Sun images method for checking alignmement of binoculars

 By Rafael Chamón Cobos

 

Created: Dec. 2006

Updated: Nov. 2007

Updated: Mar. 2008

Updated: Jan. 2013 (new Figure 18)

 

Abstract

A method for checking alignment of binoculars is described. The method uses the sun as source of collimated light and a simple  projection screen as checking device. The binocular under test  is oriented to the sun so that two sun images are projected on the screen and focused with the focusing mechanism of the binocular. Positions of sun images are compared with theoretical  positions marked on the screen. Error deviations in position of sun images are used  to evaluate misalignments of the binocular under test. Accuracy of the test can be quite high because misalignment errors in object space are multiplied by the magnification of the binocular in the image space and deviations on the screen are proportional to the distance between binocular and screen, which can be arbitrarily increased (under certain  limitations), whilst theoretical positions of sun images keep constant. The method allows to detect misalignments that are inside the tolerances specified by the collimation standards, and can be used to control  an alignment adjustment.  A simplified version of this method without the need of a marked or calibrated screen is also described. This latter allows a quick alignment test in the field during a sunny day. A strategy to adjust binoculars for so-called ‘true collimation’ without the use of  any other device  is described. Disadvantages  of the sun images method are related to the availability of the sun light and to the fact that the sun moves in the sky.

Contents

Introduction

Experimental method

Results

Discussion

Simplified version of the sun images method

Adjustment of true collimation

Conclusions

Acknowledgments

Introduction

Generally speaking, the term ‘collimation’ refers to parallelism of light rays, for instance, a ‘collimated light beam’. But this term rather indicates parallelism of both optical axes when applied to binoculars. In order to avoid confussion, we will use in this article the term ‘alignment’ referred to binoculars instead of ‘collimation’, leaving this latter term only for light beams and collimators.

If a binocular is properly aligned, then optical axes of both barrels are parallel each other. Then our eyes can effortless merge both images and vision is relaxed.

Checking alignment of binoculars is usually performed by professionals with the help of a 'collimator'. A collimator is a device that sends a collimated light beam, i. e., a bundle of  parallel light rays. Normally it consists on a puntual light surce located at the focal point of a very well corrected lens. A collimator acts as an artificial star. The collimator lens must have suficient aperture to cover both objectives of the binocular. Then, the binocular under test is put in front of the collimator and the operator verifies its alignment by observing the produced images (also star-like points), usually whith the help of some magnifying device to better detect deviations (e.g., a second, trustworthy binocular put behind the binocular under test). The best collimated light source would be a star, but for practical reasons collimators are used.

Alignment test setups are expensive and not suitable for binocular friends at an amateur level. The present article is intended to provide a method for binocular alignment tests without the use of professional components, and using only simple means like a base plate with an attached screen and some device to hold the binocular on the base plate at different distances from the screen.  

1         Experimental method

1.1       Test setup

  • The test setup consists in:
    1. The sun as collimator
    2. The binocular under test
    3. A thin wooden screen about 18x25cm in size
    4. A dick wooden board about 25x70cm in size, which serves as base plate to support the binocular and the screen. The binocular has to be somehow attached to the base plate at a distance to the screen that can be variable between 30cm and 60cm approximately. The whole set is oriented so that  the binocular projects two sun images on the screen
    5. A wooden stick about 20cm long (e. g., a pencil) to be attached on the eyepieces of the binocular to indicate the tilt of the eyepieces by its shadow on the screen

          See Figures 1 and 2

          Figure 1. Components of the test setup
           

          Figure 2. Test setup oriented to the sun

          1.2       Test procedure

          ·        Accurately measure the used interpupillary distance IPD of the binocular. To measure, use a dial caliper or steel machinist's scale to get the distance between the outside of one eyelens to the outside of the other. Then measure the diameter of one eyelens and subtract this from the previous figure. The result should give the distance between centers of the eyelenses.

          ·        Draw two vertical lines on a sheet of paper at a distance each other equal to the IPD of the binocular. Make sure that this distance is also accurately measured. Draw also an horizontal line on the bottom of the sheet of paper. Attach the sheet of paper to the screen.These three lines will be the references for the sun images positions on the screen: vertical lines for distance betwen sun images and horizontal line for tilt of sun images. 

          ·        Attach the stick (a common pencil) to the eyepieces as a “yoke” by means of a rubber tape. See Figure 3. The shadow of the stick on the screen will show the tilt of the eyepieces for a comparison with the horizontal line. Make shure that the horizontal reference line and the shadow of the stick have the same tilt.

          Figure 3. Stick attached to the eyepieces to adjust horizontal reference line on the screen to eyepieces tilt by its shadow
          • During a sunny day orient binocular and screen to face the sun in order to project two images of the solar disk on the screen. Focus the two projected images with the focusing mechanism of the binocular.
          • If the binocular is aligned, it is possible to orient the test setup so that edges of both sun images just touch the reference lines. If the binocular is misaligned sun images will show a separation error or a tilt error of both with respect to reference lines. See Figure 4.

          Figure 4. Position of sun images projected by an aligned and a misaligned binocular

           

          2         Results

          The present test method has been applied to a Dr.Wöhler Saar Septonar 7x50 binocular. This old binocular seemed to be aligned in normal use. However, for distant objects vision was not comfortable. So I supposed that the instrument  had some misalignment that was compensated by my eyes.

          Test setup data were: IPD = 65mm. Distance eyepieces-screen =  45cm.

          Following results have been found. See Figure 5.

          Figure 5. Sun images projected by the binocular under test
           
          1. The horizontal distance between sun images is smaller than IPD by 1.25cm = 0.5 inch. We say that the horizontal error on the screen is -1.25cm (negative sign is assumed to indicate distances smaller than IPD, whilst positive sign means distances greater than IPD). Therefore, there is some horizontal misalignment in the binocular, so that optical axes converge horizontally in the image space. Such misalignment is hardly compensated by forcing the eyes to diverge a little, and this is the reason why vision is not comfortable with this binocular, especially for far away located targets.
          2. The sun images does not show any tilt with respect to the eyepieces. We say that the vertical error on the screen is about 0cm, or that the instrument does not have a vertical misalignment. See also Figures 6 and 7.

          Figure 6. Measurement of horizontal error: about 1.25cm = 0.5 inch
           

          Figure 7. Measurement of vertical error: about 0cm

          A deviation error of 1.25cm = 0.5 inch on the screen is clearly discernible. This corresponds to a misalignment in the object space of  13.5 arc minutes and to a misalignment in the image space of approximately 13.5 x 7 = 95.5 arc minutes (see paragraph 4.3 Calculation of deviation error on the screen). This misalignment is clearly outside of the standards (see paragraph 4.4 Alignment standards and accuracy of the method).

          3         Discussion

          3.1       Using the sun as collimator

          The sun is so far away from the earth, that every point of the solar disk send us parallel rays. Therefore the sun can be used as collimator for checking alignment of binoculars. If a screen is located at some distance behind the eyepieces, the binocular will project on the screen two real images of the sun, that can be focused with the focusing mechanism of the binocular. Projected sun images offer good conditions to be observed on the screen, because their brightness, whilst sharp edges of sun images are the points at which deviation can be easily measured.

          3.2       Checking alignment by projection

          A binocular can project two real images of its field of view on a screen located at some distance behind the eyepieces, and these images can be focused on the screen by means of the focusing mechanism of the binocular.

          Provided that the binocular receives collimated light at the objectives, it will project two point images of the light source on the screen. If the binocular is properly aligned, these point images will show a distance each other equal to the distance between eyepieces (interpupillary distance of the binocular), and a tilt (of the straight line formed by the two images), equal to the tilt of the eyepieces. If the binocular is misaligned, either the distance between images or the tilt of images, or both, will differ. See Figure 8.

          Figure 8. Images of a collimated light source projected on a screen by an aligned and a misalignment binocular

          So, a simple screen can be used to test alignment of binoculars by measuring the distance between images on the screen when they receive collimated beams at both objectives.   

          Projection offers the advantage that angular deviations are converted in linear deviations on the screen that can be easily measured or simply compared with correct reference points marked on the same screen.  Moreover, deviations on the screen can be enlarged with the simple action of increasing the distance of the screeen to the binocular.

          3.3       Calculation of the deviation error on the screen

          Let us now calculate the value of the deviation error of projected images that produces a misaligned binocular. See Figure 9.

          Figure 9. Variables related to the deviation error

          Let us suppose that the binocular is misaligned by an angle e between both optical axes in the object space.  Then, a puntual source located at the infinite will produce two images on the sreeen whose distance each other is:

          d = IPD + e

          where e is the linear deviation error on the screen with respect to the theoretical value d = IPD.

          Being L the distance from eyepieces to the scren, the value of e can be easily calculated as:

          e = L . tan (e’)

          where e’ is the misalignment angle of the binocular in the image space.

          By definition, the magnification of the binocular is M = tan (e’) / tan (e), so we can write

          e = L . M . tan (e)

           And since e and eare small angles we can replace tangent values by arc values (in radians), so we finally have

          e = L . e         (e in radians)

          e = L . M . e    (e in radians)

          or

          e = (L . e ) / 3438       (e in arc minutes)

          e = (L . M . e ) / 3438   (e in arc minutes)

          Therefore, the linear deviation of images on the screen is proportional to the angular misalignment of the binocular in the object space e, to its magnification M and to the distance L at which the screen is located.

          Alternately, it is possible to calculate the absolute misalignment angle e of a binocular by measuring e and appliying the inverse formule

          e = e . 3438 / (L . M)   (e in arc minutes)

          Applying this formule in the case of the binocular under test (Dr. Wöhler Saar Septonar 7x50), with values e = 1.25cm, L = 45cm, and M = 7 we get a misalignment of  e = 13.5 arc minutes in the object space, which corresponds to a misalignment of approximately e = 13.5 x 7 = 95.5 arc minutes in the image space.

          The main advantage of projection is that increasing the distance L makes the deviation error e on the screen increase. Since the theoretical separation value between both images – IPD – does not change, measurements will be as accurate as the degree to which the test is setup to deliver – which could be quite high.

          In the practice, values of L between 20 and 60cm produce deviation errors of images that are clearly discernible and measurable if the screen is accurately marked and positioned and the images accurately positioned upon it.

          3.4       Alignment standards and accuracy of the method

          Existing standards for collimation (alignment) of binoculars are diverse and somewhat confusing. Misalignment angles are usually referred to the object space. However, since we perceive misalignments at the eyepieces, i. e., in the image space, where angles are multiplied by the magnification of the binocular, it is necessary to specify at what magnification the misalignment values are given. For example, a vertical misalignment of  3 arc minutes in a 7x binocular is a good value, but in a 25x binocular is inacceptable.

          Therefore, it is senseful to specify misalignment angles in the image space rather than in the object space because they are independent of the magnification. Then, misalignments in object space can be calculated by dividing given values by the magnification.

          Following alignment standards for binoculars have been found in the literature. We reproduce them below, with values referred to image space. Values for object space can be calculated by dividing them by the magnification in each particular case.

          Misalignment angles in image space (magnification taken into account)

          US standards 

          Soviet standars

          Others

          Vertical divergence

          < 35’

          < 15’

          < 40’

          Horizontal convergence

          < 50’

          < 20’

          < 60’

          Horizontal divergence

          < 100’

          < 60’

          < 180’

          It is interesting to translate the angular values specified in the standards into deviations on the screen according to the used test setup for a particular test. In our case, for  M = 7x and L = 45cm.

          Following formule applies:

          e =  (L . e ) / 3438   (e in arc minutes)

          where  e is the misalignment in the image space and L is the distance to the screen.

          According to this formule allowed deviations on the screen are:

           Deviations on screen at L = 45cm,  M = 7x

          US standards 

          Soviet standars

          Others

          Vertical divergence

          < 0.45cm

          < 0,19cm

          < 0.52cm

          Horizontal convergence

          < 0.65cm

          < 0.26cm

          < 0.78cm

          Horizontal divergence

          < 1.30cm

          < 0.78cm

          < 2.35cm

          As worst case we could consider a binocular with a moderate magnification of 6x and a moderate distance binocular-screen of 40cm. Then, maximum allowed values according to the standards are:

          Deviations on screen at L = 40cm,  M = 6x

          US standards 

          Soviet standars

          Others

          Vertical divergence

          < 0.40cm

          < 0,17cm

          < 0.46cm

          Horizontal convergence

          < 0.58cm

          < 0.23cm

          < 0.69cm

          Horizontal divergence

          < 1.16cm

          < 0.69cm

          < 2.09cm

          All these values are discernible and mesurable on the screen and it is reasonable to assume that also linear errors on the screen as small as 0.15cm = 1/16 inch are even discernible, therefore we can conclude that the method is able to easily detect misalignments inside the tolerances specified by collimation standards. This assumes that IPD is accurately measured at the eyepieces and drawn on the screen, because the degree to which the test is setup to deliver is critically dependant upon accurate measurement and charting of the IPD as it acts as a element in the equation from which arc minute error is calculated.

           In critical cases accuracy can be increased by increasing the distance to the screen.

          3.5       Using the diameter of the solar disk as angular scale

          With the sun images method there is other interesting way to directly calculate the absolute misalignment e in the object space by using the size of the sun image itself as a angular scale.

          The angular size of the sun is approximately 32 arc minutes in diameter. Therefore, a misalignment of e arc minutes in the object space corresponds to the proportion e/32 of the diameter of the solar disk. Since this proportion is kept in projected images, we can write

          e / 32 = e / D

           where e is the linear misalignment error on the screen and D is the diameter of the sun image on the screen, and therefore

          e =   (e  / D) . 32    (e in arc minutes)

          Applying this formule in the case of the binocular under test (Dr. Wöhler Saar Septonar 7x50) with values e = 1.25cm (see Figure 6) and D = 3cm (see Figure 7) we get a misalignment value of  e = 13.33 arc minutes. This value is very similar to the value e = 13.5 arc minutes found with the other formule described in paragraph 4.3:  e = e . 3438 / (L . M).

          Using the sun diameter as direct minute scale defines a more accurate and straightforward method of determining the minute arc alignment deviations than using the angular calculation method described in paragraph 4.3., because neither distance to the screen nor magnification are involved. (The exact magnification would have to be calculated separately and accurately for accurate results using the other angular calculation method). Using the sun as a ready-scaled object in image space avoids the need to calculate magnification at all.

          3.6       Disadvantages of the sun images method

          Disadvantages of the sun images method are are related to the availability of solar light during the test and to the fact that sun moves.

          Alignment tests are only possible in sunny days, provided that the position of the sun in the sky is suitable to our  work place. Possible cold wether and wind may also be a problem.

          Other problem arises by the fact that the sun moves in the sky at a rate of 1’ each 4sec (1 arc minute each 4 time seconds). This can be particularly annoying because in few seconds coincidence of sun images with reference lines is lost and displacements are comparable to the error values you are measuring. Therefore, readings have to be quickly performed and you have to constantly reorient the test setup at each new measurement.

          If you take photos by hand, without a tripod, you must foreseen the movement of sun images before you prepare the camera for a shot. Otherwise you come always “too late” and the edge of the sun images have left the reference lines. Best solution is using a tripod for the camera, and providing the screen with a calibrated chequered sheet instead of a marked sheet with the three reference lines. In this case, the instant of  taking the photo is uncritical and results can be measured with the help of the chequered sheet on the photo.

          We should also take into account possible damages in the binocular due to the effects of the sun's heat upon the optics during an alignment test, because sun rays are strongly concentred inside the barrel at the proximity of prisms and eyepieces. However, I have not detected such effects in my tests.

          4         Simplified version of the sun images method

          4.1       Quick check of the alignment of a binocular

          In its simplest version, sun images method allows checking alignment of a binocular without the need of a screen with reference lines. Holding the binocular by hand or on a tripod in front of a wall (or a cardboard), we orient it to project both sun images on the wall. On the wall we will also see the shadow of the binocular body. We adjust the distance binocular-wall so that the size of sun images is more or less equal to the size of shadows of eyepieces (in case of porro prism binoculars) or whole barrels (in case of roof prism binoculars). If we can superimpose sun images with eyepieces shadows, the binocular is aligned. See Figure 10.

          Figure 10. Checking aligment of a binocular with simplified sun images method in three cases

          On top of Figure 10 is shown a binocular facing the sun. The three bottom parts represent the shadow of the binocular in three cases. The yellow circles are the sun images. In case (A) the binolcular is aligned; optical axes are parallel and we can superimpose sun images and eyepiece shadows. In case (B) the binocular is unaligned and this superimposition is not possible. Case (C) is frequently found in binoculars. In this case the binocular is not strictly aligned, because optical axes diverge in an horizontal plane and superposition is not possible. However, the divergence is easily compensated by the eyes as a normal eyes behaviour for close distances vision.

          The alignment test with sun images is easier as we think and will detect small misalignments. The accuracy of this check is based on the fact that the bino multiplies the angular misalignment errors by its magnification. The accuracy does not depend on the magnification of the binocular, provided that the distance to the screen or wall is adjusted to produce sun images of same size in all cases (the higher is the magnification, the shorter is the distance).

          To easily check the separation and tilt of sun images it is very convenient to use a little artifact consisting in stick (e. g. a pencil) with two prinkeled pins or smaller sticks at a distance equal to the used IPD of the binocular. See Figure 11.

          Figure 11. Little artifact to use with the simplified version of the method

          Then, the shadows of stick and pins will be useful reference lines for checking. See Figure 12.

           

          Figure 12. Shadow of the stick and pins as reference lines

          Figure 12 shows the test of a ‘Nikon 12x40 CF WA’ porro binocular. Horizontal alignment of the binocular is correct, but a slight vertical misalignment can be observed. This misalignment is inappreciable in common use of the binocular.

          4.2       Checking ‘true collimation’ of a binocular (at all possible interpupillary distances)

          An interesting feature of the sun images method is that it allows to quickly check if a binocular is ‘true collimated’, i.e., if it is aligned at all possible interpupillary distances. This circumstance occurs when axle and optical axes are all three parallel each other; then the binocular is said to be ‘fully or true collimated’. Otherwise alignment is lost when IPD is changed because alignment occurs only at a determinate IPD. Then the binocular is said to be “conditionally aligned”. See Figure 13.

           

          Figure 13. Checking ‘fully collimation’ of a binocular whith the simplified sun images method in two cases

          To check “true collimation” it suffices to verify alignment at two different IPD’s with the sun images method, preferably at máximum and minimun IPD’s. If the binocular is aligned at these two IPD’s, then it is aligned at all possible IPD’s.

          5         Adjustment of true collimation

          A ‘true collimated’ binocular has both optical axes and the mechanical axis (the axle of the hinge) parallel. Sun images method allows to adjust a binocular for true collimation without the need of specific devices by using the strategy of comparing the four sun images positions that the bincular projects in two cases: when the binocular hinge is fully folded out and when the binocular hinge is fully folded in. The pattern formed by the positions of these four sun images gives sufficent information on the deviation of the three mentioned axes, and with an appropiated strategy of adjustments it is possible to bring all three axes parallel.

          5.1        Analysis of sun images positions

          Consider now a conditionally aligned binocular set at the IPD of the user. Conditionally alignment means that the optical axes are parallell to each other, but they have a certain deviation with respect to the mechanical axis. This direction of this deviation cannot be directly estimated. We just realize that sun images match the distance between eyepieces and their tilt.

          This is the main problem involved in the adjustment of true collimation: to find out in which direction the optical axes are deviated from the mechanical one, because only this information will allow us to apply the necessary corrections in order to eliminate this deviation.

          Fortunately, the sun images method provides valuable information for that, since the sun images pattern produced by the binocular at different positions of the hinge is characteristic for each deviation of optical axes with respect of the axle. Consider the following three basic positions of the hinge:

          A.     Hinge fully folded out.

          B.     Hinge folded to the IPD of the user.

          C.     Hinge fully folded in.

          And consider also four basic deviations of the optical axes when the binocular is adjusted to the user’s IPD, i. e., at hinge position B: (sense of deviations is considered as projected on the screen, i.e., when looking at the screen)

          1.      To the left.

          2.      To the right.

          3.      Upwards.

          4.      Downwards.

          Figure 14 schematically shows the patterns of the sun images projected on a screen by the binocular, combining the three basic positions of the hinge and the four basic deviations of the optical axes at user’s IPD.

          Figure 14. Patterns of sun images in a conditionally aligned binocular at three positions of the binocular hinge

          Black dots represent the intersection of the ideal (collimated) optical axes on the screen and red dots represent the intersection of the actual optical axes on the screen. Therefore, red dots can represent sun images.

          Top horizontal series shows all deviations at hinge positions A, B, C.  

          Middle horizontal series shows the four basic deviations at hinge position B (user’s IPD). In all cases the binocular is conditionally aligned at the user’s IPD .

          Botton horizontal series shows only the deviations at hinge positions A, C, and the red dots have been reinforced because they depict the four basic patterns of sun images that will provide the required information to correct deviations of optical axes.

          Basic patterns:

          We can identify the four basic patterns by means of the following quoted informal descriptions. This can help to remember them. See Figure 15.

          1.      LEFT OPENED angle”: Slanting sun images, forming a left-hand-opened angle. Distance between sun images about the same as the IPD in both, A and C cases. It means optical axes deviated to the LEFT. To correct this deviation, move objective lenses to the left.

          2.      RIGHT OPENED angle”: Slanting sun images, forming a right-hand-opened angle. Distance between sun images about the same as the IPD in both, A and C cases. It means optical axes deviated to the RIGHT. To correct this deviation, move objective lenses to the right.

          3.      PARALLEL lines with OPENED BOTTOM”: Horizontal sun images. Distance between sun images altered with respect to the IPD: smaller in A case, greater in C case. It means optical axes deviated UPWARDS. To correct this deviation, move objective lenses upwards.

          4.      PARALLEL lines wiht OPENED TOP”: Horizontal sun images. Distance between sun images altered with respect to the IPD: greater in A case, smaller in C case. It means optical axes deviated DOWNWARDS. To correct this deviation, move objective lenses downwards.

          Figure 15 shows a recapitulation of the basic image patterns and the necessary corrective movements oft the objective lenses.

          Figure 15. Corrective movements of the objective lenses in a conditionally aligned binocular in order to reach true collimation

          In the practice, any other pattern is an intermediate pattern that can be handled in two steps by using the corrections of two basic patterns.

          5.2        Use of the eccentric rings

          The eccentric rings that are located around the objective lenses in most binoculars form an ingenious system that allows small displacements of the objective lenses in any transversal direction. Since the optical axis is determinated by the objective and eyepiece centers, eccentric rings movements modify the optical axis orientation. The resulting image movements are opposite to the objective lens movements.

          There are two eccentric rings around each objective lens that can be rotated independently. Combining the angular positions of these rings it is possible to set the center of the objective lens on any position inside a small circle. See Figure 16 where the two eccentric rings have been adjusted to form an angle of 90º.

          Figure 16. Eccentric rings (dark and light gray) arround the objective lens (blue)

          Each eccentric ring produces an oriented displacement that is represented by an offset vector as shown in the figure. The vectorial sum of both offset vectors gives the final displacement of the lens center.

          The effect of the eccentric rings movements in the lens final position is not intuitive. One must think a little in order to mentally convert the rings angular positions into linear displacements.

          In order to facilitate this task it is convenient to imagine each eccentric ring as a vector that coincides with a diameter of the ring itself and that goes from the thickest point of the eccentric ring to the thinnest one. See Figure 17.

          Figure 17. Eccentric ring vectors

          The lens center position is defined by the orientation of these two eccentric ring vectors.

          Figure 18 relates the position of the objective lens to the angular positions of the eccentric rings. It will help us to locate the lens center and to move it in a controlled way. The circle schematecally represents the little circular area where the lens center can be situated (see Figure 16). This area is divided into little zones that are related to different orientations of the eccentric ring vectors. Each little  zone contains information about the orientations of the eccentric rings vectors that correspond to this particular zone.

          Figure 18. Zones diagram that relates positions of the objective lens center to the angular positions of the eccentric ring vectors

          In this figure only 12 possible angles for the orientations of the eccentric ring vectors have been considered, which are identified as the hours of a clock, being “12 o’clock” the top of the circle. For example, the information “12  3” written in a certain zone means that if we orient the eccentric ring vectors “at 12 hours” and “at 3 hours” respectively, the lens center will fall on this zone. The zone marked “X” corresponds to the center of the tube and is reached when both eccentric ring vectors are in opposition. The diagram has blank zones without angular information, that correspond to intermediate values.

           This diagram should be used as a help to make the eccentric rings adjustments. For example, let us assume that we need to move the objective lens to the right. We have first to locate the lens center. To do this, we first notice the positions of the eccentric ring vectors at the objective and we look for the little zone corresponding to these positions. Finally, we change the orientation of the eccentric ring vectors to the value that appears in a zone situated on the right side of the present zone.

          It is possible to interpolate intermediate values corresponding to blank zones with the help of the assigned colours. The coloured zones form, approximately, circumferences each of 12 zones of same colour. Same colour means same angle between eccentric ring vectors. So, if a determinated blank zone falls on the circumference line of a determinated colour, it is easy to estimate an intermediate adjustment by keeping the same angle between vectors.

          5.3        Adjusting the true collimation

          As said, to adjust the true collimation it suffices to align the binocular at two different positions of the hinge (preferably at fully folded out and fully folded in). This is much more difficult than aligning a binocular at a single interpupillary distance. The reason is clear: both adjustments are mutually dependent, they influence each other. Therefore we will need to fix collimation alternately at two positions of the hinge, again and again, but in a convergent way, till the complete collimation is reached.

          An adjust procedure is as follows:

          1.      As start point, the binocular is supposed to be conditionally aligned at the user’s IPD.

          2.      With sun images method check alignment at maximum and minimum hinge folding positions (A and C respectively, as described above) and with the help of Figure 15 deduce the deviation of the optical axes with respect to the axle, corresponding to the user’s IPD hinge aperture (B).

          3.      Set hinge at position B (user’s IPD) and write down the position of the eccentric vectors for each objective lens

          4.      Locate the center of the objective lenses using the zones diagram in Figure 18.

          5.      Move the eccentric rings in order to approximately displace the center of the objective lenses in such a way that the deviation of both optical axes with respect ot the axle is reduced. Use the zones diagram in Figure 18 to facilitate the eccentric rings movements. Write down the new eccentric ring positions.

          6.      Check alignemt at this position B by using the parallel vision method or any other method. Fine tune alignment by slightly moving the eccentric rings if necessary..

          7.       Repeat steps 2 to 6 till the binocular is completely collimated.

          If the corrections are done in the proper direction, the succession of adjustments must be convergent and finally true collimation is reached.

          It is highly recommended to mark the eccentric rings with an easily visible dot on their thinnest point, as shown in previous figures, in order to immediately visualize the vectors. It is also recommended to write down all performed adjustments, in order not to be lost during the procedure.

          The complete collimation performed in this way requires a great amount of patience!!

          6         Conclusions

          ·        The sun images method for checking misalignment of binoculars uses the sun as source of collimated light and a simple test setup consisting in a support for the binocular and a screen.

          ·        Misalignments of the binocular are detected by comparing the two sun images projected by the binocular on the screen with reference lines marked on the same screen.

          ·        From measured deviation errors on the screen it is possible to calculate absolute misalignment angles of the binocular.

          ·        Since error deviations on the screen are proportional to the distance binocular-screen, accuracy can be increased by setting the screen further.

          ·        The achieved accuray allows to detect misalignments that are inside the tolerances specified in collimation standards.

          ·        A simplified version of the method –though not so accurate- can be used in the field for a quick test without the need of a marked or calibrated scale. In this case, sun images are compared with shadows of eye pieces on any surface acting as screen. An easy to make artifact consisting in a stick and two pins improves this simplified version ot the method.

          ·        The sun images method allows checking if a binocular is ‘true collimated’, i. e., if both optical axes and the axle are parallel each other.

          ·        Adjusting  ‘true collimation’ of a binocular is possible without further devices because analysis of the sun images given by the binocular at two separated positions of the binocular hinge gives enough information on the actual direction of the optical axes and on the way they must be moved in order to match the axle direction.

          ·        Disadvantages of the method are related to the availability of solar light and to the fact that the sun moves.

          7         Acknowledgments

          1. To Ed Zarenski, moderator of the ‘Binoculars’ forum of  Cloudy Nights, who read the first description of the sun images method, made remarks to it and helped me to clarify weak points of the method.
          2. To M.Clark, member of  the ‘Binoculars’ forum of  Cloudy Nights, who made useful comments and remarks on the accuracy of this method and pointed out using the size of the sun image itself as a scale to calculate the absolute misaligment.
          3. To Bill Cook, expert in binoculars, who coined the terms “true collimation” and “conditional alignment” commonly used in binoculars literature.
          4. To Peter Abrahams, leader of the Binocular History Society, who provided references to standards for binocular collimation in diferent e-mail lists on binoculars (see “Binocular List #9: 2/3/98. Collimation” and Binocular List #246: 25 February 2003.

           

          (End of article)