SCT & EdgeHD Collimation Guide

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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

Summary of content (8 pages)

PAGE 1
SCT & EdgeHD Collimation Guide 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.
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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).
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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.
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COLLIMATION PROCESS 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 Screws Fig. 6 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.
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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.
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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. 5 6 6. Make sure the star is still centered in the eyepiece’s field of view.
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10a 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. I f the central shadow and inner ring are now further de-centered, try loosening the collimation screw instead of tightening it. Remember to recenter the star in the field of view after each adjustment. b.
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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.