Thermally Induced Reticle Drift Of EOTech Holographic Sights
BY Herschel Smith
9 years, 2 months ago
In Note To EOTech, I mentioned the issue of thermally induced setpoint drift in EOTech holo sights, relying on the post at Soldier Systems.
While there is a great deal of information in the SOUM, two glaring issues stick out. The first is the reliability of the HWS in extreme temperatures, referred to as “Thermal Drift”. The PMO has noted a +/- 4 MOA shift at -40 Deg F and 122 Deg F. Second, is the concern over the claim by EOTech that their HWS are parallax free which was the subject of a previous Safety of Use Message from the same office issued 16 March, 2015. In this case they noted between 4 and 6 MOA parallax error depending on temperature conditions. Despite the PMO working with EOTech to rectify the issues, they still have not been resolved.
I sent my article to EOTech and have yet to hear back. Commenter Lina Inverse said:
This isn’t Amy’s fault or anyone else for that matter. It’s the fault of EOTech management for failing to educate the gun-buying public on simple things. And there is nothing wrong with the EOTech. Let me explain.
Something seemed weird about the article when I went back and thought a bit about it, and I should have done my thinking before hitting publish. Occasionally I screw up. I asked Daniel, who used plenty of weapons sights in the Marines, including night vision, EOTech, scopes for DM rifles, and so on (as well as got certified in the sighting school for Scout Snipers), if thermally induced setpoint drift was a known issue with EOTech holo sights. “Of course. And not only that, you carry your rifle around on hikes and bang it, mounts come loose and things happen. And we shot hundreds of thousands of rounds (recoil impact). We were constantly re-zeroing our weapons.”
While I am a nuclear/mechanical engineer and not an electronics and computer engineer, they make all of us take courses in rival disciplines so that we will be minimally educated know-it-alls on most disciplines. I recalled my course work, as well as what I know from ECEs where I work. When electronics get hot, strange things happen. Pumps can start and stop, and valves can change position without anyone taking action. That’s why you keep electronics cool.
And that’s why you re-zero holo sights. There’s a thermally induced current with diodes, and an EOTech holo sight is a two-wire, PN-junction LED. As for that matter, so is an Aimpoint, and whatever thermally induced setpoint drift there is with an EOTech, there will be with an Aimpoint as well. I don’t have to go into the field to prove the point. I know what’s in the component, and I know that a diode controlling a setpoint will sustain drift with temperature increase. Period. No one has invented a diode that can sustain temperature increase without setpoint drift. It’s impossible for there not to be setpoint drift.
As for EOTech, they need to explain this to their customers. They need to make their literature match reality, and they need to update their web site with salient information. As for those of us who have an EOTech, and I do, as well as a flip-to-side magnifier, we need to understand that our weapons are never maintenance-free. We need to understand them, work them, maintain them, practice with them, and care for them. You don’t do things once. You do them again, and again, and again, and again.
|
|
On October 7, 2015 at 6:19 am, MattBracken said:
It’s a snap to re-zero in the field if your holo sight is riding just above your front sight.
On October 7, 2015 at 9:46 am, Lina Inverse said:
Errr, the EOTech uses a Frickin’ Laser Beam, specifically a 0.08 mW 650 nm Class II one per the manual, plus the image it produces is clearly from a laser rather than an LED.
Personally, I might be willing to deal with drift if relatively predictable, but I’m much less forgiving about the reported failures to return to zero after thermal cycling.
On October 7, 2015 at 10:39 am, Herschel Smith said:
The laser is emitted by a diode.
On October 7, 2015 at 3:09 pm, McThag said:
So temperature is making the electronics project the dot/reticule in a different physical spot on the reflector surface?
Doesn’t it make more sense that uneven expansion across differing materials is physically moving the lenses and mirrors?
On October 7, 2015 at 3:28 pm, Herschel Smith said:
Yes that’s possible, although with geometrical symmetry through the major axes expansion of the lens/mirrors shouldn’t do that (for mirrors – for lens it may be a different story since thickness of the lens effects focal point).
If EOTech would explain themselves better this would be easier.
On October 7, 2015 at 7:40 pm, hal said:
The patent for the Holographic Rifle Sight belongs to EOTech, who invented this technology. (US5483362). A great amount of information about this type of weapons sight can learned by studying the patent.
EOTech, according to their patent has designed an optical compensation scheme using a diffraction grating to correct for the laser diodes thermally induced wavelength shift and subsequent shift of the holographic aiming reticle. Perhaps it is this part of the design that does not work as intended.
But perhaps the drift in the reticle position is not caused by wavelength shift. All materials expand and contract with changes in temperature, especially metals. This sight may be holographic, but there are still mirrors and metal mechanisms that move and can change position with temperature changes. There are special metal alloys that possess tiny temperature coefficients, one being an alloy named Invar. Perhaps conventional metals were used in the aiming point adjusting mechanisms, where an allow such as Invar would have been preferred.
Taken from patent US6490060: In order to use the holographic sight as a gun sight, a means must be
provided to move the reticle pattern to coincide with the impact point
of the bullet, that is, to zero the sight. To change the angular
position of the reconstructed reticle image (windage and elevation
adjustments), the grating 20 is rotated about the vertical and the horizontal axes to change the illumination beam angle as shown in FIG. 4.
That’s my 2 cents, from a guy with 12 years of grade school, and 60 years school of hard knocks.
https://www.google.com/patents/US6490060
https://www.google.com/patents/US5483362
https://en.wikipedia.org/wiki/Holographic_weapon_sight
https://en.wikipedia.org/wiki/Invar
On October 7, 2015 at 7:46 pm, LetsTryLibertyAgain said:
I assume the author means that the electronics that control
pumps and valves can malfunction when they are exposed to heat.
However, that is not the problem with thermally induced shift in the
point of aim on an EOTech. That’s good old thermal expansion, a
purely mechanical phenomenon that should be known to every
mechanical engineer. Good mechanical design would require the
EOTech to have thermal expansion compensation, and a 4 MOA shift in
POI over a reasonable change in operating temperatures would seem to
indicate that the designer did not compensate for thermal expansion.
I’ve only owned one EOTech and I wasn’t that impressed… with the
design or the company. I bought the two N cell EOTech because I
wanted a small form factor, before the CR123 powered XPS was
available. It’d eat batteries. I wanted to leave it on the weapon
and be able to use it a month later for home defense, but the
batteries were always dead. They’d die in a few weeks, with the
power turned off. That wasn’t right. It does no good to store a
defensive weapon with the batteries out of the red dot sight so they
wouldn’t be dead. I measured the quiescent current in the OFF
state, and sure enough, there was too much current draw and the
batteries were sourcing all of the power they were rated to source.
I assumed I got a bad one. The most common cause would be a circuit
board that wasn’t cleaned after assembly and the ionic contamination
may be creating a current leak. I sent it to L3 Communications for
repair, with a letter detailing the problem including the specific
measurements of the excessive quiescent current. I got it back a
month later with the same exact problem. There could also be a
problem with too much leakage current in a field effect transistor
used to power the circuit, either from a bad part or a bad design.
Apparently it was the latter, because there were many complaints
online about this exact problem. L3 Communications offered no
repair or replacement, even under warranty. I think they sold a
known faulty product, which they apparently fixed when they made the
XPS to replace the N cell version.
Basically, I paid $500 to beta test an EOTech, it failed, I never
got any real use out of it, and I sold it for about half price on
eBay with a full disclosure of the design flaw.
I was shooting with a friend last Sunday. Her new Adams Arms piston
driven AR-15 is very nice, but it needs sights. I sent her a link
to the EOTech, but I recommended that she get a Sightron or Vortex
SPARC. That’s what I buy now.
Fool me once, shame on you. Fool me… can’t get fooled again!
On October 7, 2015 at 10:42 pm, Herschel Smith said:
” … That’s good old thermal expansion, a purely mechanical phenomenon that should be known to every mechanical engineer.”
Eh … maybe. I’m not necessarily convinced. As I said above, it has to do with whether there is geometrical symmetry in the parts, in which case the reticle shouldn’t necessarily move (depending upon how the diode is mounted – I would have to rip mine apart, which I don’t want to do).
On October 8, 2015 at 2:16 am, McThag said:
When it dies, and they all do apparently, you’ve no reason NOT to rip into it.
I’m betting that one of the screws that adjusts the reflector position for zeroing is the culprit. Especially if the 4 moa drift from zero goes the opposite direction when it’s colder than the new recommended temperature range than hotter.
On October 25, 2015 at 12:47 pm, Schoettlin Glenn said:
I understand your point about temperature changes causing shifts in the zero of electrical instruments. I always wondered about that when I read the information on the EOTech Web Site and their advertising before buying my HHS 1 system back in Jan 2014. They have in my EOTech EXPS3-4 manual dated Jan 2011 the temperature range of operation as -40 deg F to 150 Deg F. This is the Printed Manual that I got when I bought my EOTech HHS 1 System at that time.
Today Oct 25, 2015 the online manual for the EXPS3 systems has left that temperature data out of the online manual. Now that’s TELLING. IMHO they over stated the unit and forgot to warn customers about the how the temperature changes can affect the accuracy of the zero with time and temperature changes.
I always stored my optics in the house until last winter and this summer and that’s when I noticed the zero shifted from when I first zeroed the EXPS3-4 unit on my AR15.
EOTech also claims that the zero may shift by 2 MOA when one take the quick detach unit on and off the Picatinny Rail on your weapons.
And they didn’t warn me about the 4 MOA shift in POI due to parallax errors. That’s a total surprise to me today. I thought as long as you could put the reticle on a target would hit the target no matter how you were looking though the optic. IE from the side of the glass or right down the middle of the glass with a perfect cheek weld on the weapons stock. Now I have to worry about getting the right cheek weld each time I shoot the gun with this EOTech on it.