John M. Pierce's HobbyGraph Articles

HobbyGraph #6

Adjustments of Telescope Parts.

1. Squaring up the Tube with the Declination Axis.

      The tube should be on the telescope mount with the primary mirror removed. FAsten heavy threads across the ends of the telescope tube, crossing at the center of each end. Use adhesive tape to hold the threads in place. Place the declination axis horizontal, checking it with a level. Clamp the polar axis. Now stand about ten feet distant and sight through the tube at a fence or side of a building or a board stuck into the ground. An assistant with a piece of chalk should, in response to your signals, locate the point on the fence or object in line with the two sets of cross threads, and there mark a cross.
      Turn the tube end for end. If the cross still lines up with the threads, the tube is square with the declination axis. If not, put shims between the tube and the cradle to correct one-half the discrepancy; then mark a new cross, and check by reversing the tube. Repeat as often as necessary.

2. Adjusting the Primary Mirror with the axis of the Tube.

      Fasten cross threads across the face of the primary mirror, fastening them to the cell with adhesive. Place the primary mirror in position in the telescope tube. Fasten similar cross-threads over the open end of the telescope tube. Stand about 10 feet distant and look through the tube at the primary mirror. Line up the eye with the two sets of cross-threads, and have someone adjust the primary mirror until the image of the cross-threads lines up with the two sets of threads.

3. Adjusting the Prism.

      Leave the cross-threads used in No. 2 on the primary mirror. Put a paper cover over the eyepiece tube (eyepiece removed), having a 1/16" hole at its center. Point the telescope toward the sky and look through the hole in the eyepiece cover. Tip and turn the prism until the primary mirror is central in the field of view. Then adjust the prism-holder radially until the image of the prism centers up on the cross-threads of the primary mirror. Readjust the prism until all reflections are centered on the cross-threads of the primary mirror, when observed through the hole in the eyepiece tube cover.

4. Setting the Mount.

      a. For Latitude. Cut out a right traingle from thin wood or cardboard, making one acute angle equal to your latitude. Place this triangle with its hypontenuse on the polar axis of the telescope with the latitude angle at the top. Adjust the mount until the top leg of the triangle is horizontal when tested with a level. If it is impossible to apply the template to the polar axis, turn the declination axis into the vertical pane with the tube directly over the mount pointing to the north. Then bring Polaris into the center of the field in the eyepiece by turning the whole mount on the pedestal horizontally, and by using the adjusting screws for any vertical shift.
      This setting must be made when Polaris is at the same height as the true pole. Since Mizar, at the crook of hte handle of the Dipper, Delta Cassiopiea at the flattened vertex of the W, Polaris and the true pole are all in line, a time must be chosen when these are on a horizontal line. Bring Polaris into the field of view in the eyepiece by moving the telescope tube in declination, and by turning the whole mount on the base or pedestal. Then fasten permanently to the base.

5. Setting the Circles.

      Set the declination axis horizontal, with the telescope on the west end of hte axis, and in this position set the hour circle to read zero. Set the telescope tube vertical, using a level or plumb line, and in this position set the declination circle to read the angle of your latitude. You latitude may be obtained from any good map.

6. Further Adjustment of Optical Parts.

      Use a power of 100x to 150x and setup the telescope to Polaris, or some other second magnitude star. Slide the eyepiece to throw the star out of focus until the expanded image covers about one-third the field of view. If the image is not round when in the center of the field adjust the diagonal mirror or prism cautiously, both rotating it and rocking if if necessary, or possibly moving it along the optical axis of the primary mirror. If the diagonal is held in a projection of the eyepiece tube, placing paper shims under one edge of the holder to square up the axis of the eyepiece with the axis of the primary mirror may help.       If the image is round but the black spot is not central, adjust the primary mirror to center up the black spot in the image.

7. Adjusting a Diagonal Flat.

      A diagonal mirror, either rectangular or elliptical, is adjusted in the same way as a prism (#3). When finally in the best position, the center of the system of circles surrounding the crossed threads, seen when you look into the hole in the eyepice tube cover, should be slightly off-center, - a little nearer the upper end of the diagonal, the end nearest the open end of the telescope.
      If the daylight adjustment is done carefully there should be no further correction indicated when the No. 6 test is made.