How do I use the RA and Dec vernier scales on my Advanced GT (AS-GT) mount’s setting circles?
July 1, 2009
The RA setting circle on your AS-GT mount is only marked to intervals of ten minutes. To increase the accuracy of the circle, there is a vernier scale (vernier for short) that allows you to get more precise readings (down to one minute of right ascension).
The verniers for RA and Dec work in the same way. However, they do differ in scale and the verniers are different, as explained below.
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, then 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 verniers 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 using the hand control arrow keys 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 using the same technique, except first convert the minutes of declination to decimal degrees and round to the nearest quarter degree (the accuracy of the declination vernier is ¼ degree). So the declination for M42 becomes 5.5 degrees south.
The declination vernier is marked from its zero indicator out two degrees right and left. (The AS-GT Dec circles are marked every two degrees.) Use the marks to the left to add to northern declinations. (Add the vernier value to the next lower-numbered degree.) Use the scale to the right for southern declinations to the right. (Add the vernier value to the next lower-numbered degree.)
M42 is a southern declination (-05.5 deg). So the right scale will be used. Since -5.5 is between -4 and -6, you'll need to move the telescope in Dec using the hand control arrow keys until the 1.5 on the right vernier scale lines up with one of the marks on the Dec setting circle. Remember, the Dec indicator must stay between the -4 degree mark and the -6 degree mark on the Dec setting circle.Look through the telescope and the Orion Nebula should be within the field of view if you are using a low-power eyepiece.