ATN is the worst of the group, I had an ATN Thor HD 19mm 1.25-5x and it was terrible. On black hot, there was no image what-so-ever. The screen was solid white. I called ATN twice and eventually they told me to send it back and I did. Maybe I had a lemon, but plenty of other people have had trouble with ATN, I recommend not going with ATN.
The Pulsar Apex I pointed you to, lists at $2,500 but if you call Optics Planet they will give you a 10% discount. Or call other dealers and ask for $2,250 price. I doubt they will go much lower.
[URL="https://www.opticsplanet.com/...www.opticsplanet.com/pulsar-...ig-nv-scp.html[/url]
BUT, that 2.2-8.8x is not real. The 2.2x is real, that is optical magnification, as I explained above on the front lens. You can go to 2x digital, that will be 4.4x net magnification and still see things. But if you go to 4x digital at 8.8x net magnification, the image will be too fuzzy. In fact, I would never go to 2x digital, just leave it on 2.2x optical for clearest image. However, in the Pulsars, there is a PIP (Pic In Pic) feature which allows the top center 10% of the screen to show a second view of the center of the reticle at 2x digital (net 4.4x) for final aiming. You still have the FOV of 2.2x optical, but you have the shot placement/final aiming capability of 4.4x net ... the best of both worlds. It takes some getting used to, but I find it useful.
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As to FLIR/Armasight. I've had five Armasight Thermals and there was nothing wrong with the widgets. But after the merger with FLIR, we hear the entire CS staff has been laid off and FLIR is rebranding the Armasight thermals. This is different from the way Trijicon handled the merger with IR Defense. That merger seems to be going well.
So my concern with FLIR/Armasight right now, is I would want to wait and see things settle down and see the direction the new boson product line will take. The line has been delayed.
Nothing wrong with waiting if you can. There will always be new things coming out, from Pulsar and Trijicon as well. And SHOT show is in January. Of course, new thermals shown at SHOT show seem to on average, be released about 9 months later, so the wait might be a while.
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As to Clipons, the only ones near your budget that I would consider are the pulsars.
https://www.opticsplanet.com/pulsar-core-thermal-riflescope.html
There is a 38mm focal length model and a 50mm ... you can probably get the 38mm for 10% off or $3,700. Call whatever dealer you like and offer $3,700 and see what they say.
But be aware, two issues with thermal clipons
1 - Mounting, it might be possible to mount directly to the scope bell, but I don't know anyone that recommends this, including me. And mounting to the rail, you need rail space and you need to be at the right height (1-2 tenths of an inch from center line of day scope). This might require you to need new rings and or a longer rail. So mounting for clipons can be a challenge if you didn't build your gun with clipon mounting in mind.
2 - Magnification. Thermal clipons are 1x magnification. Each 2x you crank on the day scope costs you half the magnification in both dimensions on the back of the TV screen. So for a 320x240 device you are now a 180x120 and you've lost 75% of your pixels. And that is about the limit. Personally, for a 320 device I find even 3x to be too fuzzy on the day scope. So you can run a thermal clipon like these in front of your NF 2.5-10x ... at 2.5x at minimum power And that is it. Any higher and the image will be too fuzzy.
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So due to those two issues, for short distance hunting (out to 300yds depending on target size), I recommend a thermal scope, like you were originally thinking. Then you have optical magnification like 2.2x on the front and you get all your resolution with the 2.2x and if you crank up to 2x digital (4.4x net) then you still have a usable image. Better image and more magnification with a dedicated scope.
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You can probably find a used Apex 50mm for around $2,500, I've seen one go for that recently. But if you want to buy new with a warranty, then the Apex 38mm is a good hunting scope, I've had both and still have the 38mm and I recommend it as a good entry level hunting thermal.