Clip-on or dedicated scope?
A clip-on mounts in front of the day scope you already own, so you keep that optic for daylight and add thermal only after dark. A dedicated thermal scope replaces the day optic. Guide's Orion 2-in-1 models also come off the rifle and work as a handheld monocular for scouting.
How to choose
Sensor
A 640-class sensor shows more detail at distance than a 400-class one. The Athlon Cronus ATS Pro 35C-640 and 35CL-640, the Guide Orion O635CS, the Alpen Apex 640 and Guide's TB clip-ons carry 640-class or larger sensors. Inside about 250 yards, a 400x300 sensor resolves what you need.
Rangefinder
Some clip-ons add a laser rangefinder: the Athlon 35CL models, the Alpen Apex 640 50mm and Guide's TB650P and TB1260LS, whose rangefinders sit inside the objective on the TB650P. Between Athlon's 35C-640 and 35CL-400, the choice is a sharper sensor or a rangefinder.
Frequently Asked Questions
Which clip-on is lightest?
The Guide Orion 2-in-1 models, at about 10 oz. The Athlon clip-ons weigh 16.5 to 17.2 oz and the Alpen Apex 640 50mm 20.5 oz.
Can a clip-on work as a handheld?
The Guide Orion 2-in-1 models can: they work as a clip-on or as a handheld monocular for scouting.
Is thermal hunting legal?
Rules differ by state and by species. Check your state wildlife agency before you hunt with thermal.
What does NETD mean?
Thermal sensitivity: the smallest temperature difference the sensor can show, in millikelvins. Lower is better, and it decides how the image holds up in fog, rain and humidity.
For each maker's clip-ons in depth, read Guide Sensmart Thermal Lineup and Athlon Cronus ATS Thermal Lineup.