Celestron Omni CG-4 Telescope Mount Astronomy Basics - Page 6
Ligning, Etting, Ircle, Ernier
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move to the star near the eastern horizon. The star should be 20 degrees above the horizon and within five 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 deepsky 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. Aligning the R.A. Setting Circle Before you can use the setting circles to find objects in the sky you need to align the R.A. setting circle. The declination setting circle is aligned during the process of polar alignment. In order to align the R.A. setting circle, you will need to know the names of a few of the brightest stars in the sky. If you don't, they can be learned by using the Celestron Sky Maps (#93722) or consulting a current astronomy magazine. To align the R.A. setting circle: 1. Locate a bright star near the celestial equator. The farther you are from the celestial pole the better your reading on the R.A. setting circle will be. The star you choose to align the setting circle with should be a bright one whose coordinates are known and easy to look up. 2. Center the star in the finderscope. 3. Look through the main telescope and see if the star is in the field. If not, find it and center it. 4. If you purchased an optional motor drive, start it now so that it will track the star. 5. Look up the coordinates of the star. 6. Rotate the circle until the proper co-ordinates line up with the R.A. indicator (the zero mark on the vernier scale). The R.A. setting circle should rotate freely. If the circle does not move freely, loosen the thumbscrew to the right of the scale. NOTE: Because the R.A. setting circle does NOT move as the telescope moves in R.A., the setting circle must be aligned each time you want to use it to find an object. This holds true even if you are using an optional motor drive. However, you do not need to use a star each time. Instead, you can use the coordinates of the object you are currently observing. Once the setting circles are aligned you can use them to find any objects with known coordinates. The accuracy of your setting circles is directly related to the accuracy of your polar alignment. 1. Select an object to observe. Use a seasonal star chart to make sure the object you chose is above the horizon. As you become more familiar with the night sky, this will no longer be necessary. 2. Look up the coordinates in a star atlas or reference book. 3. Hold the telescope and release the DEC clamp. 4. Move the telescope in declination until the indicator is pointing at the correct declination coordinate. 5. Lock the declination clamp to prevent the telescope from moving. 6. Hold the telescope and release the R.A. clamp. 7. Move the telescope in R.A. until the indicator points to the correct coordinate. 8. Lock the R.A. clamp to prevent the telescope from slipping in R.A. The telescope will track in R.A. as long as the motor drive is operating. 9. Look through the finderscope to see if you have located the object and center the object in the finder. 10. Look in the main optics and the object should be there. For some of the fainter objects, you may not be able to see them in the finder. When this happens, it is a good idea to have a star chart of the area so that you can "star hop" through the field to your target. This process can be repeated for each object throughout any given night. Using the R.A. Vernier Scale To increase the accuracy of the R.A. setting circle, the mount comes with a vernier scale. This device allows you to get more precise readings down to one minute of right ascension. Before we go into the specifics on how to use the vernier, let's take a look at the scale and learn how to read it. First, the zero (0) mark on the vernier is the R.A. indicator and will hereafter be referred to as such. It is on the extreme right end of the vernier scale with the other numbers increasing as you move toward the left. If the R.A. indicator is right on one of the marks of the R.A. setting circle, then that is the coordinate the telescope is pointing at. The problem arises when the R.A. indicator (zero mark) is in between two of the marks on the R.A. setting circle. If this is the case, you will notice that along the vernier scale, one of the marks will line up with one of the marks on the setting circle. This mark indicates the number of minutes that should be added to the R.A. reading of the indicator. Since the indicator is between two R.A. marks, add the minutes to the lower value that the R.A. indicator falls between. 6
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