Red Dot Not Lining Up With Front Post

[Redberens]

I have a question about the relationship of the red dot and the front sight post. I recently added an Aimpoint T1 to my Colt. After zeroing the Aimpoint at 50 yards, I checked the zero on my iron sights. Both shot exactly where I need them to, but the odd thing is they don't exactly line up with each other.

I've noticed when I look through the rear aperture, the dot is on the right side of the front post. Not by much, but it's definitely over. At lower power settings I can see that if the front sight post were divided in two, the dot would be completely on the right half.

I’m just wondering if anyone else has experienced this? The only explanation I can come up with is that the Aimpoint allows for much finer adjustment, whereas the Larue BUIS has less ability for adjustment?

[Belmont31R]

Two independent sighting systems.



To be exactly in the same spot you would have to assume every tolerance is exactly where it needs to be, and there is zero design or manufacturing variables.



You can use a zero'd iron sight or RDS to help get the other (vice versa) on paper...but thats about it.

[subzero]

Presumably you zeroed the irons shooting with one eye.

But the red dot you zeroed with both eyes open.

That's the difference. Chances are you have a slight astigmatism.

[shootist~]

Unless you cant the gun, (maybe later at home?), they should line up or be very very close. How could you have two sighting systems point at two different points and still shoot the same POI?

[sevin8nin]

absolute co-witness or lower 1/3 co-witness.
Just curious.

I notice the same thing on my rifles, but they have the same POI for me.

[Quiet-Matt]

Both shot exactly where I need them to, but the odd thing is they don't exactly line up with each other.
I've noticed when I look through the rear aperture, the dot is on the right side of the front post. Not by much, but it's definitely over.
I’m just wondering if anyone else has experienced this?

I eperience same exact thing with the Amipoint Micro. The dot is just to the right of the post. I just figured that it had something to do with the smaller glass on the micro vs optics with larger glass. I never had this occour with Eoechs or the bigger aimpoints. Just zero them independantly and roll on.

-Matt

[Hmac]

Presumably you zeroed the irons shooting with one eye.

But the red dot you zeroed with both eyes open.

That's the difference. Chances are you have a slight astigmatism.

Huh?

Could you explain how that works?

[subzero]

I'm no eye doctor but I'm told it's got to do with the shape of a person's eye. Each eye is slightly different from the other, and this can cause an image shift or blur.

The way I found out about it was trying to shoot an occluded eye sight (an RDS with the front lens cover on, in this case). With my right eye unable to look through the lens to build the image, I was relying on my right eye to see the dot and my left eye to build the image of the target. When I superimposed the dot over the target, I was hitting way left thanks to my messed up eyes.

[Hmac]

I understand astigmatism, I'm trying to understand how it would cause POI/POA difference in one-eye vs both-eye aiming. I'm not sure your proposal is correct but it's an interesting point to think about. I have a buddy that's an ophthalmologist and a shooter - I'll ask him next time I see him.

[JSantoro]

Nearsightedness/farsightedness is caused by the physical shape of an individual's eyeball(s), which, in layman's terms (i.e., the only ones I'm capable of truly understanding, too :p ), allows the focal intersect of whatever the eye is seeing to lay in front of or behind the fovea (where all the rods/cones are, part of the retina), instead of upon it.

http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/19511.jpg

Astigmatism is the result of an irregularly shaped lens or portion of the cornea over the lens, which can put the focal intersect ANYwhere.

http://eyezone.in/Images/astigmatism1.jpg

What subzero said is possible -- metaphorically, using only Camera 1 while lining up three items in one's focal plane and focusing on the one in the middle can net different results than using both Cameras 1 and 2 while lining up two items in one's focal plane and focusing on the one at the far end -- but his conclusions as to why it is possible are incorrect. What he described is more likely to be a simple matter of relative perspective, and can alter based upon one person's eyes being set wider or narrower on their face than another.

An astigmatism might exacerbate the effect, but would unlikely to be the cause. It'd more likely change the perceived shape of whatever is being focused upon, but would have to be pretty damned bad to alter it's relative position.

They are two separate sighting systems, and therefore should be zeroed separately. They MAY line up with one another, but are in no way required to IOT be effective. A slightly canted FSB or a forend rail that's close-to-but-not-quite installed perfectly plum/level/square, upon which is mounted a front BUIS (and therefore not in exact plane with a receiver-mounted RDS) may have the same effect while having nothing to do with what the eyes of the shooter may or may not be doing to his visible field.

[Belmont31R]

Unless you cant the gun, (maybe later at home?), they should line up or be very very close. How could you have two sighting systems point at two different points and still shoot the same POI?




Because you would have to have all 3 sights be in perfect alignment with each other and the target.


There are manufacturing variables and tolerances. For instance the rear sight can be IIRC 9 clicks to either side of mechanical zero and be in the acceptable range. That doesn't mean the RDS is going to be on the same exactly plane and height as the irons, and thus when aiming at a target the two will not perfectly line up. Just enough for the dot to sit slightly to the side of the FSP.


If everything was made to perfectly line up there would be no need to have adjustments on sights and scopes. You could just slap any scope on the gun, and no need to zero it. Aside from trajectory scopes have adjustments to compensate for manufacturing variables in each rifle.


With the eyes thing that is why each gun should be zero'd to the shooter. You cant just zero a gun, and have anyone hit to the same POI.

[shootist~]

Because you would have to have all 3 sights be in perfect alignment with each other and the target.


There are manufacturing variables and tolerances. For instance the rear sight can be IIRC 9 clicks to either side of mechanical zero and be in the acceptable range. That doesn't mean the RDS is going to be on the same exactly plane and height as the irons, and thus when aiming at a target the two will not perfectly line up. Just enough for the dot to sit slightly to the side of the FSP.


If everything was made to perfectly line up there would be no need to have adjustments on sights and scopes. You could just slap any scope on the gun, and no need to zero it. Aside from trajectory scopes have adjustments to compensate for manufacturing variables in each rifle.


With the eyes thing that is why each gun should be zero'd to the shooter. You cant just zero a gun, and have anyone hit to the same POI.


Assuming both are actually sighted at the same 50 Yd target - at the same time: How could you be on the bench (or prone, etc); co-witnessing the dot and the irons with the two not aligned? I'm talking about actually putting shots on target, co-witnessed.

Irons sighted before the RD is installed could possibly result in a parallax issue, but that's not at the same time. Neither is fooling around later with the rifle canted at a different angle (or different cheek weld) than used for the sighting session.

I do agree with "just sight them separately and move on" if that's what works for someone. However I would be headed back to the range to find out what's going on.

[Hmac]

An astigmatism might exacerbate the effect, but would unlikely to be the cause. It'd more likely change the perceived shape of whatever is being focused upon, but would have to be pretty damned bad to alter it's relative position.

Granted, I'm not an ophthalmologist, and I haven't been taught or studied anything about these kinds of eye issue since medical school, but that corresponds with my understanding of astigmatism.




They are two separate sighting systems, and therefore should be zeroed separately. They MAY line up with one another, but are in no way required to IOT be effective. A slightly canted FSB or a forend rail that's close-to-but-not-quite installed perfectly plum/level/square, upon which is mounted a front BUIS (and therefore not in exact plane with a receiver-mounted RDS) may have the same effect while having nothing to do with what the eyes of the shooter may or may not be doing to his visible field.

I think that makes more sense and pretty much nails this issue.

[markm]

Ditch the Red Dot. Problem solved. :cool:

[Belmont31R]

Assuming both are actually sighted at the same 50 Yd target - at the same time: How could you be on the bench (or prone, etc); co-witnessing the dot and the irons with the two not aligned? I'm talking about actually putting shots on target, co-witnessed.

Irons sighted before the RD is installed could possibly result in a parallax issue, but that's not at the same time. Neither is fooling around later with the rifle canted at a different angle (or different cheek weld) than used for the sighting session.

I do agree with "just sight them separately and move on" if that's what works for someone. However I would be headed back to the range to find out what's going on.


There is parallax, and depending on the manufacturing variables the LOS may not line up between both sighting systems.


If you had the irons out of the way, zero'd the RDS, took the RDS off, zero'd the irons, and then used both together they don't really have to line up as long as both are zero'd. To assume they would line up perfectly you have to assume both LOS's are on the exact same plane and intersect right at the same POI.


You're basically making adjustments within the RDS affecting your LOS through the optic. Parallax is what allows the dot to always seem centered within the RDS despite making internal adjustments. If there was zero parallax the dot would be off centered in appearance within the tube.

If people don't think there is parallax then look through it with a FSP in the way, and move your eye round. The dot will move in relation to the FSP. What happens is they set a parallax distance where there is minimal parallax at that range. Usually 100Y or 100M. Its a bit of marketing spin to say an RDS has zero parallax.

Now how that relates to the irons is because you've made adjustments to two sighting systems to get your LOS aligned with the POI you create two LOS's. Depending on manufacturing variables and deviances the two might not line up. I don't really think theres anything wrong with it but it what it is. You have two iron sights placed on different points along the gun, and an RDS somewhere in the middle.


The only thing that matters is when you use the sight system you are aiming with it hits where you are aiming. Worrying about two different systems not being alignment if they still both hit the target when being used doesn't really matter. They are not meant to be used at the same time in the first place.