How to Fit a Thermal Clip-On to Your Day Scope

A thermal clip-on turns the day scope you already own into a night-capable rifle without re-zeroing — but only if it's mounted correctly. Fitting one comes down to two matched parts and one careful measurement. This guide explains how a clip-on attaches, what you need to buy, and the mistakes that cost people their zero.

Quick answer

A clip-on mounts in front of your day scope's objective lens and feeds it a thermal picture, which your day scope then magnifies optically. To fit one you need two matched parts: an adapter that threads to the back of the clip-on, and a ring sized to your day scope's objective housing diameter. Measure that diameter accurately, match the adapter to it, and choose a quick-release version so you can take the clip-on on and off without disturbing your rifle's zero.

How a clip-on actually works

Unlike a dedicated thermal scope, a clip-on doesn't replace your sight — it sits ahead of it. The clip-on builds a thermal image on a small internal display, and your existing day scope looks through that display and magnifies it. Two things follow from this that matter for fitting and using one:

• Your zero stays with the day scope. Because you aim through the same scope, a properly aligned clip-on lets you switch from day to thermal without re-zeroing — the whole appeal of the format.
• Magnification comes from the day scope, optically. You don't use digital zoom on the clip-on; the day scope enlarges the clip-on's screen image. That's why a higher-resolution clip-on holds detail better when your scope is wound up — there are simply more real pixels in the picture being magnified.

The two parts you need to fit one

1. An adapter to the clip-on's rear thread

The back of the clip-on carries a thread (HIKMICRO's Thunder clip-ons, for example, use an M52×0.75 thread). A clamp such as the Rusan Clip-On Direct Thread Clamp or Modular Adapter Clamp screws onto that thread and provides the mounting interface.

2. A ring sized to your day scope's objective

The adapter then clamps over your day scope's objective bell. This ring has to match the outside diameter of your scope's objective housing — too loose and it won't hold alignment, too tight and it won't seat. This is why adapters come in a stepped range of millimetre sizes (for the HIKMICRO Thunder, Rusan offers 42, 47, 48, 57, 59, 60, 61 and 62mm options). You pick the one that matches your scope.

How to measure for the right adapter

1. Take the outside diameter of your day scope's objective housing (the metal bell around the front lens) with vernier calipers — not the lens glass, the housing.
2. Match that measurement to the adapter size. For a HIKMICRO Thunder, that's the 42–62mm Rusan range.
3. If your measurement falls between two sizes, size up and use the adapter's clamping range, or talk to us before buying.
4. Prefer a quick-release adapter (such as the Rusan Q-R one-piece options) so you can remove and refit the clip-on repeatably without touching your zero.

Tips for a clean fit

• Get it square. The clip-on must sit concentric with the day scope's optical axis — a cocked adapter shifts point of impact.
• Don't over-magnify. Clip-ons pair best with the day scope at moderate power; wind it up too far and the screen image starts to look coarse (covered in our digital zoom guide).
• Match resolution to your scope. The more you magnify, the more a higher-resolution clip-on pays off in retained detail — see our 384 vs 640 guide.
• Want it handheld too? A monocular adapter turns many clip-ons into a handheld spotter, so one device does both jobs.

FAQ

Do I have to re-zero when I fit a clip-on?

No — that's the point of the format. Because you still aim through your day scope, a correctly aligned clip-on lets you go from day to thermal without re-zeroing. A repeatable quick-release adapter is key to keeping it that way.

How do I know which adapter size to buy?

Measure the outside diameter of your day scope's objective housing with calipers and match it to the adapter range — for the HIKMICRO Thunder that's Rusan's 42–62mm options. If you're between sizes, size up or ask us.

Does a clip-on reduce my magnification or detail?

The clip-on shows a thermal image that your day scope magnifies optically. Detail holds up best at moderate magnification and with a higher-resolution clip-on; pushing the scope to maximum power makes the screen image look coarse.

Can I use one clip-on across several rifles?

Yes, if each rifle's day scope shares the same objective diameter or you buy the matching adapter for each. The clip-on itself carries no zero — the day scope does.

Related: Thermal Clip-Ons · Best Thermal Clip-Ons 2026 · Choosing a Rusan Adapter for a HIKMICRO Thunder · How to Zero a Thermal Scope · How Thermal Imaging Works