A refractor telescope uses a convex lens to capture and magnify light, projecting images directly onto an optical objective for viewing. They are typically less expensive than reflector telescopes and work well for viewing celestial objects close to Earth. Meade Instruments Corporation manufactures a number of refractor telescopes, including the LXD and DS series refractors and their high-end ED series of apochromatic (APO) telescopes. Powerful refractor telescopes are sensitive to optical errors that can produce fuzzy images. In order to eliminate optical error, the front lens cell needs to be set at an exact 90-degree angle to the optical tube assembly (OTA). The process of adjusting the front lens is called collimation.
- Skill level:
- Moderately Challenging
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Things you need
- Foam blocks
- Hobby knife
- Small photo tripod
- Laser pointer
- 24- by24-inch white posterboard
- Painters tape
- Small screwdriver
Tape the sheet of white posterboard to a wall about six feet away from a table or other stable work surface. Make sure the centre of the posterboard is at the same level as the table top.
Mount or tape the laser pointer to the head of the tripod. Place the tripod in front of the posterboard, and point the laser toward your work table. Adjust the height of the tripod so the laser is roughly at the same height as the table top. Use the level to make sure the laser pointer has no vertical tilt.
Cut shallow V-notches in the top of the foam blocks. The notches should be slightly narrower than the width of your telescope's optical tube assembly (OTA).
Place the foam blocks on the table. Place your telescope's OTA in the notches on the blocks and turn it so that the front lens cell points at the laser. Check that the tube assembly sits level on the foam blocks. Trim the foam blocks as needed to level the tube assembly.
Activate the laser pointer and adjust it until the beam strikes the centre of your telescope's front lens cell. The laser beam reflected from the lens will illuminate a point on the posterboard.
Rotate the telescope tube a full turn, being careful not to move the foam blocks. If the reflected laser beam does not move on the posterboard, the lens is square with the tube and does not need to be adjusted. If the laser moves on the posterboard, collimation adjustment is required.
Mark the spot where the laser strikes the posterboard with a pencil. Rotate your telescope's tube a half-turn. Mark the laser's new illumination point on the posterboard.
Adjust the collimation screws on the front of your telescope until the laser beam falls halfway between the two pencil marks. The number and location of collimation screws will vary depending on your exact model of telescope, but there are typically three screws spaced evenly around the lens on the outside of the tube.
Mark the new impact point of the laser on the posterboard. Repeat the process of rotating the tube and adjusting the collimation screws until the laser beam does not move around on the posterboard. When the laser beam stops shifting as you rotate the telescope, your telescope lens is properly squared to the optical tube.
Tips and warnings
- The front lens cell on Meade's apochromatic (APO) series of refractors also needs to be centred -- a separate procedure -- before it can be accurately collimated.
- Do not point the laser at your eyes, or stand where the laser may be reflected into your eyes. Even relatively low-powered laser pointers can cause eye injuries.
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