I cannot recommend the Zeus Pro 100mm ... I had one and the image was worse than the Zeus standard 75mm I had ... maybe I had a "lemon" ... the Zeus Pro had a mil reticle in it ... one of the reasons I got it ... but the reticle was fuzzy ... and barely visible even on 2x digital (8x net). 500yds was about the limit on heated steel and I expected more. So sold it and re-read the IR-V thread about high end thermal clipons and went on that journey and I think that is the right journey if you want to go long with thermal.
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Now, can you get the job done with PVS-30 or SIMRAD and a good illuminator? Listening to KSE as I have over the eons, I'd have to say I believe it totally. Now 600yds is still a LOOOONG way at night. But if you are determined to make it work, it should work.
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As to advantage with high end scope and clipon ... IR-V said yes in the article in question ... I have low end (Burris xtr2 3-15x and 5-25x) and high end NF 7-35x ATACR ... and the UTC-x looks fine through both to my olde eyes. So he says glass matters with a high end thermal clipon ... I'm not 100% convinced ... but you certainly want tracking.
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Any thermal can be shaded to the rear. Some come with rear light shades, for others you need to buy/make separately. It can be done. With clipons it is a good idea to wrap something (I use tape) around the joining between the rear of the clipon and the objective of the day scope. The keeps rain etc. out of those lenses and also light from the moon out. And on the rear of the day scope, you can mount a light shade as well.
From 600yds away your "christmas tree" will be pretty dark and small ...
Here is a clipon with tape as a light shade, in the center ...
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I think "renting" a T-75 will be tough. The FLIR folks servicing the DIRECT channel are just shipping them out. They are not a store front. And these are too expensive for any dealers to have them in stock. There might be 1-2 out there like Vic said, but you'll be hunting for months, like I did with the UTC-x. The good news, is I suspect if you hated it, you could sell it.
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AS to "clipons for dummies" .. this sight is the place ... we've debated the lining up of the optics many times.
Short answer is, yes it matters, but it matters for some clipons more than others. The military clipons are FAR more tolerant and that is the magic. Move them around from gun to gun and no adjustments needed or even possible. The civilian clipons, you will be adjusting the hecque out of.
For example, the PVS-30 is 1.5 center. And KSE attested to me that he ran it on 1.3 to 1.5 center rings with no POI offset.
The UTC-x is 1.31 center and I attested to myself that it would work 1.3 to 1.4 center. So, when I got new rings with the RAPTAR(and Radius) mounts, I went for 1.3 tp 1.4 centers and I'm fine.
So yes, worst case, you might need new rings and worst case you might need a higher chin piece ... But get a clipon and test it first.
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As to availability of LWTS-LR ... no ... and I don't put much stock in predictions either ... I work in a tech company and one of my jobs is managing projects putting new product features into production and yes we have estimates ... but those are like weather predictions ... they are always wrong. And we, like most companies, can't afford to release a new feature half done ... so ... it is DONE when it is DONE. And sometimes that takes months longer than we thought. So in summary, "in the future"
But, there is always a better widget coming ... that seems to be a constant ...
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Hope I touched all the questions ...