Autocollimator Calibration

Calibration : interpretation of output from Autocollimator Controller

What we have:Output from controller i.e. numbers that quantify the lateral displacment (Δx, Δ y) caused by tilts in arbitrary unit

What we search:A coefficient that transform the lateral displacment arbitrary unit in µm

Autocollimator Calibration

The idea

•Generate a tilt of a known angle…

•…measure it…

•…calibrate the output!

Autocollimator Calibration

Generate a tilt of a known angle

Micrometric translation stage

Response spring

Rotating rail

autocollimator

Micrometric translation

Known angle

Rail arm+

=

Autocollimator Calibration

Procedure

Step of T.S.20 μm

Min angleΔθ = 0,0571 mrad

Average Δy read 74 measure of 20 μm step

Δy = 56,783 a.u.

Lateral displacementΔy = 32 µm

)2tan( fy

Average of conv coeff: 0,565082 μ/a.u.

Error: 0,068163 μ/a.u.

ACCURACY [%] 12,06248

i.e. Δy=100 a.u. read Δy = 56.5 μm

Why this error?

Autocollimator Calibration

mmF 280 mmR 350Focal length Rail length

mmR 1

Positioning (per step)

mP 20mP 2

Data Acquisition (per step)

YXYX

Average on100 measSt. dev. on 100 meas

Shift Angle

2

22

2

2

2

2

11

arctan

RR

P

P

RR

P

R

P

RP

YDifference between the two Y values

21 YYY

Average of Y

48.0

78.56

y

Y

)2tan( FLLinear Shift

))(2(cos

2

))(2(cos

2

122

1

2

122

2

FFL

12 Difference between the two next angles

Conversion Coeff

2

2

2*

/

Y

L

Y

YLC

YLC

Autocollimator Calibration

Next Step

Improve AccurancyReduce Min. angle Δθ +

• Using Exapod

• Using Theodolite+ large mirror