OPO - Monitoreo de la reflectividad y dispersión de los recubrimientos del telescopio

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Application note #4

Basics of straylight definition and measurement

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Thank you for your interest!

Copyright:

Author:
Daniel Malaise, Dr Sc
First issue: September 1996

Last review: August 2020

This document is the sole property of OPO and, except to learn about our work, may not be used without our written consent.

There is some theory in the note: just what is necessary in order to understand the meaning of scattering. The main part of the note deals with practical matters related to the measurement.

It is also used negatively when considering optical surfaces that are used for

The latter can be used to measure the severity of those defects.

Let us consider a sphere of radius R centered on the point O ; we define the solid angle Ω subtended by a surface S which does not contain O, as the region comprised between the point O and the straight line generator through O and leaning on the surface contour. Its numerical value expressed in steradians (str), equals the area A of the surface S projected on the sphere divided by the square of the radius of the sphere, i.e. Ω = A/R

Let us consider two very small surface elements dA1 and dA2 (the d is there to indicate that the surfaces are extremely small and plane) separated by a distance R<; the normals N1 and N2 to these surfaces are indicated on the figure. The vector radius R makes respectively the angles θ1 and θ2 with these normals. The beam etendue is the quantity:

d
^{2}
E = da
_{1}
.cos(θ
_{1}
).da
_{2}
.cos(θ
_{2}
)/R
^{2}

But we have seen that da.cos(θ)/R

d
^{2}
E = da
_{1}
.cos(θ
_{1}
).dΩ
_{2}
= da
_{2}
.cos(θ
_{2}
).dΩ
_{1}

This radiant energy is produced by sources and received or measured by detectors; these two components are characterized geometrically by their useful area. In nature, there is no such thing as a “point”, but there are emitting (or receiving) volumes. These will not be considered here; they can always be treated by enclosing them completely in a surface and applying the surface formalism.

The quantity of energy through a unit area per unit time is the

The

The

All these quantities can be defined spectrally, that is when the radiation is not approximately monochromatic, one consider the quantity per unit wavelength (usually µm

Notice that the radiance is also the power per unit etendue and one could also write :

d
^{2}
P = L.da.cos(θ).dΩ = L.dE = I.dΩ

for the power going through the two surfaces when there is no loss (or production) of energy between the surfaces (no absorption, no sources).

The total power absorbed by the detector is described by the differential relation dP = L.ε.dE that has to be integrated over the etendue of the beam or practically over the area of the detector and the solid angle of the source surface as seen from the detector. Note that the etendue being symmetric with respect to the two surfaces, one could also integrate over the surface of the source and over the solid angle of the detector as seen from the source. If the medium between the two surface has a bulk transmission of T, the relation is simply dP = L.ε.T.δE.

Note that usually the coefficient ε is included in the transfer function of the detector (A/Watt) which transform the optical signal in amperes per incident watt (and not per absorbed watt). In this case the absorption coefficient should not be repeated explicitly in the formula.

The coefficients L, ε and T depend in general on the wavelength and on the incidence angle. When there is no dependence on the angle, the surface is said to be “lambertian”. When there is no dependence with the wavelength, the surface or the medium is said to be grey.

Diffusing surfaces (ground glass, velvet paint, lamp filament) show a more or less lambertian character; when one wishes to produce a true lambertian beam, one uses different special tricks: a few paint or materials are very close to lambertian (sodium salicylate, spectralon of lambdasphere); a hollow sphere painted inside with a lambertian paint and observed through a small hole is extremely close to a lambertian source and is uniform. All these bodies, when lit by any beam of light re-emit a lambertian beam. Hence, if one sends a uniform laser beam of power P on a lambertian surface of area a and albedo A, the total scattered power of the surface is:

and the constant radiance of the surface is:

If one observes the whole lit surface from a direction at an angle θ with the normal to the surface, the observed intensity will be:

I(θ) = P.A.cos(θ)/π

The power received by a detector of area S at a distance R (S<<R

W = I.T.Ω = P.A.ε.T.cos(θ).S/π.R
^{2}

since Ω = S/R2 and T is the transmission of the medium separating the surfaces. Here, the detector is assumed to be perpendicular to the measured beam (fig.5a). In this relation, all the factors are constant with the direction of observation except for the cosine factor; if one plots the power versus the angle in a polar diagram, one obtains the well-known lambertian diagram fig 4a. Note that this diagram is obtained only if the detector sees all the lit surface; if, the surface were illuminated uniformly with collimated light (Fig.4b), the diagram of the power measured by the detector would be a constant.

The situation of Fig.4a is the one encountered in

When the detector is parallel to the emitting surface (Fig.5b) we are in the situation of a detector in the focal plane of a lens; then, Ω=S.cos(θ)/R

W = I.TΩ = P.a.ε.T.cos
^{2}
(θ).S/π.R
^{2}

dP = - κ.P.dx

with

dx → 0

which simply tells us that the loss of power from the beam by crossing a very small thickness dx of the medium is proportional to the incident power P and to the crossed thickness; since absorption will decrease the power of the beam the proportional coefficient is negative and is written –κ. If the medium is homogeneous, κ is a constant and the differential equation can be integrated, yielding:

where P is the energy flux at x, P

When considering a glass plate of thickness d crossed at an (refracted) angle θ by a parallel beam, the external transmission is the ratio of the flux after and before crossing the plate.

where R

There are basically two related ways: angle-resolved scattering or

The distribution function is mostly utilized in its simplified form limited to the incidence plane (φ=0):

In this expression (another distribution function is sometimes used; it is known as the angle resolved scattering function (ARS); it is simply the

One can rearrange the

one sees, by comparing with the definitions on p. 3, that the radiance is :

(the radiance is what remains constant in a "lambertian" source).

The

Usually it is very difficult and inaccurate to measure directly the scattering at angles smaller than 2 or 3°. This is due to straylight within the instrument. The latter is mainly produced by dust and stains on the front collimating lens and, hence is essentially variable. If it is not too important, and if we measure the transmission scattering, the instrument stray light can be approximately removed by calibration, measuring without sample. This calibration procedure, of course does not work for reflective measurements since then, no measurement is possible without sample. Also, portable instruments are bound to utilize short path beams with relatively large apertures; hence, baffling for small angles is impossible.

In reflective measurements of mirrors before aluminizing, the measurement is usually polluted by a certain amount of volume scattering by the bulk of the blank. This difficulty disappears of course when the mirror is aluminized.

Diffraction spreads the power transported by a perfect beam onto a small area. The diffracted angular deflection with respect to purely specular directions are very small; they depend on the beam diameter D and the wavelength so most of the power falls inside a circle of radius θ = 1.2*λ/D (Airy disk for a circular opening). Since this is the minimum image dimension for perfect conditions, it is used as a natural scale in our discussion.

If the surface is not perfect and that the defects are quite smaller than the wavelength (and smaller than the coherence length of the source), some additional power will fall outside the Airy disk. This type of scattering yields to mathematical analysis, allowing to compute the relation between the physical shape of the defects (micro-roughness) and the resulting optical scattering. This is typically the case of a good optical surface such as a clean optical mirror.

If the defects are large compared to the wavelength, power is scattered at large angles with respect to the specular direction. There is no theory for accounting for this type of scattering, but only empirical relations (between physical defects size and distribution, and scattering). One can visualize the surface as covered by thousands of microscopic mirrors of all shapes and various dimensions placed at all angles with the mean surface. There cannot be a theory because there are as many scattering functions as there are distributions of micro-mirrors. This is the case of dusty optical mirrors or of ground glass. Hence, it is improper to convert the scattering measurements of a dusty mirror into µ-roughness values.

One can think of a surface scatterer in terms of a superposition of many ruled gratings of various pitch and ruling directions. The first theoretical analysis of scattered light was simply made in term of Fourier analysis of the frequency and amplitude content of the scattering surface. From these early (but conceptually very easy to understand) studies, one should remember the two basic relations :

The grating equation relating the ruling pitch d, the wavelength λ of the light and the incidence (i) and diffraction (θ) directions :

p is the integer diffraction order.

The relation giving the diffracted intensity for small angles (i~θ):

for a depth a of the grooves.

Of course, one cannot go very far with this treatment. But on top of being very easy to grasp, it already shows characteristic features which will stay with us in the most advanced theories of surface scattering.

• The correct independent angular variable is: sin(θ) - sin(i) which the specialists of scatterometry usually write : β

• If you change the incidence angle, the scattered intensity drops as the square of the cosine of the angle.

Considerable amount of work has been devoted to surface scattering. Finally, the most exact treatment is known as the Rayleigh-Rice perturbation theory.

This yields the following result :

where T is the transmission of the sample and k = 2π/λ.

This equation is somewhat simplified :

• it is written for small angles and projected in the scattering plane

• it does not account for polarization effects

It is remarkable that we find again the two features we had pointed out from the grating theory. A third useful feature is comprised in the Rayleigh-Rice relation: the fourth power dependance on wavelength. It can be used to compute the

Another important result that can be drawn from this theory is the relation between the µ-roughness of the surface and the total scattering of this surface.

If the surface profile is z(x) measured at N equidistant points x

The

One deduces from the vectorial model :

The approximation holds for 4.π.δ<<λ which is also the condition for the theory to be senseful. In this case the total reflectivity and the specular reflectivity are very close together and one can use the specular reflectivity easier to measure; but one should keep in mind that this is invalid as soon as scattering exceeds a fraction of 1%, i.e. as soon as we deal with a dirty mirror.

The following table shows numerical values for a wavelength of 650 nm and various ratios δ/λ.

One sees that for a µ-roughness of 40 nm, the

Dust or pollution scattering has nothing to do with the Raleigh theory and cannot be described in terms of µ-roughness.

Normal astronomical mirrors exhibit a mirror scattering loss ranging from 0.1% up to 1% at most if they have been properly commissioned.

A dirty or scratched mirror will exhibit a value of

As we already pointed out, this is in fact all we need in order to build the most economic and reliable instrument. The Harvey model of surface scatterer follows the law :

Where b is a conventional “pivot” angle, usually very small, and m is the slope of the log-log diagram of the

Y = Y
_{0}
+ m*X

where Y = log(BSDF), X = log(|β

One notice that if two

The Harvey relation turns out to be an excellent representation of the scattering behaviour of well-behaved surface scatterers such as clean polished mirrors with small defects. If the scatterer is not well behaved (as a dusty mirror for instance), the Harvey law still represents good portions of the angular dependency of the

Some typical Harvey curves are reproduced hereafter. One sees that, for normal polished glass, the Harvey relation is quite accurate in a very wide angular domain. The cyan curve is for a not well behaved scatterer as a narrow angle diffuser. The smallest scattering angle is 15 arcmin and the two arrows show the

A typical astronomy application would be the following. Assume that you measure a faint stellar-like object close to a bright star with a pixel detector; that the pixel size is .5"*.5", the seeing is .3". The bright object is mag 2; what amount of (parasitic) light from the bright object is polluting your faint object if the distance between the two object is 5" and the

One finds the scattered power at angle θ from specular (taken as normal incidence for simplicity):

δP
_{s}
= BRDF(0,θ).P
_{i}
.δΩ

which in our example evaluates to :

The main drawback of the Harvey representation is that it breaks down at very small angles since its mathematical formulation exhibits a singularity at

The Harvey relation in non-logarithmic form reads :

where m is a negative number with a value usually comprised between –1 and -3, so that for s=i the expression yields an infinite value.

This makes the result of the preceding section approximate, since the BRDF at .5" has been deduced from measurements made at 15° and 45°: the result should not be used in correcting the photometry of the faint object, but just to make a preliminary estimate of what can be expected. For instance, such computation will show immediately whether the mirror cleanliness is critical for a given observation. The actual photometric calibration of the scattering should be performed by measuring an empty field at the same distance of the bright star.

The Harvey model has been modified in order to remove this singularity. This new model is known as the ABg or Harvey-Shack model; it is defined by the relation:

Unfortunately, if you measure the

While the log-log plot of the Harvey relation is a straight line with slope –m, the ABg log-log plot flattens out for small angles (less than 1° or at most 2° from the specular direction). This roll-off takes place for scattering angles such that B=(β

For β

The ABg system is interesting if we want to estimate the diffusion at 1" to 10" from a bright object as in our example. Here, we deal with values of β

If the mirror is dirty, the value of A and B are larger. The following diagram shows the fit of ABg curves to measured BRDF samples.

ES is a clean aluminized mirror : Fp is the same with a plain finger print; Mean is a glass with µgrit 12 scratches; Bad is the same but badly damaged; Calc is a glass with conspicuous traces of calcium due to repeated water evaporation. One can see that the ABg formalism can represent accurately the scattering data. The ABg parameters and the

One sees that a small value of A indicates a low global scattering, a low value of B indicates a clean mirror scattering at small angles; a large value of B indicates a dirty or scratched mirror scattering at large angles; the g exponent has generally a value around 2 : larger for a clean mirror, somewhat lower for a dirty one.

One word of caution is necessary if you want to perform quantitative measurements and computations using the BRDF and the

It has the advantage of being defined dependently of the reflectivity of the surface.

The

In this setup, The detector R is positioned alternately to measure P

It is very important to be noted that

This is a sample mirror with very heavy scattering (28%) the various reflectivities have been entered and one can read : A= 89.65% and R=64.71% : this is just the power in the central peak of the figure divided by the incident power, measured with a very small detector (1 arcmin); this power is not dependent of the radius of the detector between 10 arcsec and about 10 arcmin. When increasing the size of the detector further, scattered power progressively adds its contribution to the specular power and the apparent reflectivity increases; figures are given for three detectors of current acceptance angle: 1.5°, 2.0° and 2.5°. One sees that the error on ‘reflectivity’ measurements is quite substantial and it really makes no sense to measure directly the reflectivity of a sample with heavy scattering; the measure depends much on the exact value of the acceptance angle of the instrument and will differ from one instrument to the other.

The beekman theory is a trial for obtaining a theory of scattered light when λ << σ.

For normal incidence, the following relation is given :

BRDF(s) = π*R*e
^{-[sin(s)/(2S)/(1+cos(s))]^2}
*{2πS*(1+cos(s))}
^{-2}

Where s is the scattering angle, R is the

The following plot compares the ABg curve to the Beekman result, for a common value of scattering at small angles : one should notice that the Beekman relation having only one free parameter, when using this parameter to adjust the scattering at small angles, the rest of the curve is entirely determined. The value of S used is 0.118, and the corresponding Abg values are respectively .00028; .001; 2. The angles range from 0.17 arcm (-4.31) up to 71° (-.02); the roll of point of the Abg curve is around 30 arcm (-2.06),and of the Beekman curve around 8° (-0.86). It is apparent that apart for the common value of the scattering at very small angles, the two curve bear no resemblance at all.

The Beekman curve has not been observed nowhere; from our discussion at the beginning of this note (what causes optical scattering?), this failure of a “theory” of scattering for σ>>λ is not surprising.

A very simple way of representing a scattering function is to use a Gauss distribution; this procedure has the merit of being easy and fast to introduce in ray tracing computations.

The Gaussian scattering function is defined by :

where is the projection on the scattering surface of the difference between the scattering unit vector and the specular unit vector (so that, if the icidence angle is 0°, is the sine of the scattering angle. P is the total power scattered for a given value of , the azimuthal distribution being uniform), and σ is the width of the distribution (a Lambertian distribution has σ > 4; most dust scatterers have 0.25<σ<1). The Gauss distribution is however far from a Harvey distribution: it has no range of the angular variable with a constant slope so that it is too flat for small angles and too steep for large angles. It is useful when considering a very limited angular range and when estimating scattering due to mounts in optical instruments.

If you want to do spectroscopy of faint stars, scattering by the mirrors and even by the atmosphere is not so detrimental as if you run after direct detection of planets around stars. But of course, a large telescope has to provide the required service to a large community of astronomers and since you cannot switch the mirror’s coatings from poor to excellent depending on the program of the night, the coating should always be ready for the best performance. The most reasonable attitude is to take into account that there will necessarily be some decay of the coatings performance between two maintenances and to arrange the program following the planned maintenance: The most demanding programs should be scheduled soon after maintenance, and the less demanding towards the end of the maintenance period. This of course supposes that the maintenance is carefully planned and that the mirrors have been monitored so that a good prediction of their state is possible.

As we have seen from the discussion of the BRDF,

The reflectivity is of course the main parameter which is of interest, but one should understand the

For instance, if we consider a pure aluminum coated mirror with various

The first column is the

Practically however, I cannot imagine that a telescope mirror would be allowed to become so dirty as to show more than 5% scattering and therefore one can use a narrow angle reflectometer, using a scatterometer to get a feeling of how accurate the reflectivity measurement is, that is how close it is to the specular reflectivity.

However, the total scattering is not easy to measure accurately with a portable instrument. One can estimate its value by measuring the scattering at one or, better, two angles assuming a model for the scattering function. This is a good practice, because the scatterer is always the same at a given location, although it might differ from one place to the other. We now turn to the task of estimating the

The general structure is shown on the drawing : the source sends a laser focused beam on the sample at an angle of 45°; the detectors at -15° and -45° are not represented : they bring no difficulties. The detector at 2° should discriminate between the

The frame and positioning balls are sketched on the figure; these are important elements for maintaining precisely the angle relations between the beams. A three-points contact is always a perfect positioner on any surface; even with a parabola, the angular difference at the incidence point is much smaller than 1° and the return beam to the detector will always be correctly positioned. This would not necessarily be the case if the positioning interface were an o-ring for instance.

First of all let us examine the source because light scattered at 2° from the specular image should be dimmed by a factor of the order of 10

The source comprises of a laser diode, a lens and baffling; the latter is the most important part for controlling stray light. First, the laser diode has an emitting junction which is very small (1x5µm) and can be seen as a point source. But it emits light on both sides and part of this light is scattered and reflected around by metallic parts and by the output window; finally the point source is surrounded by a diffuse source of the size of the window; this is maybe 10

Then the lens itself is completely bright if AR coating is not very efficient (no coating means a large source with a total brightness of the order of (0.04*0.04 = 0.0016) times the point source. Then the other baffling builds up a series of black boxes killing grazing incidence high reflectivity on the tube (lower part of the figure). Finally, this source configuration has been abandoned because it turned out impossible to reduce the projection on the detector of the scattering inside the laser diode window and maintain the laser stability. In the final configuration, we use a pigtailed deep blue diode and define the parallel beam by two pin holes just large enough to avoid diffraction. Intermediate larger diaphragms take care of grazing incidence internal reflections.

Let us now turn to the detector cavity. Light is entering from the right through the 2.5 mm opening of the first stop; the total length of the cavity is 19 mm.

The first three small cavities (1; 2; 3) separated by stops are there to trap the stray light and prevent grazing incidence reflections. The last cavity is where the

Copyright:

Author:
Daniel Malaise, Dr Sc
First issue: September 1996

Last review: August 2020

This document is the sole property of OPO and, except to learn about our work, may not be used without our written consent.

OPO sprl Address: Crawhez 9, 4890 Thimister-Clermont, Belgium Enterprise nr: 0478.624.724 Email: [email protected] Tel +32 87 68 89 89 IBAN: BE38 0689 0364 6872 BIC: GEBABEBB