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, so we have developed some hints to help you to determine which type of 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, “CELESTRON” imprint in upper case lettering and “9.25 (235mm)” in the center, slots for an auxiliary secondary screw cover, and a lip around the secondary housing perimeter. (Our knobs are shown in the photo.) This is the most common black Phillips configuration.
Type of collimation screw: Metric
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Example 2: Black Phillips Collimation Screws
Smooth secondary surface with serial number sticker and a lip around the secondary housing perimeter.
Type of collimation screw: Standard
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Example 3: Black Phillips Collimation Screws
Fastar secondary with “Trifid” rotary collimation screw cover and stick-on label. Collimation screws are deeply countersunk. Scope may or may not have EdgeHD optics.
Type of collimation screw: Metric
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Example 4: Silver Phillips Collimation Screws
Smooth secondary surface with or without a serial number sticker, and a lip around the secondary housing perimeter.
Type of collimation screw: Standard
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If you decide to remove a Celestron 9.25” SCT collimation screw, here's how to determine if the screw has standard or metric threads. 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 14 thread turns over its 7/16 inch length, and the metric screw has about 20 thread turns over its 10 mm length.
Please contact us using the link below for guidance if you have further questions.