A buddy has just ordered a FLIR T60 thermal optic to put in front of his optics on various rifles, going with the notion that no-rezeroing is needed because the thermal sight doesn't change the zero of the original daylight optic. This is the position of FLIR as well. The T60 does not mount in contact with the original sight, but in front of it. On top of that, the thermal sight clips on and off the rail, which is a handy feature. The claim is you don't have to re-zero. My buddy wants the capability of shooting long range at night, such as long range hog hunting where regular night vision may not work so good and wants the option of being able to do it with various of his more precision setup rifles...several of which he likes for 600-1000 yard shooting.
The problems I see with this are numerous and I am trying to figure out why I think the problems exist, but apparently FLIR and others don't. I must be missing something.
As I see it, the optical axis of each optic will need to be alignged with absolute parallel. Any deviation high, low, left, or right from parallel will result in looking at a location not aligned with the original zero point of the daylight optic. The shots will not impact where intended. Deviation off of the axis by 1/128" from the rear to the front of the thermal optic which is about 6" long such as produced by the flexing of a floated handguard on which the thermal optic would be mounted would produce an error of about 4.7" at a target 100 yards distant, or a ~4.7 MOA error. Handguard flexing, of course, can occur from resting the rifle's handguard on something like a sandbag, for example. The more forward the resting is on the handguard, the more likely and the greater distance the handguard is apt to flex.
Misalignment of the two optics would undoubtedly result from various causes:
1. Mounting tension (which may be alleviated to being consistent with FLIR's clip-on system
2. Possible debris on the rail not allowing for perfect/replicated seating of the optic
3. Rail misalignment between the receiver and the floated handguard rail
4. Flexing of the handguard rail. This may not be an issue if the handguard isn't used as a rest or holding point such that it does not support the weight of the gun.
Most of the problems I see are with 3 and 4.
All of these things can put the forward thermal optic looking at a slightly different location than the rearward daylight optic. If the forward thermal optic is looking in a different location, then the image seen through the daylight optic via the thermal presents an invalid sight picture. Original zero is no longer maintained.
It would appear that FLIR shows the scope mounted on a non-floated rail system. http://www.flirt60.com/
My goal here is to save him considerable frustration and possible safety issues. Have I missed the boat on this and am concerned inappropriately? He has a couple single long rail setups, but is also talking about use on AR15 and AR10 platforms where the daylight optic is mounted on the receiver and the forward positioned thermal optic would be on a float handguard rail.
Comments are appreciated. If I am wrong on the errors produced by misalignment or why it may or may not occur, please explain where I have erred, preferably in small words as I am not up on all the optical/physics lingo.