the altitude adjust control on the wedge to make appropriate changes in alti- tude, based on your observations above. Again, track the star for a period of time to be certain that Declination drift has ceased.

The above procedure results in very accurate polar alignment, and minimizes the need for tracking corrections during astrophotography.

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