ForkArms

I am an avid user of Digital Setting Circles. In my light polluted environment, they help me rapidly locate objects in the night sky. Properly used, the setting circles will position the object within a one degree field of view.

What is the problem

The setting circles will only be able to do this if the fork arm of the telesccope is at right-angles to the base. This point is a bit obscure, and is often missed by the would-be happy digital settng circle owner.

The digital setting circle computer translates one co-ordinate system into another. The first coordinate system is the one currently being used by the telescope. I say currently, because the setting circle imposes no restrinction on the orientation of the telescope. The only information the setting circle computer wants to know is:

1) where is DEC=0 (sometimes 90), 2) where is star A and 3) where is star B.

The first question is very important, and is usually not well understood. If the telescope is not polar aligned, then what is DEC=0? The answer is both complex and it is simple at the same time.

First the simple answer. Dec=0 is the setting at which the OTA is at right angles to the Fork Arms.

Now, the complicated Answer: Dec=0 is the setting at which the OTA is parallel to the base at all points of movement about the azamuth base, and the point at which the OTA is at right angles to the fork arms.

In order for the digital setting circles to work, the computer requires that the fork arms be at right angles to the azamuth base.

How to determine if there is a problem: If the setting circles seem to work in an area of the sky close to the stars used to set the computer, but not work so well at greater distances, you can be sure your fork arms are not set at right angles.

Testing for squareness

This procedure works for my C8. A variation of the process will probably work for your scope.

1) Use shims to make the kitchen table level in all directions

2) Set the C8 on the kitchen table

3) Using masking tape, make three lines across one fork arm parallel to the floor. Make the first tape mark at the bottom of the arm, the second at the top, and the third in the center.

3a) Take another piece of masking tape and place it vertically down the fork arm crossing each of the three horizontal pieces of tape.

4) Using a ruler, find the center of each of the horizontal tape pieces. Make a black dot at that place on each of the three pieces of tape.

5) Using a ruler, line up the three dots and draw a line up the middle of the fork arm. Now you have a vertical line to test squareness of your mount.

6) Use a carpenters square and place it on the table. Now line the square up with the vertical line on the fork arm. Does the square line up with the vertical line? If not, you need to adjust the fork arms.

Setting the Squareness

Do this procedure only if set 6 above showed a problem.

1) Loosen the Alan screws that hold the fork arms to the drive base of the scope. These are found toward the bottom of the fork arms. There are probably 4 of these alan screws, two on each arm, and they are probably quarter inch diameter screws. Loosen the screws until it is possible to move the fork arms.

2) WIth the carpenter square in place, adjust one fork arm until the vertical line and the square agree. Now tighten one Alan screw.

3) Check the square again, and tighten the second screw if all is good, otherwise re-adjust the fork arm and tighten again.

4) Check the fork arm again to make sure the fork arm is at right angles to the table top.

5) If all looks good, tighten the two alan screws for the adjusted fork arm down tight.

6) Loosen up the two alan bolts on the other arm, and then tighten them down tight. The only reason to do this is to make sure the second arm isn't carrying torque.

7) Place a level on top of the OTA, and adjust declination until the bubble indicates level.

8) Loosen the declination circle knob

9) adjust the circle to say 0 exactly

10) Tighten the knob, and re-adjust the circle to 0.

11) Make this knob as tight as possible, all the while making sure that the bubble stays level, and the circle says 0. You can now answer the question DEC=0? correctly.

Good Hunting

Note: This procedure assumes that the OTA and the optical line are parallel. If the optics are not properly positioned inside the OTA tube, then the above procedure will provide a good first approximation.

What to do if things are still not right?

The first thing to do is to re-check the procedures identified above. The problem may be that the optics are not set parallel to the OTA tube, and that the procedures used to set DEC=0 are not quite right.

To adjust for this eventuality, set up the Digital Setting circle in the usual way. Now use a finder scope to find a bright star above DCL=0 (Northern Celestial Hemisphere) for which the declination is known. Use a high power eyepiece and set the star in the center of the field.

Now change the mode of the Digital setting circle to read out the RA and DEC of the currently centered target.(Your Star). The difference between the Dec value shown on the Digital Setting Circle and the known declination of the star is your optical error.

Adjust your Mechanical setting circle as follows:

If the Digital Declination is greater than the known declination of the star, the optical axis is pointing too low.

to fix this:

1) set the scope to reflect the mechanical setting circle DEC=0.

2) Turn the scope to point South

3) Raise the southern end of the scope toward the north by the number of degrees that represent the absolute (positive) difference between the Digital Setting Circle Declination, and the known star Declination.

4) Clamp the declination axis

5) Loosen the mechanical Declination circle screw and reset declination circle to 0 and re-tighten.

6) Tighten as much as possible assuring that the mechanical declination circle remains at 0.