Celestron EDGEHD User manual

To get the best performance from your Schmidt-Cassegrain (SCT) or EdgeHD telescope, its optics must be collimated—
properly aligned within the telescope. Your telescope was collimated at the factory when it was assembled, and its
collimation was QA-checked before it was shipped. However, if the telescope has been dropped or jarred during
transport or has undergone considerable temperature changes, it may need to be re-collimated.
You can achieve correct collimation by adjusting the tilt of the secondary mirror housed at the center of the Schmidt
corrector (see Figure 1). This is the only adjustment available to the end user and should be the only adjustment required
to bring the telescope’s optics into excellent collimation.
NOTE: Do not attempt to make adjustments to the Schmidt corrector’s orientation or positioning. Doing so will likely degrade
collimation and optical performance and may require return to Celestron to achieve proper optical alignment.
SCT & EdgeHD Collimation Guide
Optical Tube Side ViewSchmidt Corrector Side View
Optical Tube Front ViewSchmidt Corrector Front View
Secondary Mirror
Fig. 1
Schmidt Corrector Front View
Collimation Screws

2| SCT & EDGEHD COLLIMATION GUIDE
CHECKING COLLIMATION –
HOW DO I KNOW IF MY OPTICS
NEED ADJUSTMENT?
Checking optical collimation is quick and easy. We recommend
taking a few moments to check collimation before each observing
session. To do this, you’ll need to examine a star image through
the telescope’s eyepiece.
1. To begin, set up your telescope as you normally would for an
observing session. Connect a higher power eyepiece (ideally,
10mm focal length or less). Before you begin, be sure that your
telescope is in thermal equilibrium with its surroundings. Allow
45 minutes for the telescope to reach equilibrium if you move it
between large temperature extremes, like from a heated car or
house into the cold night air.
2. Choose a star at least 20 degrees above the horizon to
minimize the effect of atmospheric seeing conditions and slew
the telescope to it. Center the star in the field of view.
3 Defocus the telescope by about one turn of the focus knob
until the star image appears doughnut shaped. There will be
brighter rings of light surrounded by a dark hole (i.e. the shadow
from the secondary obstruction) in the center (see Figure 2).
If the telescope is collimated, the hole should appear centered
relative to the rings. If the hole is not centered, then adjustment
to the collimation is needed (see Figure 3).
4. Next, check the other side of focus by reversing direction of
the focus knob so that the centered star is focused and then
defocused again. The hole should still be centered in the rings.
If it isn’t, some collimation adjustment is needed.
If you are unsure if the telescope needs adjustment or not, the
collimation is likely OK. In this case, it’s best not to make any
adjustments, as doing so can easily make the collimation worse,
not better.
Fig. 2 Out of focus star with proper collimation
Fig. 3 Out of focus star with collimation needed

SCT & EDGEHD COLLIMATION GUIDE | 3
COLLIMATION SETUP
As you did when checking collimation, make sure your system
is acclimatized to the outside temperature before you begin the
collimation process.
To collimate your SCT or EdgeHD, you will need a pinpoint light
source. A bright star works great. A bright star near the zenith is
ideal since there is minimal atmospheric distortion in that part of
the sky. Make sure that your mount’s tracking is turned on, so the
star won’t drift as you are making collimation adjustments. Or, if
you are using a roughly polar aligned equatorial mount and you
do not want to power up your telescope, you can choose Polaris
as your collimation star. Its position relative to the celestial pole
means that it moves very little, eliminating the need to track it.
If seeing conditions (i.e. air steadiness) are turbulent, collimation
will be difficult to judge (see Figure 4). This is because the
extra-focal diffraction patterns that need to be evaluated will be
smeared and blurred. Wait until a better night if it is turbulent or
aim to a steadier part of the sky. Look for a part of the sky with
steady—not twinkling—stars. If stars are twinkling, then conditions
are poor, and it may be hard to achieve proper collimation. If
seeing conditions are good (see Figure 5), you can use a high-
powered eyepiece (i.e. focal length less than 10mm) to achieve
the most precise optical alignment. If seeing conditions are ideal,
use an eyepiece with even more magnification (i.e. focal length
between 2.3mm and 7mm) for the best results.
Next, if you have one attached, remove the dew shield from your
telescope. This will provide best access to the collimation screws.
NOTE:If you are outside in heavy dew conditions when attempting
to collimate, you may need to consider using an optional
dew heater system; this will prevent dew from forming on
the Schmidt corrector while you adjust collimation.
When connecting the eyepiece to the telescope, make sure
all thumbscrews are firmly tightened, as this helps ensure the
eyepiece is best mechanically aligned with the optical axis. Also,
make sure the retaining ring that connects the visual back to
the rear cell of the telescope is securely tightened. If you will
be subsequently using the telescope for imaging with a camera,
consider collimating the telescope with the eyepiece inserted
directly into the visual back, without a diagonal in place. Doing so
will remove any source of alignment error which may arise from
the diagonal itself.
After making each collimation adjustment, you will need to
re-center the bright star in the eyepiece. If using a computerized
mount, you should use your telescope’s hand control to move
the telescope (use a slow speed rate) and re-center the star. So,
make sure the hand control is in a handy location nearby.
Fig. 4 Out of focus star under turbulent
conditions
Fig. 5 Out of focus star under good seeing
conditions

4| SCT & EDGEHD COLLIMATION GUIDE
To collimate your telescope, you’ll use a Phillips-head
screwdriver to make adjustments to the collimation
screws on the back of the secondary mirror assembly.
These three screws are clearly visible when looking at
the front of most telescopes (see Figure 6).
COLLIMATION PROCESS
Collimation
Screws
There are three collimation screws; each can be adjusted to change the tilt of the secondary mirror. All
three screws provide a “tip/tilt” adjustment system, if you loosen one screw, then the other two screws
should be tightened. Conversely, if you tighten one screw, then the other two screws should be slightly
loosened (see Figure 8). In practice, since each screw will only be loosened or tightened by a very small
amount at a time, it is not necessary to adjust all three screws at once, as there is some flex in the mechanical
assembly. However, if you do tighten (or loosen) one of the screws by a significant amount, then the other two screws
will need to be loosened (or tightened) at some point in order to provide the proper screw tension.
WARNING: When using the screwdriver to make adjustments to the collimation screws, stay focused and be careful
not to inadvertently scratch the Schmidt corrector. Use a red-light flashlight, if needed, to illuminate the
collimation screws so you can make sure you put the screwdriver tip in the head of the intended screw.
The goal of making adjustments to the collimation screws is to center the central shadow (i.e. the hole) in the defocused
star image. Make only small 1/10th-of-a-turn adjustments at most to each of the collimation screws.
Some older SCT telescopes have an orange cap
that covers the collimation screws (see Figure 7).
Carefully use a flat-head instrument or other tool to
pry the cap off. There are a couple of tabs on the
plastic cap that connect it to the rear of the secondary
mirror assembly. Once the cover is removed, it’s easy
to see and access the three collimation screws.
Fig. 6
Fig. 7
Fig. 8
Secondary Mirror Secondary Mirror
Front View Side View Side View
Loosen
Collimation
Screw
Tighten
Collimation
Screw
Tighten
Collimation
Screws (2)
Collimation
Screws Loosen
Collimation
Screws (2)

SCT & EDGEHD COLLIMATION GUIDE | 5
Be sure to re-center the star in the eyepiece’s field of view by moving the telescope on its mount after making any
adjustments to the collimation screws. If the star is not centered in the field of view, interpretation of the defocused star
images will not be accurate to determine proper collimation. As discussed previously, using shorter focal length eyepieces
will increase magnification and improve the centering precision of the star within the field of view of the eyepiece.
To collimate the SCT or EdgeHD, follow these steps:
1. Center the bright star in the eyepiece’s field of view. Insert the highest
power eyepiece that can be used with good results under current seeing
conditions.
2. Defocus the star until you can see a dark “hole” in the center (i.e. the
shadow of the secondary mirror) and 2-4 diffraction rings of light around
it. If you defocus the star too much, it may be difficult to determine precise
collimation, and it will also become dimmer and harder to see. If you defocus
the star too little, you won’t be able to distinctly see the hole and rings.
3. Inspect the out-of-focus (“extra-focal”) diffraction pattern.
4. Next, turn the focus knob in the opposite direction. This will cause the
image to pass through focus and become defocused on the other side of
focus. Again, defocus the image until you can see the central hole and 2-4
rings of light around it.
1
2
3
4

6| SCT & EDGEHD COLLIMATION GUIDE
5. Inspect the extra-focal diffraction pattern on the other side of focus to see if
one side of focus provides a slightly better appearance of the light and dark
diffraction rings. Whichever side of focus has clearer light and dark diffraction
rings should be used for collimation purposes.
6. Make sure the star is still centered in the eyepiece’s field of view. Note the
direction in which the central shadow and the diffraction inner bright ring are
skewed relative to the bright outer diffraction ring
7. Place your finger along the edge of the front cell of the telescope pointing
towards the collimation screws, taking care not to touch the Schmidt corrector.
The shadow of your finger should be visible when looking at the extra-focal
diffraction pattern in the eyepiece. Rotate your finger around the tube edge
until its shadow is closest to the narrowest portion of the rings (i.e. the same
direction in which the central shadow is skewed)
8. Locate the collimation screw closest to where your finger is pointing. This is
the collimation screw you should adjust first. If your finger is pointing exactly
between two collimation screws, adjust the one that is further away from your
finger.
9. Use the screwdriver to turn the collimation screw identified in Step 8. Very
slightly tighten the screw. Usually, 1/10th of a turn of the screw is enough to
notice a change in collimation. Then, look in the eyepiece and re-center the
star in the field of view by moving the telescope mount. Use a slow speed rate
to do this, as the field of view will be quite narrow.
NOTE: If you turn the collimation screw too much, the star may no longer appear
in the field of view, especially if you are using a very short focal length eyepiece. If
this occurs, use the telescope’s finderscope to find and re-center the star. Make
only very small adjustments to each collimation screw, and the star should always
appear somewhere in the field of the eyepiece before re-centering.
6
5
7
8
9
Shadow of Finger

SCT & EDGEHD COLLIMATION GUIDE | 7
10. Confirm that the diffraction pattern’s central shadow and inner bright ring
have moved closer to the center relative to the bright outer ring.
a. If the central shadow and inner ring are now further de-centered, try
loosening the collimation screw instead of tightening it. Remember to re-
center the star in the field of view after each adjustment.
b. If the central shadow and inner ring have moved closer, continue to make
adjustments in the same way until the shadow and inner ring are as centered
as they can get using that collimation screw. Remember to re-center the
star in the field of view after making each adjustment.
NOTE: If you notice while making adjustments that one of the collimation
screws becomes very tight, slightly loosen the other two collimation
screws by an equal amount. This will relieve pressure on the screw that
was tight, and you should be able to continue to make adjustments
by tightening the screw. Conversely, if you notice that the collimation
screw becomes too loose when loosening it, tighten the other two
screws (refer to Figure 8).
11. If the central shadow and inner ring are still not centered relative to the outer
ring when centered in the field of view, you will need to make adjustments
to one of the other collimation screws. Go back to Step 6 and repeat the
process. This time, you will make adjustments to a different collimation screw.
Proper Collimation
You have achieved proper collimation when the central shadow and the bright inner diffraction ring are concentric
with the bright outer diffraction ring, and the thin concentric dark ring (in between these two bright rings) extinguishes
uniformly and simultaneously as you slowly pass through focus (see Figure 9). Proper collimation yields a star image
diffraction pattern that is concentrically similar just inside and outside of focus. In addition, proper collimation delivers
the optimal optical performance specifications that your telescope is built to achieve.
WARNING: If you cannot collimate your SCT or EdgeHD using the collimation screws, do not attempt to make any
other adjustments to the optics or mechanics of the telescope! Doing so will not only likely put your telescope further
out of alignment, it will also void your warranty. Instead, contact Celestron Technical Support by submitting a ticket here:
celestron.com/pages/technical-support. Your telescope may need to be returned to Celestron for warranty evaluation
(or paid repair, if the warranty has expired).
10 a
10b
Fig. 9

8| SCT & EDGEHD COLLIMATION GUIDE
CONCLUSION
Collimating your telescope is an important part of ensuring its best performance and best images. While you should
quickly check your telescope’s collimation before each observing session, it is unlikely that collimation adjustments will
be required on a regular basis. The exception would be in cases when the telescope is transported and not well-secured
during a bumpy ride.
Collimation is not necessarily difficult to do but requires some learning and some patience. After adjusting your telescope’s
collimation a few times, you’ll get the hang of it. The process will become easier and less time-consuming. Good luck
and clear skies!
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