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To verify collimation, view a star near the zenith. Use a medium to high power ocular — 12mm to 6mm focal length. It is
To verify collimation, view a star near the zenith. Use a medium to high power ocular — 12mm to 6mm focal length. It is
important to center a star in the center of the field to judge collimation.
important to center a star in the center of the field to judge collimation.
symmetry of the star. If you see a systematic skewing of the star to one side, then recollimation is needed.
symmetry of the star. If you see a systematic skewing of the star to one side, then recollimation is needed.
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.
T T
o accomplish this, you need to tighten the secondary collimation screw(s) that move the star across the field toward the
o accomplish this, you need to tighten the secondary collimation screw(s) that move the star across the field toward the
d d
irection of the skewed light. These screws are located in the secondary mirror holder (see figure 8-1). To access the collimation
irection of the skewed light. These screws are located in the secondary mirror holder (see figure 8-1). To access the collimation
sc sc
rews you will need to remove the cap that covers the secondary mirror holder. To remove the cap, gently slide a flat head
rews you will need to remove the cap that covers the secondary mirror holder. To remove the cap, gently slide a flat head
sc sc
rewdriver underneath one end of the cap and twist the screwdriver. Slide the screwdriver underneath the other side of the cap
rewdriver underneath one end of the cap and twist the screwdriver. Slide the screwdriver underneath the other side of the cap
a a
nd 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
nd 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
m m
oving the scope before making any improvements or before making further adjustments.
oving the scope before making any improvements or before making further adjustments.
T T
o make collimation a simple procedure, follow these easy steps:
o make collimation a simple procedure, follow these easy steps:
1 1
.
.
While looking through a medium to high power eyepiece, de-focus a bright star until a ring pattern with a dark shadow
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.
appears (see figure 8-2). Center the de-focused star and notice in which direction the central shadow is skewed.
2 2
.
.
Place your finger along the edge of the front cell of the telescope (be careful not to touch the corrector plate), pointing
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
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
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).
which the central shadow is skewed).
3.
3.
Locate the collimation screw closest to where your finger is positioned. This will be the collimation screw you will need to
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 scr
adjust first. (If your finger is positioned exactly between two of the collimation scr
screw opposite where your finger is located).
screw opposite where your finger is located).
4 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
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.
the central obstruction of the star image is skewed.
Figure 7-3
A collimated telescope
should appear
symmetrical with the
central obstruction
centered in the star's
diffraction pattern.
pattern.
Perfect collimation will yield a s
Perfect collimation will yield a s
delivers the optimal optical p
delivers the optimal optical p
If seeing (i.e., air steadiness) is tu
If seeing (i.e., air steadiness) is tu
steadier part of the sky. A steadie
steadier part of the sky. A steadie
5.
5.
While looking through the eyepiece, use an Allen wrench to turn the collimation screw you
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.
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
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
skewed, than you are turning the collimation screw the wrong way. Turn the screw in the
opposite
opposite
direction, so that the star image is moving towards the center of the field of view.
direction, so that the star image is moving towards the center of the field of view.
6.
6.
If while turning you notice that the screws get very loose, then simply tighten the other two
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
screws by the same amount. Conversely, if the collimation screw gets too tight, then loosen
the other two scr
the other two scr
ews by the same amount.
ews by the same amount.
7.
7.
Once the star image is in the center of the field of view, check to see if the rings are
Once the star image is in the center of the field of view, check to see if the rings are
concentric. If the centr
concentric. If the centr
al obstruction is still skewed in the same direction, then continue
al 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
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
different direction, than simply repeat steps 2 through 6 as described above for the new
direction.
direction.
tar image very symmetrical just inside and outside of focus. In addition, perfect collimat
tar image very symmetrical just inside and outside of focus. In addition, perfect collimat
erforma
erforma
nce specifications that your telescope is built to achieve.
nce specifications that your telescope is built to achieve.
rbulent, collimation is difficult to judge. Wait until a better night if it is turbulent or aim to a
rbulent, collimation is difficult to judge. Wait until a better night if it is turbulent or aim to a
r part of the sky is judged by steady versus t
r part of the sky is judged by steady versus t
Slowly cross in and out of focus and judge the
Slowly cross in and out of focus and judge the
ews, then you will need to adjust the
ews, then you will need to adjust the
winkling stars.
winkling stars.
38
ion
ion
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