SDSS 2.5-m primary support system measurements

Sloan Digital Sky Survey Telescope Technical Note 19981019

Contents

• Introduction
• Measurements
• Conclusions
• Acknowledgment

Introduction

The support of large optics in telescopes is complicated by the extreme sensitivity of such optics to deformation. While such optics may actually be quite rigid by normal engineering standards, their deformations must be exceedingly small to be acceptable. A satisfactory solution for monolithic optics is provided by separate systems for defining the orientation and location of the optic with respect to the telescope mount, i.e., the 6 rigid body degrees of freedom, and supporting the weight of the optic. Typically, it is called on to react to wind-induced forces of the optic at frequencies higher than about 1 Hz. Such forces are generally less than 1% of the weight of the optic. Since the definition system applies only small forces to the optic, it cannot cause significant deformation of its surface. The support system, on the other hand, must apply large forces to the optic to support its weight. This is accomplished using a large number of force actuators. Each force actuator need only apply the relatively small force required to support the weight of that portion of the optic that is near the actuator. Since each portion of the optic is supported locally, bending stresses are small as is the deformation of the surface of the optic.

In the case of the 2.5-m telescope primary mirror, the support actuators are air pistons. The 48 axial and 18 transverse air pistons support the axial and transverse components of the mirror weight vector respectively. The pressure of the air that is supplied to the pistons is controlled so that the forces applied by the definition system (as sensed by load cells) are small.

Table 1: Air piston parameters. Piston diameters are set so that the maximum pressure required is similar for the two sets.

	Axial	Transverse	Unit
Piston area	2299	3910	mm^2
Piston number	48	18
Piston force	156	416	N
Pressure	0.068	0.106	MPa

The air pistons are simple, high performance, reliable and inexpensive (Figure 1 and Figure 2). They consist of an acetyl piston (blue) that is captured in a machined aluminum housing (black). The piston seal is made by a reinforced silicon elastomer formed into the shape of a hat and folded as shown in the Figures (magenta). As the piston extends and retracts, the elastomeric seal rolls in and out. Since the seal is quite flexible, the force exerted by the piston is to first order independent of its axial displacement ("Design of the Apache Point Observatory 3.5 m Telescope III. Primary mirror support system", E.J. Mannery, W. A. Siegmund and M.T. Hull, Proc. of S.P.I.E., 628, 1986, p. 390). As reported by Mannery et al., axial parasitic forces can be 1 N or even less depending on the diameter, stroke and maximum pressure (which affects the strength of the seal that is required and thereby its stiffness).

Figure 1: Axial air piston. The piston consists of a two-part aluminum housing (black), an acetyl piston (blue) and a reinforced silicon rubber seal (magenta). Silicon rubber bumpers (cyan) support the mirror when the pistons are not pressurized and prevent full compression of the pistons. A cylindrical base (green) and washer (red) complete the assembly.  

Figure 2: Transverse air piston. The piston consists of a two-part aluminum housing (black), an acetyl piston (blue) and a reinforced silicon rubber seal (magenta).

Measurements

To measure the response of an air piston to transverse forces, a transverse piston was loaded with 11 kg lead bricks and pressurized using a syringe (Figure 3). To provide stable support for the bricks, an aluminum plate was placed on the piston. It was supported at the corners opposite the piston by two ø15 mm steel balls. A spring scale was used to apply a transverse force. The transverse displacement of the piston was measured using a dial indicator.

The data demonstrate that the piston behaves like a spring with hysteresis (Figure 4). The spring rate is proportional to the load on the piston (Figure 5). The data of Figure 4 were taken with the piston extended 6 to 7 mm, i.e., about 50%. With the piston extended 3 mm (25%) and a load of 223 N, no significant change in the spring rate or hysteresis behaviour was observed.

 

Figure 3: A spring scale was used to apply lateral forces to the air piston as a dial indicator measured its lateral displacement. The axial load was supplied by lead bricks. The piston was pressurized with a syringe.

 

Figure 4: The air piston acts like a spring with hysteresis in response to a side force. The spring gets stiffer as the axial load increases from 111 to 445 N. The data were taken with the piston extended 6 to 7 mm, i.e., about 50%.

 

Figure 5: The spring rates of Figure 2 are proportional to the axial load on the piston. The points at 0 N are much less reliable than the other data.

Conclusions

Measurements of the primary mirror transverse air pistons indicate the following.

• In response to a sideways force, the piston behaves like a spring with hysteresis.
• The spring rate is proportional to the load on the piston. As the piston is decentered, the seal fold is squeezed higher on one side and stretched lower on the opposite side. Thus the pressure in the piston acts on more area on one side than the other. This effect amounts to a spring rate of about 0.2 N/mm, much too small. Instead, stress stiffening of the membrane of the seal fold may be responsible, particularly in that part of the membrane that deforms due to shear.
• If the transverse air pistons could be allowed to exert up to +/-25 N axially on the mirror, then the mirror may be moved axially up to +/-0.5 mm. It is likely that the mirror can be positioned open loop to this accuracy. Finite element modeling could be used to investigate the deformation of the mirror due to these forces.
• Even if the air pistons are centered, their hysteresis is such that they may exert up to 10 N axially, depending on their history.

Acknowledgment

We are grateful to Jon Davis of Apache Point Observatory for making the first measurements of the lateral stiffness of an air piston.

Date created: 10/19/98
Last modified: 10/26/98
Copyright © 1998, Walter A. Siegmund
Walter A. Siegmund