How do I use the RA Vernier scales on my Omni XLT (CG-4) mount’s setting circles?

The RA setting circle on your CG-4 mount is only marked to intervals of ten minutes. To increase the accuracy of the circle, there is a Vernier scale that allows you to get more precise readings (down to one minute of right ascension).

To use the RA Vernier, note that the zero (0) mark on the right-hand side is the same as the RA indicator. Numbers on the scale increase to the left.

If the RA indicator is right on one of the marks of the RA setting circle, that is the RA coordinate where the telescope is pointing. What if the indicator is between two of the marks on the RA setting circle? In this case, one of the marks on the Vernier will line up with one of the marks on the setting circle. This mark on the Vernier is the number of minutes that should be added to the RA reading of the indicator. Since the indicator is between two RA marks, add the minutes to the lower value that the RA indicator falls between.

For example, if the RA indicator is just left of the 5h 40m mark, then the RA value is between 5h 40m and 5h 50m. If you look down the Vernier scale, you will see only one mark is in line with a division mark on the RA setting circle. Let’s say that the “4” is the only mark to line up with any of the marks on the RA setting circle. This means that you are 4 minutes to the left of the 5h 40m mark or more simply at 5h 44m.

Here’s how to use your Vernier when observing:

Look up the coordinates of the object you want to observe. If you want to see the Orion Nebula (M42), then it’s at 5h 35m right ascension (RA) and -05 degrees 27 minutes declination (Dec).

Release the RA clamp and rotate the telescope until the RA indicator is between the 5h 30m mark and the 5h 40m mark on the RA setting circle. Lock the RA clamp to hold the telescope in place.
 Move the telescope in RA until the five on the Vernier scale lines up with one of the marks on the RA setting circle. Remember, the RA indicator must stay between the 5h 30m mark and the 5h 40m mark on the RA setting circle!

Set the declination directly by estimating between the marks at every two degrees on the Dec circles.

Look through the telescope and the Orion Nebula should be within the field of view if you are using a low-power eyepiece.
 


Updated 12/21/13