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From: (Bart Bobbitt)
Subject: Re: [SCOPE]Backlash Hysterises
Organization: Hewlett-Packard Fort Collins Site

Dion Hollenbeck ( wrote:

: I think backlash hysterises is the right term.  Anyhow, anyone have
: any good tricks for changing scope elevation or windage and then
: returning it to its original position.  I have tried writing down the
: number of clicks and then reversing direction to get it back, but it
: never seems to work.  This would be mighty useful for different loads
: in the same gun.

I've been using an older Weaver T20 for highpower competition for several
years.  About twice a year, I mount it on my .30-.338, intall a collimator
in the muzzle, then adjust the scope to the top center corner of the X of
the collimator reticule.  With a 7X monocular on the scope's eyepiece, I
refine the adjustment with the magnified view to a reference point on the
reticule.  Then, I move the adjustments a given number of clicks in both
directions and back the same number.  So far, the adjustments have been
repeatable to about .05 MOA, or 1/5th of a quarter-minute click.

I even plotted the errors on paper to get the hysterises loops visible.
An interesting chunk of data.

I've also done this test with Leupold 24X target scopes made since 1984;
they also have about the same repeatability.  And in testing other scopes,
none are as repeatable as these two makes/models.

An interesting thing about most scopes is that they seem to need a
`running start' to get the reticule back to the same place it started
from.  For example, from a given starting point, let's say you want to
move the reticule 5 MOA.  By moving the adjustment about 3 MOA more than
that, then moving the adjustment back to the desired setting, they seem
to be more repeatable.  And always comming to the setting from the same
direction seems to help too.  Like moving the adjustment about 3 MOA in
the plus direction to each new setting.  When this is done, the mechanics
seem to have all the slack and lost motion removed.  But the mechanics of
most scopes are such that a repeatability of less than half a MOA is
just not there.  Plus the mechanic's ability to keep the reticule in the
same place from shot to shot after an adjustment has been made varies a
lot across scope brands/models.

But I get a kick out of seeing ads/articles in print showing four 5-shot
groups; each at a corner of a square supposedly showing how good a scope's
repeatability is.  The groups are typically about 3/4 to 1 MOA in size
and about 10 or so inches apart.  All to demonstrate how repeatable a
scope's adjutments are.  I still haven't figured out how these folks can
claim repeatability of about 1/4 MOA when their groups are 3 to 4 times
that size.  It's kind of like measuring the thickness of a sheet of paper
with a yardstick.  I guess these claims are impressive to the ignorant.


From: (Bart Bobbitt)
Subject: Re: [SCOPE]Backlash Hysterises
Organization: Hewlett-Packard Fort Collins Site

Bob Westover ( wrote:

: My testing plan is to shoot 5 groups into the four corners of the square,
: in the following manner: a - b - c - d - a, the letters representing the
: four corners, so corner "a" will be a 10-shot group. If anyone else can
: suggest any additional exercises to subject the scope to, I'm interested!

If your scope has 1/8th MOA adjustment clicks, I think you'll need something
to measure them that has an error of less than the per-click error.  For
example, if you want to see if the scope's adjustments have no more than
a 25% error, you need something that will measure the adjustments to that
same percentage of the adjustment value.  For a 1/8th MOA click, that is
1/32nd MOA.  I don't think you'll be able to verify the accuracy of 1/8th MOA
within 25% by shooting 4-corner groups unless the rifle/ammo/shooter and
conditions produce at worst, .03125 MOA groups.

I suggest using the collimator-monocular method I mentioned in my earlier
response.  One nice thing it does is check adjustment tracking.  By tracking,
if you put the vertical crosshair as close as possible to a reference
point on the collimator reticule, you can move the elevation knob throughout
its range, then watch for horizontal movement of the vertical crosshair.
Some scopes don't track very well; as you move elevation, a small amount of
horizontal movement takes place.  This typically means the reticule cell
isn't aligned exactly with the adjustment axes, or the crosshairs are not
perpendicular to each other at exactly 90 degrees.  On some scopes, the
flat surface on the adjustment head is rough enough that you can watch the
crosshair jump a bit back and forth at right angles to the adjustment axis.

There's another way to check scope adjustments that doesn't require a
collimator.  Take a ruler and place it exactly 25 yards in front of the
scope's objective lens.  Solidly mount the scope, like in a vice in your
garage (or even on the kitchen counter like I've done) and place the ruler
outside at right angles to the line of sight.  With another telescope (or
pair of binocs, etc.), look through the scope and align the reticule on
the ruler.  Some scope focusing may be needed, both objective and parallax.
But you can watch the scope's reticule move across the ruler as you make
adjustments.  It's easy to record the adjustment value vs. the mark on
the ruler.  If the scope really has 1/4th MOA clicks, the crosshair will
move 1/16th inch mark on the ruler for each click.

In checking several scopes, it's really interesting when you're looking
through a scope with a 7X monocular and get great/nice magnification of the
reticule's image on the test target.  Then move the adjustment one click
and nothing moves; putting the adjustment back and still no movement.  And
with another scope, the reticule moves exactly what's expected and it
tracks great.  No shots fired; no data contamination due to wind or shooter
or ammo error.  If a scope doesn't adjust and track well under these
static conditions, I don't think it's worth shooting with.


From: (Bart Bobbitt)
Subject: Re: scope for .22-250, loading, general info.
Organization: Hewlett-Packard Fort Collins Site

Eric E. Snyder (eesnyder@boulder.Colorado.EDU) wrote:

: However,
: at $449 locally, rumors of this sort of problem sound discouraging.
: Can anyone confirm or deny the existance of this problem?

Virtually all zoom scopes change point of impact as their power changes.
But the Leupolds typically change less than the others; their mechanics
are excellent.  It's possible that Leupold let some out that were at
the edge of specifications, or even beyond.  For best, and most repeatable
accuracy, the fixed-power scopes tend to be more reliable.  They don't
have 3 or 4 extra lens elements to maintain optical alignment through
a range of mechanical movement.

If you want to watch something interesting, put a collimator in the muzzle
of a rifle with a variable power scope on it, then adjust the scope onto
the collimator's reticule.  Look through the scope and vary its power;
that reticule will make figure 8s, loops, or some other strange movements.
I've checked some scopes that have had two MOA of reticule movement through
their power range.


From: (Bart Bobbitt)
Subject: Re: scope for .22-250, loading, general info.
Organization: Hewlett-Packard Fort Collins Site

I meant to mention that virtually all scopes have their reticules mounted
in the focal plane of the eyepiece.  This is done to keep it centered
as adjustments are made.  Adjustments move one end of the erector lens
group; the other end is anchored to the scope tube.  Another reason for
mounting the reticule in the focal plane of the eyepiece is so the
eyepiece can be focused on it to obtain a sharp, clear image of the
reticule.  Adjusting the knobs only moves the erector lens assembly which
moves the target image on the fixed reticule.  It's the erector lens
tubes that has the spiral slots in it to move different lens groups
back and forth to change the magnification.  With a little math, you
can calculate how much the erector tube's adjustment end needs to move
for a given adjustment.  Or, you can measure how much an adjustment knob
moves per revolution, then divide that distance by the number of clicks or
marks on the knob to find out what the actual movement per click/mark is.
After you've done this, it's easy to see why a few ten thousandths of
an inch slop in the mechanics causes inaccurate adjustments and the scope
not being repeatable.


From: (Bart Bobbitt)
Newsgroups: rec.guns
Subject: Re: : Rifle Scopes
Date: 28 Feb 1994 19:46:46 -0500

Norman F. Johnson ( wrote:

: I am curious how you define "accuracy" in a rifle scope.  

Rifle scope accuracy, to me, involves five areas:

* Round to round repeatability.

  As the scopes innards vibrate around from recoil, it's important that
  the erector tube goes back to the exact same place after each shot.
  All scopes have an erector tube to invert, or erect, the target image
  that is focused on the reticule.  The reticule is at the back end of
  the scope and is in effect just like the film plane in a camera.  The
  erector tube is anchored to the main tube next to the reticule.  At
  the front end of the erector tube are the adjustment screws.  A hard
  metal ring on the erector tube rides on the flat surfaces of each
  adjustment and are held hard against these adjustment flats by a spring
  for each adjustment.  After each shot, vibrations in the scope loosen
  the front end of the erector tube.  If the design and parts quality
  are such that the front end of the erector tube is at a different 
  place after each shot, the lens groups in the erector tube now are
  at a different angle.  That changes where these erector tube lenses
  position the target image on the reticule.  Which means the barrel
  won't be aligned to the same angular offset from the line of sight
  for the next shot. 

* Adjustment repeatability.

  For each click, the adjustment should move the erector tube's front
  end exactly the same amount.  And in both directions; up/down and
  left/right.  When 4 clicks/graduations are made, the erector tube
  moves exactly the same amount each time.  To be repeatable in this
  department, the adjustment screws must be perfectly fit and have
  enough preload by the spring opposite them that pushes the erector
  tube fron end against 'em. 

* Adjustment backlash.

  When 2 clicks are made in one direction, then 2 clicks are made in
  the opposite direction, the erector tube puts the target image exactly
  back where it was before.  Or if 20 or 200 clicks are made, when 
  reversed, all is as it was before.  This means the springs holding 
  the erector tube's front end against the adjustment screw flat must
  overcome any looseness of fit between adjustment screw and housing.

* Power change vs. image movement.

  There's probably some scope-industry term for this (and the others, too)
  but I don't know what it is.  But it pertains to target image movement
  on the reticule as a variable's power is changed.  When a variable scope
  is `zoomed,' two lens groups in the erector tube get moved.  Each group
  moves forward or backward different distances; this is how the power, or
  magnification is changed.  Screw-cam mechanics are used; pins in each
  lens group's cell ride in spiral grooves in the zoom tube that gets
  turned by the power-change ring.  The closer the tolerances in these
  mechanics, the smaller distance the target image moves about the reticule
  during zooming.  But it'll never be zero as manufacturing tolerances are
  required to keep prices down.  A thousandth of an inch slop in this
  area can easily mean a half MOA of target image movement on the reticule.

* Adjustment tracking.

  When an adjustment is moved throughout its limits, the target image on
  the reticule tracks the reticule's axis.  In other words, if an elevation
  adjustment is made, the target image moves exactly parallel to the
  vertical reticule element.  And there is no movement at right angles to
  the adjustment axis.  This means that when making an elevation change,
  the target image doesn't move back and forth in windage as the erector
  tube's point sliding on the windage adjustment surface due to micro-grooves
  on it from the way it was machined and ground about mirror smooth.

The physical movement an erector tube's front end moves per click (or per
mark for the non-clicker scopes that have friction-tight adjustments) is
quite small.  A few ten-thousandths of an inch is normal when a 1/4th MOA
change is made.  Yet us shooters expect everything to come back into
perfect alignment after belting this opto-meco assemblage of parts with
the recoil from a magnum that we insist is the only thing to shoot.  And
the two lens groups floating in spiraled grooves get the same belting force
as the adjustment mechanics get.  All of which to me means that absolute
repeatability is oft times closer to a dream than a reality.

No wonder Cecil Tucker (a fine Texas gentleman) puts his extra-strength
adjustment preload spring on Leupold target scopes.  The factory springs
just aren't strong enough to keep the erector tube's front end hard against
the adjustment screw flats consistant and repeatable enough to satisfy
some folks.  Well, considering the space in the 1-inch tube to put a leaf
spring opposite each adjustment is rather small, an internal leaf spring
is not going to put much force on the tube.  Mr. Tucker has a 3/4ths inch
long tube outside the adjustment turret at a 45-degree angle.  With a heavy
duty coil spring inside of this tube, the erector tube's front end is well
and consistantly positioned against both adjustment screw flats.  This also
preloads the adjustment screw threads against their mating threads in the
adjustment housing; there is no backlash whatsoever.

Real, true, top-of-the-line rifle scopes used to win matches and set records
in competitive shooting disciplines have just about zero error in four of
the above five areas of scope accuracy.  The one area they don't have any
concern with is power change vs. image movement; variables aren't used.  A
decent competition scope will have all of the other three accuracy areas
combined total no more than about 1/50th MOA (0.02-in. at 100 yards) error.
If they had more repeatability and backlash error than that, those screamers
(sub 1/10th MOA benchrest groups) would probably never happen. 

Making such scopes demand extreme attention to detail.  And that precision
manufacturing processes ain't cheap.

: Incidentally, my Widener's flyer offers the KT15 for $165.  

Sounds like a good price to me.


From: (Bart Bobbit)
Newsgroups: rec.guns
Subject: Re: Weaver Scopes question
Date: 9 Oct 1994 16:09:41 -0400

David Post ( wrote:

: But I would have thought the engineers could have eliminated this source
: years and years ago.  Wouldn't anti-backlash technology have taken care of
: most of this source of error?

Anti-backlash technology did take care of MOST of this source of error.
The problem is, it didn't take care of all of it.  Machining and fitting
the lens cells into the erector tube's power change mechanics costs
money to do it right and have minimum repeatability errors.  Customers
end up paying for that.

On another related topic, the objective (front) lens cell gets moved
back and forth to focus the scope at different ranges; just like a
camera lens.  That lens cell is also subject to being non-repeatable
in its position.  And if it's not in the same place from shot to shot,
it will focus a different part of the target on the reticule center
for each shot.  Fortunately, its sideways movement is typically much
better controlled than power change lens cells.  There's virtually
no movement other than front-back and that keeps the target image
very repeatable on the reticule.  But there are some scopes whose
adjustable objective isn't very repeatable and therefore the scope
isn't capable of really good accuracy.  


From: (Bart Bobbit)
Newsgroups: rec.guns
Subject: Re: Scope technical details
Date: 28 Nov 1994 11:20:47 -0500

David Post ( wrote:

: Thanks for the posting on the advantages of 30mm tube scopes.  I wonder, if
: I asked a reputable manufacturer how much more rugged and repeatable their
: 30 mm scopes are compared to their 25 mm scopes, what kind of answer I would
: get?

I have no idea.  Ask the manufacturer.  

: Do they have internal shock and vib specs like some well known electronics
: manufacturers, such that they could say "well, for such and such g's shock,
: the 30mms hold 0.3 moa and the 25mms hold 0.5 moa under foo conditions".

Many manufacturers do shock test their scopes.  I've never seen any test
results published in such a way to be meaningful.  Sometimes, they advertise
such stuff, but the way it's done is plain old marketing hype.  The one I
laughed the most at was where five 5-shot groups were shown indicating how
well the scope's adjustments worked.  This is called `boxing' and is not a
good way to check a scope's adjustment repeatability; it introduces way too
much rifle/ammo/shooter error, and, if the scope is a variable, the error
caused by the power-changing mechanics not being repeatable compounds the
data error.

: Has anyone more knowledgeable than I ever approached them about stuff like
: this?

I don't know.

: Then I have another question.  Several manufacturers claim "return to zero"
: afer dismounting and remounting from special bases.  One manufacturer even
: implies you might want to remount a different scope, for example if you
: change terrain in which you are hunting or if the good scope is damaged, you
: might want to mount a backup in the field.

The most repeatable or return-to-zero scope mounting systems are those that
do not have any changes to the base during the remove/replace process.  Any
base that has screws or levers whose position has to be changed to remove
or replace a scope tend to not be very repeatable.  Mounting systems based
on the Weaver way tend to be the most repeatable.  These include Tasco,
Bushnell, Kelby, Jewell and Weaver.  These clamp-on types are favored by
competitive shooters and will be repeatable to 1/4th MOA or less; good
enough indeed.  They may be others, too; I don't know all of them.

: The catalogue does not have any return to zero specs.  But do they measure
: this and spec it, at least to themselves?

They might.  Ask 'em.


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