Astigmatism/Killflash Question

[wtm75]

I was having a hard time zeroing at 50 yards with a magnifier. Sometimes the dot looked like a slash, sometimes like a cluster of grapes, and sometimes like a figure eight. My groups were horrible.

I put duct tape on the end of the magnifier where you look through and poked a tiny hole with a pen. Well to my surprise, I saw a perfect dot and was finally able to zero correctly.

[bp7178]

Unless you shoot with that duct tape all the time I don't know if i'd zero that way because of the possibility of perceived POI shift when you aren't using it.

[jmp45]

I have a little astigmatism also. With shooting glasses it becomes more noticeable than without. I'll be looking to upgrade those again this year. I've tried a few. Even if the glass isn't distorted it still increases the issue when I have a good cheek weld. Good tip using the rear buis to help eliminate the issue. Using an Aimpoint Pro.

[Jgraves1107]

I find that paying more attention to the target instead of the dot helps. I can zero better that way. Glasses fix it, but when you start to sweat they het in the way! I was taught that your natural hand eye will line you up all the time. It's just getting use to it that's hard to do.

[JSantoro]

possibility of perceived POI shift

No. An aperture isn't a medium. All it does is alter the length of the focal plane, not bend/refract light like another layer of glass would. His aperture is simply one of tape, instead of a peep sight.

[Kokopelli]

An aperture will certainly help with astigmatism and "old eye" focus issues. Of course anything like this will slow target acquisition accordingly.. Ron

[bp7178]

No. An aperture isn't a medium. All it does is alter the length of the focal plane, not bend/refract light like another layer of glass would. His aperture is simply one of tape, instead of a peep sight.

With a SR-15E3, I shot a group with irons at 50 yards, then with a T-1 at 50 yards.

I just flipped up the rear sight, and the dot was very nice and round, but the POI was about 3" low and 2" left.

Would this be more of a parallax issue as the dot was used in the bottom 1/3 of the T-1 or related to using it with the aperture of the rear sight? The rear sight is a KAC 600m.

Without a doubt a big POI shift. Shot it w/o the rear sight flipped up and like magic went right back to zero.

[JSantoro]

Damn, that's a bit of a puzzler....THAT great a shift at that distance is outside the trend I'm used to seeing.

On the surface, it's in the same vein as how putting tape or the like over the FO on an ACOG keeps the lit portion of the *insert reticle shape, here" from blooming. A group shot with the pre-tape reticle will differ from that of a tighter, more defined post-tape reticle shot group because blooming can and does make the apparent position of a reticle differ from that of the actual, non-bloomed shape of the reticle on the target.

We're not talking about ACOGs, of course, but it's still a lit reticle and the principle applies. Given that some folks are seeing bunch-of-grapes shapes, slashes, etc., it adds a variable that somewhat fits what you're describing...with a shift that great being what prompts me to make s Scooby-Doo interrogative noise... under the same principle, BUT (of course, there's a "but...!")...

I'm not an eye doctor, and gynecology's only a hobby. There's nothing in my head that tells me an aperture would or should create the effect you're describing, and it's telling me that I need to dig deeper into how/whether parallax can be induced by something other than passage through a medium.

And, we've still got the individual eye behind the sight to account for...

That's a long way of saying what amounts to "I got nuthin'," innit?

[bp7178]

I didn't have that much time to test it after I noticed that much of a shift.

Next range trip I'll verify everything is on zero and try it again, and have another shooter give it a try.

The group with the shift was tighter than the group w/o the rear flipped up, all shots at 50y (there was only five per group) were touching but strung vertically. No other group I shot that day was strung vertically, so that was a bit odd as well.

The T-1 mount is a lower 1/3 Larue mount. I'm wondering if the results would be different with a absolute co-witness.