Scope Checker Usage Caution

[bichettereds]

Update 1/31/19

Our optics guru has frozen one donor scope using its original erector with original lenses but with new etched fine cross hair reticle and new reticle mount. He is prepping mounts for the second donor which will include new ED erector lenses and a new fine etched cross wire reticle while awaiting the new lenses.

The remaining two of us are prepping target designs to use initially to test the two frozen scopes then mounts and other scopes for POA shifts. To that end, we would appreciate input as to what would constitute a minimum detectable POA shift. Please specify as 0.XXX MOA or 0.XXX inch at XXXX yards or X.XX millimeter at XXXX meters.

Here are some targets I made up for scope testing. 1/4” grid marks.

PDF of the actual target can be found in post #11 of this thread.

http://forum.accurateshooter.com/threads/scope-checker-uses-how-to-thread.3966472/#post-37373310

I’m curious how little movement some of you guys are able to detect as well. I would think with the typical BR scopes we use, 1/16” @ 100 would be pretty obvious.

 

[tom]

Tom - I agree with you in principal.

However, we are trying to devise a test protocol that will be both practical and useful. The limiting factor will be the nominal resolution of the scopes and vernier resolution in combination with the test targets. For example the ring tests our group did had a nominal resolution of 0.05 MOA (scope) and a vernier resolution (target/conditions) of 0.01 MOA. This means any POA movement had to be more than 0.01 MOA to be detectable under favorable conditions. Note that the size of the SRBR current record group is 0.00735 MOA so we are not yet able to detect a POA shift that big.

It sounds like we're on the same page. Using 1/4" grid paper (i like the cover, dark blue with white grid lines), I had no doubt I was seeing as low as about 1/64" (.015"ish). If we can't detect it, then it isn't what's costing us at a match is how I've been looking at it anyway. I look forward to reading about some results, and different problems/solutions you might come across with the testing itself.


Tom

 

[Fred Bohl]

To that end, we would appreciate input as to what would constitute a minimum detectable POA shift. Please specify as 0.XXX MOA or 0.XXX inch at XXXX yards or X.XX millimeter at XXXX meters.

When I asked for the above, I was trying to define a measurement error circle that below which would constitute probable "NO" POA shifts or at least acceptable non-detectable shifts.

However, us three SRBR guys have come to the realization that our current test error limit of 0.01 MOA is too big, at least for our discipline. We need to get down under 0.005 MOA or less for SRBR group shooters. This doesn't appear practical by the frozen scope and test scope on a scope checker. We will continue to tilt at this windmill for awhile though, at least until spring and warmer weather.

 

[Fred Bohl]

It sounds like we're on the same page. Using 1/4" grid paper (i like the cover, dark blue with white grid lines), I had no doubt I was seeing as low as about 1/64" (.015"ish). If we can't detect it, then it isn't what's costing us at a match is how I've been looking at it anyway. I look forward to reading about some results, and different problems/solutions you might come across with the testing itself.

Tom - we are on the same page. In order to get down to 0.01 MOA from 0.015 MOA we had to design and print better high contrast targets (to take advantage of vernier/hyper-acuity vision), test only on overcast (no mirage) days and only do it ourselves (to ensure well practiced/experienced with high-power scopes observers).

Testing problems:

Scope Under Test (SUT) mounting to ensure its initial crosshair alignment is parallel to that of the Frozen Reference Scope (FRS). Our present plumb line method works but is tedious and for multiple SUTs require mounting to be done at the range. Note: we found some SUTs that either rotated slightly in the rings or had crosshairs that tilted within the scope due to recoil. Still looking for a better method.

SUT "co-witness" adjustment to the POA of the FRS. Same POA is limited to one click (typically 0.125 MOA) so we have usually used a target copy to mark the pre and post recoil SUT POA for later measurement. I am working on a method to photograph rather than draw the results to expedite and reduce error potential.

 

[Fred Bohl]

Update 2/28/19

Our optics/scope guru finished the two frozen fixed scopes with new fixed erector replacement including ED lenses and SFP reticle mount (allows changing reticle - initially an extra fine crosshair).

He was able to borrow the F & C checker we used for the last tests in return we agreed to test some scopes and rings for the owner (more on those results later). To verify the two fixed scopes, each was tested against the #1 scope and both were tested against each other - no POA shifts detected.

As promised, the test we promised for the checker owner were done and showed no POA shifts detected for either of his scopes. However, one of his scopes in one set of his rings had more offset from "co-witness" than the same scope in his other set of rings. After doing some ring shifting, fixed side left to right (same offset change but opposite direction) and reversed front and rear ring positions (no change in offset), we decided that the one set of rings on this F & C checker rail produced an offset of the scope axis that was detectable and parallel to the frozen fixed scope axis.

Yet another testing problem?