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Submitted

REPORT OF PRELIMINARY IALS

OF OPTICAL ALIGNMENT

REPORT TO PROJECT M - NO. ABA-35

STANFORD UNIVERSITY SUBCONTRACT S-128

UNDER AEC CONTRACT AT(04-3)-363

by f rrn!4d T'

Approved by

AETRON-BLUME-ATKINSON A Joint Venture

ARCHITECT-ENGINEER-MANAGER 1455 California Avenue Palo Alto, California

ABA-35

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SUMMARY

Three preliminary trials of precise alignment by

line of sight were made on a line adjacent to

the accelerator site. Results indicate that this

method offers an attractive method of measuring

horizontal strain movements in this area.

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t measurements U.S.C. & G.S. es 1

the stence of horizontal strain movements of earth's

crust in the eastern block adjacent to the San Andreas Fau

These strain movements extend back for at least 20 miles from

the fault and appear to be a shear displacement as the western

edge of the eastern block is displaced north westerly by move-

ment of the western block. The U.S.C. & G.S. measurements o <-

were ob~ained from resurv~ys of the first order triangulation

net which straddles the San Andreas Fault. Their measurements

(and Dr. Byerly's estimate) indicate a horizontal strain in the

vicinity of the accelerator site of about 0.2 seconds

(10- 6 radians) per year. The accuracy of conventional first

order surveys is of such an order that these measurements require

intervals of 10 to 20 years between surveys in order to obtain

significant measurements of this strain.

Since the estimates of horizontal strain represent long !

term ~oerages over extensive areas it is ~ery desirable to

obtain more precise rates of horizontal strain at the accel-

erator site. To this end a very cise on-

triangulation horizontal survey net is being established at

the site. An optimistic estimate of the precision which can be

6 obtained by this type of survey is on the order of 1:10 •

Assuming this precision it will requlre a one year interval be:t~SYtl~

berveys $orveY$fyo verify the rate of horizontal

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strain if es rate is correct uni

Similarly, if actual rate ould be trip es ted

rate, surveys at four month interva

chan

would show si ificant

Measurement of horizontal displacement from linearity

along the accelerator site a prime aim of the trilateration-

triangulation surveys. An optical alignment system would

measure this quantity directly and eliminate many operations

of the network survey, hence it appeared reasonable to assume

that more preclse measurements of horizontal strain could be

made by a line of sight than by the network survey. The purpose

of the preliminary trails of optical alignment was to verify this

assumption as far as possible on a limited budget and determine

if further work on a monumented line of sight is justifiable.

DESCRIPTION OF TRIALS

The trials were conducted onaa line roughly parallel to the

accelerator center line. Drawing SK-C-20lll-A shows a plan and

profile along the line. For all three trials the sighting

instrement was at A, a nal target ~as at C, an

mediate target was repeatedly lined In at B by radio co~munication

between A and B. TH~ VaRiations in the aligned position of the'

target at B, measured horizontally at right angles to the line of

sight, were taken as a measure of the accuracy for each set of

observations.

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first was 7th at weather was

clear and co with a north to northwest wind 20 to 25

A Ve Maksutov astronomical type te cope was used as a si ing

instrument. This instrument was selected on the basis that it

has the best optics obtainab in a readily portable instrument.

It has a 24 inch long telescope barrel, a si~ inch objective.

lens and provides a resolution of 0,8 seconds which is the best

theoretically obtainaBle with a six-inch objective lens. The

instrument ~sed was borrowed from the Vega Instrument Company

of Palo Alto, who are the manufacturers of this instrument.

Small battery powered light bulbs were used as both the terminal

and intermediate targets. Attempts were made to use a terminal

target of two black vertical lines on a white background illumina-

ted by a spot light but it was unsatisfactory. A slit target

was also tried as an intermediate target, but results were not

promising. One set of 7 sightings and one set of 5 sightimgs

were taken.

The second triil was made ln the early mornlng on October

21st in fuhe period immediately before and after sunrise. The

. f 0 0 weather was clear. Temperatuee varled rom 41 to 57 F, and

the nd was 0 to 5 s ast. Sighting instruments

were a Wild T-3 theodolite and a Vega Mak6Gtov telescope. The T-3

telescope provides a resolution of about 2.25 seconds. The small

light bulbs were again used as targets. Readings were taken in

sets of ten. Four sets were taken using the T-3, and g ve sets

using the Vega. Two observers alternated between the two instru-

ment •

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The third trial was made at night on October 27th. ~A .\~ '-...VJto--~ v.-)CL/) ci.-~~ G~~ L'Y',.Luv)~-1I.L,-,V \\~JkV

Cfcbret ~ rd:P1imz:t:'i:r~ i rwti.EtthniLt'rtteI'blit ~Irtt s 1i~ awe re -::. J ') C ( , 2\ ~ ~~. Wind was 5 to 15 mph from the northwest. A Wild

T-3 was the sighting instrument. Both targets were backlighted

translucent plastic with black tape in the following pattern:

Linear dimension-{Fvwv~~~j intermediate i~ter~=rtjlte Angular width

seconds radians -?

dimension a 3.0 1.5 xl~e55 ;c target t~aBg~t

0.9 in. 1.8 in.

dimension b 5.0 2.4 x 10- 5 1.4 In. 2.8 in.

These targets were to be split by the instrument crosshair

so as to appear as shown below to the observer. f---------vertical crosshair

~-distant target (black tape on translucent white plastic)

The dimension "a" was chosen a little too small for the

resolution ability of the T-3 however, and it was found that

arranging a vertical coincidence of the two targets gave a

sharper definition of the aligned targets. They then appeared

as shown below.

~-----intermediate ~arget

~-----terminal target

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RES

The following is a tabulation of the results of the preliminary

trials.

(f) Standard Probable Error of h 0 SDsFlcil.arcil.on Hean Position of

lH 'M Deofa$B:d:m set O+J

ttl+J od! (f) awt (f) (f) H :> Q.J "0 (f) "0 Q.JH(f) Q.J C Q.J S 0

,.QQ.J .£:; Q..- .£:; ~ 'M E -(f) C 0 0 0 0 +J ~::1 ,.Q".~" ~ ~C Q.J h Q.J ttl zo H Cf) H \Q Q:; E~m~:t:k5

22:25 If 7 Oct. 7 0.232 0.81 0.059 0.20 1:10 6 Vega 22:15 fit 7 Oct. 5 0.250 0.88 0.075 0.26 1:7.9xl0 5 Vega

06:00 21 Oct. 10 0.381 1. 34 0.081 0.28 5 T-3 1:7.3xl0

5 06:13 21 Oct. 10 0.421 1. 47 0.090 0.30 1:6.5xl05

Vega 06:21 21 Oct.". 10 0.492 1. 73 0.106 0.36 1:5.5x10

5 T-3

06:25 21 Oct. 10 0.890 3.12 0.192 o .65 1:3.1x105

Vega v {

06:36 21 Oct. 10 0.917 3.22 0.290 o • 98 1:2.0x10 5 Vega, Stinu] 06;39 21 Oct. 10 0.900 3.16 0.195 0.66 1:2.0x10 T-3

/06;47 21 Oct. 10 1.153 4.04 0.250 O. as" 1:2.4x10~ Vega OfJ:lS 21 Oct. 10 0.796 2.79 0.172 0.58 1:3.4x10 T-3 07:19 21 Oct. 10 1. 59 5.58 0.344 1.17 1: 1. 7x10 5 Vega

• 1

18:15 27 Oct. 10 0.226 0.79 0.048 0.16 . 6 T-3 1.1. 2xl06 18:37 27 Oct. 10 0.150 0.53 0.032 0.108 1:1.8x106

T-3 18:43 27 Oct. 10 0.087 0.31 0.0185 0.063 1:3.2x10 T-3

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expected, the homogeneity of the air, through its influence

on re on, was the most important single tor limiting the

accuracy of the trials. The decreasing accuracy following sun-

rise on 21 October clearly demonstrated this effect. In a very

short time the "boiling" effect, where the line of sight skimmed ,~

the ridge on which th~ intermediate target was located, had .

appreciably $n~mmased the errors. By the same token, the presence

of the light rain or drizzle on 27 October produced a nearly

homogeneous atmosphere for the line of sight and by far the most

6 accurate results, with a probable error of about 1:3xlO •

These preliminary trials had no provisions t for maintaining

permanent reference marks at the three points hence represent

'apparent" accuracy of alignment since no check of a stable .

horizontal refraction error was made. On the other hand the line

of sight did not involve any of the typical conditions which

produce large horizontal refraction errors, and hence can

tentatively be considered reliable. Typically questionable lines

of sight are those which pass close to a steep slope or diagonal

cross a cu vated field or building. tance sup-

position is that air temperature differentials are located as

a lens which bends the light bnrizba~~ao~~aDeplane.

The homogeneity of the air and resultant accuracy of horizon­

tal sightings in no manner implies a lack of vertical refraction.

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No was to measure or s Ii

cal 2 ng Is, it seems reas Ie

to lieve that the ve refraction al 1 si t

was as stab as the "a " ac of horizontal

sightings. This implies that at s to ver2 calculated

vertical refraction d most successful under the same

conditions that give the best results for horizontal alignment.

The optics of the sighting instrument can also limit the

sighting accuracy. The trials indicated the T-3 telescope gives

results as good or better than the Vega, but it is believed these

results are misleading. The T-3 was specifically designed for

sighting under moderately good conditions, and its balance between

resolution and magnification is well chosen for this purpose.

It appears that the eyepieces used with the Bega gave too much

magnification and were not well matched for the sighting conditions

of the second trial when the two instruments were compared. This

assumption is based on the more rapid increase in sighting error

by the Vega telescope as sighting conditions deteriorated during

the second trial.

Target trials indicated the backlighted targets used for

the ird 1 were the most satis coruscation

effect of the small light bulbs increased their apparent size

and seems to nullify their actual smallness. Apparently there

is little advantage in target features smaller than the resolution

of the sighting instrument. Again, however, the limited scope

of the trials did not allow a thorough investigation of target

si

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Optical ali techn 11 probably provi a

more pre se measurement zontal e strain at the

site than the horizontal network survey (triangulation­

trilateration). The primary source of error in optical

alignment is refraction. The refraction effects act in two ways:

(1) a systematic bemding of the line of sight; and (2) a

limitation on pointing ability due to random refraction. The

network survey is subject to the same problems of refraction

and introduces additional sources of error in the graduation

of the horizontal circle, and the reading of the circle at

each pointing. To compensate for the latter errors the network

provides a redundancy of measurements which allow for a rational

distribution of errors. The positive advantage of optical

alignment, however, is that a relatively short time is required

for a complete independent set of readings, hence, results can

be based on nearly ideal sighting conditions. A complete set

of independent readings of the network survey requires many

nights of observations, and therefore, sighting conditions can

no tter the avera ob ne at ni t.

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RE

A permanent monumented sight line should established

at the site and further trials conducted on this line to

further develop techniques best suited to our site conditions.

These trials should include the following:

1) Additional comparisons of sighting instruments

2) Additional comparisons of aarious target designs

and arrangements

3) The use of different colored lights to determine if

actual measurements of refraction can be obtained at

the same time the sightings are being made.

4) Repetitions under various sighting conditions to

determine optimum conditions.