Special Note: The Celestron SCT uses either standard 6-32 or metric M3 collimation screws. It is important to order the correct knobs for your telescope. Allen and set-style factory collimation screws have standard threads. For Phillips collimation screws, the general rule is: Silver are standard; Black are metric. There are exceptions to this rule for black Phillips screws, so we have developed some hints to help you to determine which type of black Phillips collimation screw your telescope uses. Moreover, there are even exceptions to the hints, so if you are unsure of the type of screw in your telescope, you can either 1) pull a single collimation screw and check its threads using the guidelines at the bottom of this page; or 2) order both standard and metric knobs and return those that aren’t needed for a full refund.
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Example 1: Black Phillips Collimation Screws
Textured secondary surface with serial number sticker and “CELESTRON” imprint in upper case lettering. Collimation screws are surrounded by a thin ring of smooth plastic and are slightly countersunk into the secondary housing. This is the most common black Phillips configuration.
Type of collimation screw: Metric
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Example 2: Black Phillips Collimation Screws
Textured secondary surface with serial number sticker and "Celestron" imprint in upper and lower case lettering. Collimation screws are surrounded by normal surface texturing and are not countersunk.
Type of collimation screw: Standard
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Example 3: Black Phillips Collimation Screws
Nearly smooth secondary surface with serial number sticker but no “Celestron” imprint. Screws are slightly countersunk.
Type of collimation screw: Can be either Standard or Metric
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Example 4: Black Phillips Collimation Screws
Smooth secondary surface with serial number sticker and “CELESTRON” imprint in upper case lettering. Screws are slightly countersunk.
Type of collimation screw: Can be either Standard or Metric
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Example 5: Black Phillips Collimation Screws
Fastar secondary with label. Scope can be either with or without EdgeHD optics. (Note: EdgeHD and non-EdgeHD require different knobs.)
Type of collimation screw: Metric
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Example 6: Black Phillips Collimation Screws
Hyperstar compatible secondary. Collimation screws are deeply countersunk into the secondary housing.
Type of collimation screw: Metric
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If you decide to remove a Celestron 8” SCT collimation screw, here's how to determine if the screw has standard or metric threads. (Our knobs are shown in the photograph.) Both factory screws have about the same thread length below the screw head. The metric screw is slightly smaller in diameter, but that isn't really helpful without having samples of both screws. The best way to determine screw type is to count threads. The standard screw has about 10 thread turns over its 5/16 inch length, and the metric screw has about 16 thread turns over its 8 mm length. (Exception: The C8 EdgeHD with black Phillips collimation screws and the Fastar secondary has a longer 12 mm black Phillips secondary collimation screw. This screw is metric with 24 thread turns over its length.)
Please contact us using the link below for guidance if you have further questions.