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Figure 8-2 -- Even though the star pattern appears the same on both sides of focus, they are asymmetric. The
dark obstruction is skewed off to the left side of the diffraction pattern indicating poor collimation.
To accomplish this, you need to tighten the secondary collimation screw(s) that move the star across the field toward the direction of the
skewed light. These screws are located in the secondary mirror holder (see figure 8-1). To access the collimation screws you will need to
remove the cap that covers the secondary mirror holder. To remove the cap, gently slide a flat head screwdriver underneath one end of the
cap and twist the screwdriver. Slide the screwdriver underneath the other side of the cap and twist until the cap comes off. Make only
small 1/6 to 1/8 adjustments to the collimation screws and re-center the star by moving the scope before making any improvements or
before making further adjustments.
To accomplish this, you need to tighten the secondary collimation screw(s) that move the star across the field toward the direction of the
skewed light. These screws are located in the secondary mirror holder (see figure 8-1). To access the collimation screws you will need to
remove the cap that covers the secondary mirror holder. To remove the cap, gently slide a flat head screwdriver underneath one end of the
cap and twist the screwdriver. Slide the screwdriver underneath the other side of the cap and twist until the cap comes off. Make only
small 1/6 to 1/8 adjustments to the collimation screws and re-center the star by moving the scope before making any improvements or
before making further adjustments.
To make collimation a simple procedure, follow these easy steps: To make collimation a simple procedure, follow these easy steps:
1. While looking through a medium to high power eyepiece, de-focus a bright star until a ring pattern with a dark shadow appears (see
figure 8-2). Center the de-focused star and notice in which direction the central shadow is skewed.
1. While looking through a medium to high power eyepiece, de-focus a bright star until a ring pattern with a dark shadow appears (see
figure 8-2). Center the de-focused star and notice in which direction the central shadow is skewed.
2. Place your finger along the edge of the front cell of the telescope (be careful not to touch the corrector plate), pointing towards the
collimation screws. The shadow of your finger should be visible when looking into the eyepiece. Rotate your finger around the tube
edge until its shadow is seen closest to the narrowest portion of the rings (i.e. the same direction in which the central shadow is
skewed).
2. Place your finger along the edge of the front cell of the telescope (be careful not to touch the corrector plate), pointing towards the
collimation screws. The shadow of your finger should be visible when looking into the eyepiece. Rotate your finger around the tube
edge until its shadow is seen closest to the narrowest portion of the rings (i.e. the same direction in which the central shadow is
skewed).
3. Locate the collimation screw closest to where your finger is positioned. This will be the collimation screw you will need to adjust
first. (If your finger is positioned exactly between two of the collimation screws, then you will need to adjust the screw opposite
where your finger is located).
3. Locate the collimation screw closest to where your finger is positioned. This will be the collimation screw you will need to adjust
first. (If your finger is positioned exactly between two of the collimation screws, then you will need to adjust the screw opposite
where your finger is located).
4. Use the hand control buttons to move the de-focused star image to the edge of the field of view, in the same direction that the central
obstruction of the star image is skewed.
4. Use the hand control buttons to move the de-focused star image to the edge of the field of view, in the same direction that the central
obstruction of the star image is skewed.
5. While looking through the eyepiece, use an Allen wrench to turn the collimation screw you located in
step 2 and 3. Usually a tenth of a turn is enough to notice a change in collimation. If the star image
moves out of the field of view in the direction that the central shadow is skewed, than you are turning the
collimation screw the wrong way. Turn the screw in the opposite direction, so that the star image is
moving towards the center of the field of view.
5. While looking through the eyepiece, use an Allen wrench to turn the collimation screw you located in
step 2 and 3. Usually a tenth of a turn is enough to notice a change in collimation. If the star image
moves out of the field of view in the direction that the central shadow is skewed, than you are turning the
collimation screw the wrong way. Turn the screw in the opposite direction, so that the star image is
moving towards the center of the field of view.
Figure 7-3
A collimated telescope
should appear
symmetrical with the
central obstruction
centered in the star's
diffraction pattern.
6. If while turning you notice that the screws get very loose, then simply tighten the other two screws by
the same amount. Conversely, if the collimation screw gets too tight, then loosen the other two screws
by the same amount.
6. If while turning you notice that the screws get very loose, then simply tighten the other two screws by
the same amount. Conversely, if the collimation screw gets too tight, then loosen the other two screws
by the same amount.
7. Once the star image is in the center of the field of view, check to see if the rings are concentric. If the
central obstruction is still skewed in the same direction, then continue turning the screw(s) in the same
direction. If you find that the ring pattern is skewed in a different direction, than simply repeat steps 2
through 6 as described above for the new direction.
7. Once the star image is in the center of the field of view, check to see if the rings are concentric. If the
central obstruction is still skewed in the same direction, then continue turning the screw(s) in the same
direction. If you find that the ring pattern is skewed in a different direction, than simply repeat steps 2
through 6 as described above for the new direction.
Perfect collimation will yield a star image very symmetrical just inside and outside of focus. In addition,
perfect collimation delivers the optimal optical performance specifications that your telescope is built to achieve.
Perfect collimation will yield a star image very symmetrical just inside and outside of focus. In addition,
perfect collimation delivers the optimal optical performance specifications that your telescope is built to achieve.
If seeing (i.e., air steadiness) is turbulent, collimation is difficult to judge. Wait until a better night if it is turbulent or aim to a steadier part
of the sky. A steadier part of the sky is judged by steady versus twinkling stars.
If seeing (i.e., air steadiness) is turbulent, collimation is difficult to judge. Wait until a better night if it is turbulent or aim to a steadier part
of the sky. A steadier part of the sky is judged by steady versus twinkling stars.
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