Spiders and Swing Arms: A Summary

Piero Salinari and Ciro Del Vecchio

Osservatorio Astrofisico di Arcetri

and

J. M. Hill

Steward Observatory

Large Binocular Telescope Project
Technical Memo
OAA-93-02
October 28, 1993 / October 4, 1994
http://medusa.as.arizona.edu/lbtwww/tech/oaa9302.htm

Abstract

1. Requirements

2. Why cantilever swing arms?

3. Mechanics and Operation

4. Obstruction and Emissivity

5. Diffraction

6. Another swing arm

Abstract

This memo summarizes the present status of the studies on the swing arms used to support the secondary and tertiary mirrors of the Large Binocular Telescope. These mechanisms have evolved considerably in recent time and are still in evolution. Historical designs have evolved from conventional tensioned spiders to moveable bridge spiders known as ``trolleys'' to the present single-sided bridge structures known as ``swing arms''. More detailed memos are in preparation on aspects that will be touched here only very briefly, such as the optimization of the diffraction pattern.

The main text of this memo was written in the Fall of 1993, but some delays have occurred in preparing all the figures for publication.

1. Requirements

1.1 Rapid Exchange

The Large Binocular Telescope (LBT) has established the goal of exchanging between all the optical configurations of the telescope in a short total time (minutes or tens of minutes). This requirement is the one that has guided not only the design of the supporting structures of secondary and tertiary mirrors (that we will improperly call ``spiders'' for brevity), but also the design of the telescope and enclosure. This is because the exchange operation deeply influences the telescope geometry and the possibility of accessing and maintaining the various parts of the telescope. LBT has chosen to achieve observing versatility and flexibility by exchanging secondary optics rather than by changing instruments during the night. Usually the change of secondary optics implies a change to an instrument already mounted at another focal station.

1.2 Stability

The basic mechanical requirement for the spiders is to guarantee the stability of the supported optical elements within optical tolerances in the frequency domain between about 0.01 and 10 Hz, i.e., in the region where the wind can cause vibration problems. Displacements at lower frequency, due to gravity and to thermal deformations of the spider itself and of the entire structure, can be sensed and corrected by the ``active optics'' alignment system.

The adopted performance specification is that the lowest resonant frequency should be above 25 Hz for all the spiders. In terms of displacement this corresponds, for instance, to a total deflection moving from zenith to horizon of < 0.36 mm, or to a deflection of < 3.6 microns for a mirror unit with 1/2 ton mass, 1 m2 section subject to a wind pressure change of 50 Pa. Pressure changes as large as 50 Pa on short time scales should be avoidable under most observing conditions by appropriate use of the wind-shields.

The mechanical design was guided by the following priorities:

2. Why cantilever swing arms?

We had to reject more conventional spider solutions based on tension vanes attached to a ``top ring'' (in compression) or directly to the telescope structural elements because of the severe implications on telescope and enclosure posed by handling of the top rings and because of poor performance in the case of not having a top ring.

For a long time we developed a binocular telescope design based on ``bridge'' (non-tensioned) spiders that could be displaced sideways on the telescope and could be parked in the central area between the primary mirrors when not in use (see Figure 1 for a global view of that telescope design). These moveable bridge structures became known as ``trolleys''. The main problem we met was that of achieving a sufficient rigidity of the structure to provide adequate support to the spiders in all directions. In particular this was true for the "back wall" of the telescope, whose only purpose was that of supporting on that side the spiders and the rails for their displacement. It became evident that, rather than supporting the spiders, the back wall was instead depending on the spiders for its stability in the front-back direction! The first telescope resonant frequencies were relatively low (about 8 Hz for the front-back mode) and we could not obtain a first resonance above about 12 Hz for the spiders. The bridge spiders themselves, on the other hand, could be designed to achieve high rigidity (> 30 Hz on fixed constraints) and reasonably low obstruction.

View Figure 1 here

It became soon clear that the symmetric geometry of the spider, that is necessary for vanes in tension attached to a ring in compression, is not a necessity in a telescope with an open top end where the spider elements are not preloaded. We found that removing altogether the back wall and halving the spiders, so that they were attached only to the front telescope wall, was of great benefit for both telescope and spiders. This is because one could increase the thickness of the front wall, with a great gain in its rigidity and no increase of the telescope mass, while the halved spider could preserve the original rigidity and mass and could take advantage of the increased stiffness of the front wall.

These considerations were confirmed by results of the structural analysis of the latest telescope design. The spiders reach resonant frequencies in much better agreement with the specifications, and the telescope resonant frequencies are higher. Moreover the entirely open structure of the telescope allows to us simplify the handling problem by using a bridge crane to access all the parts of the telescope.

3. Mechanics and Operation

3.1 Spiders for various mirrors

Figures 2 and 3 show more in detail the mechanical design of the F/15 spider and of the mechanism used to rotate the unit and to preload it when in working (Figure 3) or in rest position (Figure 2). The tertiary mirror spider is shown in Figure 4. The F/4 spider, supporting the secondary mirror and also the wide field corrector and the fiber feed, is shown in Figure 5. These last two figures are derived from the global telescope picture and are not updated with the latest details of the spider mechanics. Figure 6 shows the most recent geometry adopted for all spiders with element thickness referred to that of the F/15 and tertiary spiders. These thicknesses are optimized to control both the local and global vibrational modes of the spider.

View Figure 2 here

View Figure 3 here

View Figure 4 here

View Figure 5 here

View Figure 6 here

3.2 Finite element results

The various spider designs have been studied in detail by finite element analysis (FEA). Table 1 reports masses and values of the first resonant frequency for the three swing arm units. The masses do not include the mechanisms that actuate the rotation and it must be noted that the F/4 arm was only considered for an initial estimate of the problem and no optimization was attempted because of the still very preliminary definition of its central units. The frequency reported in parentheses is obtained by dynamical analysis of a simplified arm structure attached to the complete model of the telescope. One can notice that the finite rigidity of the telescope causes a modest reduction of the resonant frequency. Figure 7 shows a telescope structure with the secondary and tertiary mirrors mounted on swing arm spiders.

spider mass (Kg) lowest res. freq. (Hz)
F/15 932 29.8 (27.52)
M3 694 29.2 (27.82)
F/4 4237 19.9 (18.33)
Table 1: Estimated mass and calculated lowest resonant frequency for each of the spiders.

View Figure 7 here

3.3 Mechanisms for motion

A single mechanism is used to rotate the swing arms to their working and parking positions and to preload all the interfaces between the rotating and fixed parts. The spider is moved by a ballscrew which preloads the structure against stops on a rigid rod. The applied preload, necessary to achieve the required reproducibility of position and rigidity of the articulation, is larger than twice the maximum load induced by gravity at each interface when the spider is in the working position, while it is comparable with the gravity load in the parking position. A complete, full-sized, prototype of the F/15 swing arm with its rotation mechanism has been constructed for checking the practical mechanical performance of real joints versus FEA results. A drawing of this prototype spider is shown in Figure 8. A photo of the assembled prototype is shown in Figure 9. The dynamical response of this unit is now being measured in the laboratory.

View Figure 8 here

View Figure 9 here

3.4 Utilities and servicing

The mechanism is dimensioned to drive the arms under maximum gravity load, i.e., from parking to operating position when the telescope is pointed to the horizon. This is unnecessary during observations, but useful for the maintenance and of no significant impact on design and cost. Maintenance of the spiders and of the supported units can take place with the telescope pointed to the horizon and the spiders in rest position, so that all parts become accessible from a platform placed on the observing floor. For major maintenance the central units or the entire spider will be removed using the bridge crane.

Electrical and service connections for the central units are permanently connected and subject to a bending of about 90 degrees when the arms are rotated. The central units supported by the swing arms have to be designed for accurate reproducibility, but still the unavoidable difference in the telescope deformation sensed by the different units will require an ``active'' refinement of the alignment on a field star after an exchange of configuration.

3.5 Thermal properties

The thermal time constant of the spider structure is about 1/2 hour in still air and the surfaces will be covered with low emissivity Aluminum. Because of relatively thin-walled steel structures, these passive measures are sufficient to keep close equilibrium with ambient air temperature (within < 0.2 °C) in most observing conditions. In the most adverse conditions (large positive (dT air / dt ), no wind and maximum radiative loss) the spider can become up to about 1 °C cooler than air, but this can be easily corrected, if experience will dictate, by adding electrical heaters.

4. Obstruction and Emissivity

4.1 Requirements

Different requirements apply to each of the three spiders concerning obstruction:

The obstruction problem is severe for the tertiary spider because it is crossed by the optical beams twice (in the incoming parallel beam and in the converging beam between the primary and the secondary). Moreover the tertiary is used in conjunction with the Gregorian secondary in the optical train of the combined focus for which emissivity and diffraction effects have to be carefully controlled.

The F/15 spider has to be infrared optimized for use at the direct Gregorian focus and is in the combined focus train, but it is crossed only by the incoming parallel beam. To avoid increasing the obstruction in the combined beam it must have the same projection as the tertiary spider in a direction parallel to the telescope axis.

The F/4 spider is crossed twice by the optical beam like the tertiary, but the F/4 station is for work at optical wavelengths. The trapped F/4 focus has intrinsically a large central obstruction, due to the necessary sky baffle of about 2.5 meters diameter. It is therefore relatively easy to limit the light loss due to the F/4 spider to a comparatively small term.

To reduce the obstruction of the tertiary spider in the converging beam we adopted a radial geometry for the main structural elements of the swing arm, so that the obstructions in the parallel and in the converging beams overlap, and reduced the number and cross-section of non-radial elements in the design reported here. The resulting projection on the pupil was therefore adopted for the F/15 spider so that the obstructions of the two spiders in the incoming beam overlap again.

4.2 Emissivity

With the present design the total spider obstruction is of about 2.0% at the combined focus and 1.07% at the Gregorian focus. This is the equivalent obstruction of a conventional ``X'' spider with vanes of 39 mm width and compares well with obstruction values of other large telescopes. Still the spider obstruction contributes about 1/5 of the telescope IR emissivity and therefore a further reduction of the obstruction is highly desirable. It must be noticed that the thickness of each structural element is determined by the requirement of obtaining a local (i.e. of the element itself) resonant frequency above 25 Hz. As the local resonant frequency is proportional to the square of the element length and inversely proportional to its thickness, a further improvement can be obtained by using a larger number of shorter (and therefore thinner) elements in the spider. Refinements in this direction are in progress. A reduction of the emissivity can also be obtained by placing on the parts of the spider facing the primary reflective elements of appropriate shape to reflect the sky via the primary.

4.3 Chopping

Do the swing arm spiders produced an asymmetric background that would cause problems with chopping in the thermal infrared?

The simple answer is that you could chop in the direction normal to the axis of the swing arm and still have a symmetric background from the spider. A 5 arcsec chop throw on the sky moves the beam only 0.25 mm on the spider vanes so the ~ 1% obscuration of the spider is varying by only a part in 15000 or so. Thus the unbalance signal would be very small even chopping parallel to the swing arm (in elevation). This is to be avoided in any case because in elevation one gets the atmospheric unbalance signal that changes with the airmass while one tracks, while at least the spider unbalance is constant if the temperature distribution is stable. So the optimum is to chop parallel to the elevation axis (in azimuth).

5. Diffraction

What is the diffraction pattern produced by spiders such as those considered here? Each element of the spider produces a spike of diffracted light in the PSF whose integrated intensity is proportional to the area of the element and whose direction is perpendicular to the element. Reducing obstruction is therefore the prime measure to reduce the amount of light diffracted out of the central peak. A second measure is that of changing the number of different directions of the spider elements to change the number of spikes in the PSF. This affects the PSF mainly at some distance from the central peak. More in general one can optimize the PSF acting on the entire geometry of the spiders (relative positions, angles and lengths of all elements) to obtain a complex Fourier transform of the pupil function such that its squared modulus is closer to the desired PSF. Until now we have considered the first two more obvious measures and we are presently exploring the possibility of further ``phasing''.

Figure 10 is a three-dimensional logarithmic plot of the central part of the PSF, normalized to its maximum, of the 8.4 m diameter pupil without any spider but with the central obstruction determined by the 89 cm diameter hole in the primary. Figure 11 is a grey-scale image of the same circular PSF. Figures 12, 13, 14, 15, 16 and 17 are similar plots showing the effect of inserting two versions of the old F/15 ``X'' spider and the new F/15 swing arm. One of the ``X'' spiders considered (Figure 12) has vanes 100 mm wide to obtain about the same resonant frequency of the swing arm, the other (Figure 14) has vanes 39 mm wide, and of course doesn't satisfy the requirement on local resonant frequency, but allows a direct comparison of the effects of ``X'' and of swing arm geometry with equal obstruction. Figure 16 shows the PSF of the nominal swing arm obstruction shown in Figure 6 and Figure 17 shows the corresponding grey-scale plot. Figures 18, 19 and 20 are on linear scale and show the difference of the PSF of the pupil with no spider and of the PSF with the three above spiders. These plots show the effect of the diffraction by the spiders in the vicinity of the central peak, where the difference is zero because we subtracted a normalized PSF.

From a comparison of the figures one can see that the swing arm is equivalent to the ``X'' spider of equal obstruction in the vicinity of the central peak and better (i.e. is departing less from the unobstructed pupil) at a larger distance.

View Figure 10 here View Figure 11 here View Figure 12 here View Figure 13 here
View Figure 14 here View Figure 15 here View Figure 16 here View Figure 17 here
View Figure 18 here View Figure 19 here View Figure 20 here

6. Another swing arm

In addition to those supporting auxiliary optics, a fourth swing arm on each side of the telescope is used for supporting the mirror cover. The mirror cover is a fan-shaped structure made of thin aluminum panels enclosed between two layers of plastified canvas. Inflatable pockets inserted in the outer cavities of the fan structure are used for deploying and closing the cover. A two meter diameter model is in construction to verify the reliability of the device, that has an essential role in preserving the integrity of the primary mirrors in case of failure of the enclosure shutters and consequent possible exposure of the mirrors to bad weather. In more normal circumstances the mirror covers provide protection from dust and from accidental impacts. Figures 21 and 22 show the conceptual design of the mirror covers and of the supporting arms respectively in closed and in deployed position. For clarity the top and bottom canvas closures and the pneumatic pockets are not shown.

View Figure 21 here

View Figure 22 here