Collimation

Introduction

The Astro Optics collimator is a simple, fast and low cosl tool to easily collimate Newtonian rcllcctors. The retroreflective surface is housed in a anodized, precision CMC machined aluminum housing, insuring accuracy and long life.

The first two steps, preparation and mechanical alignment, need only be performed the first time the telescope is assembled and collimated or if the telescope is disassembled for any reason. Thereafter, only the optical alignment is needed to collimate your telescope. Don't underestimate the importance of mechanical alignment. Most troublesome collimating jobs can be traced back to mistakes or assumptions regarding the placement of components.

Position the telescope

Place the telescope on a horizontal surface or move it to the horizontal. Position the focuser so it is easily accessible. You will be moving back and forth from the rear to the front of the tube assembly to check progress and it helps to have things positioned for convenience.

Assemble the proper tools

Gather the necessary tools required to adjust the primary mirror holder, secondary mirror holder, spider and focuser base.

Center spot the primary mirror

It is necessary to "spot" the center of the primary mirror. To facilitate collimation, a white mark on the primary is most visible. The ideal spot can be made from any type of adhesive backed white paper or label. Cut a square 1/2" on a side or a triangle 5/8" on a side and use a paper hole punch to make a smooth 1/4" hole in its center. An adhesive backed paper reinforcing ring such as the type used on notebook paper also works. The spot is placed at the center of the primary mirror.

NOTE: To avoid possible damage to your optics, remove rings, watches, bracelets, pens, pencils and any other loose articles from your person or around your work area that could fall on or scratch your mirror. It is also advisable to use a plastic or wooden ruler when measuring your optics.

The center of the primary mirror can be determined by first finding its diameter and dividing by two. Use this radius to measure in from the edge at six or eight positions around the mirror. After several measurements, you will begin to zero-in on the exact center. Use a fine tipped permanent marker to mark the center and then accurately position and attach your paper "spot." Use acetone or alcohol on a cotton swab to remove the permanent marker in the center of the spot.

NOTE: Don't worry about placing a spot on the mirror, the center of the primary is shadowed by the secondary mirror.

Preparation / Mechanical Alignment

Mechanical Alignment

Mark the tube opposite the focuser

Mark a spot on the side of the tube opposite the focuser by carefully measuring from the front of the tube down an equal distance (on the outside of the tube) at the center of the focuser drawtube. A piece of adding machine paper works well to find the dimension around the tube (or upper cage). Wrap this paper around the outside, measure its center, rewrap the paper and drill or punch a tiny hole through to the inside. Place a white spot centered on the hole for visibility.

Mechanical Alignment

Mechanical alignment is the accurate positioning of components in the tube assembly. The importance of mechanical alignment lies in the fact that all the collimation steps that follow are either easy or difficult, depending on how well the telescope components are mechanically positioned.

NOTE: Truss tube owners are proceeding on the assumption that when the telescope is assembled the upper cage is centered over and square to the mirror box. Measuring to verify this will save time and frustration later.

Center the primary mirror in the mirror box

With open end mirror boxes, it is a simple task to measure in from the mirror box sides to the side of the mirror. With closed tubes, centering the mirror relative to the mirror mount and then centering the mirror mount relative to the tube works well. Mirror cells supplied with a sling can be adjusted in combination with cams or locating pads to center the primary mirror. It's a help to first turn the collimating screws so they all have the same amount of travel in both directions.

Square the focuser to the upper cage (or tube)

With the secondary mirror and holder removed, insert the Eagle Eye collimator into the focuser. The focuser mounting screws are loosened and the adjusting feet, if the focuser is so equipped, are used to tilt the focuser. By moving in and out you can position the focuser so the mark opposite the focuser is centered. On focusers not provided with tilt, adjustment shims made of cardboard or other suitable material can be placed under the focuser base.

Square the spider

The spider is next squared to the optical axis. This is checked by sighting through the spider bore towards the primary. The spider bore should look directly at the mirror center spot. Adjustment can be made by drilling the mounting holes slightly oversize and shifting the spider vanes to square. Also be sure the spider screws are properly tensioned to hold the secondary securely.

Position secondary laterally under focuser

Replace the secondary mirror and holder and position it under the focuser so the secondary is visually centered. Washers or adjustment nuts can be used to position the secondary (see Figure 1).

Mechanical / Optical Alignment

Position the secondary mirror rotationally under the focuser

Look through the focuser drawtube. Rotate the secondary holder and mirror until the reflection of the primary mirror is centered all around. Move towards or away from the focuser until the edge of the primary is just seen in the secondary. Insert the Astro Optics collimator and when the primary mirror reflection is centered, lock the secondary in place. This completes the mechanical alignment.

Figure 2. Rotational adjustment Adjust tilt of secondary mirror

Insert the Astro Optics collimator into the focuser. In this case you are using the collimator as a peep sight to keep your eye centered, or on axis. Adjust the secondary tilt so that the reflection of the primary mirror appears centered front to back and side to side in the secondary. If not, adjust the secondary lateral or rotational position to center, and then readjust tilt. Aligning the secondary becomes an iterative process, with each successive adjustment to lateral, rotational and tilt generating smaller changes to the placement of the secondary. When it is set up correctly, the reflection of the primary should appear centered in the secondary.

Adjust the tilt of primary mirror

Now that the secondary mirror is positioned, it's time to adjust the primary mirror to point in the correct direction so that the optical system becomes closed. By closed, we mean that the image at the focuser (your eye) reflects off the secondary to the primary and then the primary reflects light back to the secondary, which in turn reflects the light 90 degrees back to the focuser.

Optical Collimation with the Astro Optics Collimator 5

The open sky or a white wall work well to evenly illuminate the view. You can also illuminate the system by shining a flashlight directly at the primary mirror center spot. It helps to hold the light close to the center of the tube, next to the secondary mirror.

The primary collimation screws are now used to adjust the tilt of the primary so the reflection from the Astro Optics is centered on the primary mirror spot. By introducing a circle of light at the focuser, the reflection of that light will appear centered on the primary center spot when collimated. This is precisely what the Astro Optics collimator does. At this point, it becomes obvious why the center spot was cut square or triangular. It is much easier to see the corners of a square or triangular spot extend equally around the illuminated Astro Optics reflection when the two become superimposed. Just be sure the hole in the primary mirror spot is accurately centered to use the outer corners for final adjustment.

Primary square spot

If you have aligned your secondary correctly, you will only need to tweak the primary at this time. Adjust the collimation screws until the illuminated circle in the Astro Optics is centered on the primary spot. When you get through, the small black dot in the center of the collimator will appear superimposed in the dot on the primary center spot. Figure 3 illustrates the collimated view. After using the Astro Optics , your Newtonian is now well collimated.