Basic Telescope Parameters

Sloan Digital Sky Survey Telescope Technical Note 19970514

Contents

• Introduction
• Telescope Parameters
• Wind Baffle Parameters
• Procedures

Introduction

The Sloan Digital Sky Survey 2.5-m telescope was modelled using finite element analysis code by Terry M. King of L&F Industries, Huntington Park, CA. As part of this analysis, the volume, mass, center of gravity and mass moment of inertia of the telescope and optics support structure (OSS) were computed.

Telescope Parameters

Prior to installation of the telescope, the manufacturer provided estimates of the masses of the largest pieces of the telescope. This table was used to plan installation and the pieces can be seen in the installation images. (This table is from a memo by Terry M. King dated 11/23/96.)

Table 1: Masses of the largest pieces of the telescope (used to plan installation).

Component	Mass (kg)
Primary support w/pillow block	5215
Fork assy	4308
Sec. truss w/cage assy	499
Rotating floor framing	3084
Wind baffle support assy w/out counterweights	1678

(The following tables are from a file provided by Terry M. King dated 11/23/96.)

In the following discussion, the following coordinate system is defined.

• X-axis is the telescope altitude axis.
• Y-axis is perpendicular to the x and z axes.
• Z-axis is the azimuth axis for the telescope or the optical axis for the optics support structure (OSS).

The reference point (XR,YR,ZR) is at ( 0.000, 0.000, 0.000)

Table 2: Volume, mass and center of gravity of the telescope.

Volume (m^3)	Mass (kg)	XC (m)	YC (m)	ZC (m)
2.999	19570	0.001	-0.0163	-1.722

Table 3: Mass moment of inertia of the telescope with respect to X-Y-Z axes at (XR,YR,ZR) azimuth axis. IZ, the moment about the azimuth axis, is an important parameter for the telescope control system.

IX (kg m^2)	IY (kg m^2)	IZ (kg m^2)	Comment
127127	134233	33855	OSS AT ZENITH
119422	123478	32114	OSS AT 45 DEGREES
118732	119840	34594	OSS AT HORIZON

Table 4: Volume, mass and center of gravity of the optics support structure (OSS).

Volume (m^3)	Mass (kg)	XC (m)	YC (m)	ZC (m)
1.365	4957	0.006	0.076	-0.001

Table 5: Mass moment of inertia of the optics support structure (OSS) with respect to X-Y-Z axes at (XR,YR,ZR) altitude axis. IX, the moment about the altitude axis, is an important parameter for the telescope control system.

IX (kg m^2)	IY (kg m^2)	IZ (kg m^2)
10405	9458	11384

Wind Baffle Parameters

The masses and moments of inertia of the wind baffle and circular floor panel (aka rotating floor) are important for uplift and dynamic calculations (Table 6). The moment of inertia is equal to mass * (effective distance from axis)^2. If all the mass of a component were concentrated at its effective distance, its moment of inertia would be unchanged. Masses were mostly taken from drawings. Effective distances were estimated for the components. For the subtotals and totals, effective distances were computed from the mass and moment of inertia sums in order to complete the table.

The total moment of inertia of the wind baffle and telescope in altitude should be correct to 20%. The total moment of inertia of the circular floor panel, wind baffle and telescope in azimuth should be correct to 30% but is dependent on estimates of the mass of equipment supported below and above the circular floor panel. This moment of inertia decreases about 14% as the telescope moves from the horizon to the zenith.

Table 6: Masses and mass moments of inertia of the wind baffle and circular floor panel components. The moment of inertia is about the azimuth axis. The telescope is pointed at the zenith.

Component	Mass (kg)	Effective distance from axis (m)	Moment of inertia (kg m^2)
Rotating floor frame	3084	2.6	20847
Floor panels (est)	363	2.6	2453
Baffle forks	454	2	1814
Hanging equipment	1814	2.5	11338
Photometric camera, cart, etc.	635	2.5	3968
Baffle support assy w/out counterweights	1678	2.53	10741
Baffle counterweights	1532	2.53	9806
Wind baffle	907	2	3628
Flat field screen	227	2	907
Wind baffle assy	10693	2.48	65502
Telescope	19567	1.32	33855
Wind baffle / telescope assy	30260	1.81	99357

Table 7: Masses and mass moments of inertia of the wind baffle and circular floor panel components. The moment of inertia is about the azimuth axis. The telescope is pointed at the horizon.

Component	Mass (kg)	Effective distance from axis (m)	Moment of inertia (kg m^2)
Rotating floor frame	3084	2.6	20847
Floor panels (est)	363	2.6	2453
Baffle forks	454	2	1814
Hanging equipment	1814	2.5	11338
Photometric camera, cart, etc.	635	2.5	3968
Baffle support assy w/out counterweights	1678	2	6712
Baffle counterweights	1532	2.95	13332
Wind baffle	907	3.69	12350
Flat field screen	227	4.97	5608
Wind baffle assy	10693	2.71	78422
Telescope	19567	1.33	34594
Wind baffle / telescope assy	30260	1.93	113016

Table 8: Masses and mass moments of inertia of the wind baffle components. The moment of inertia is about the altitude axis.

Component	Mass (kg)	Effective distance from axis (m)	Moment of inertia (kg m^2)
Baffle support assy w/out counterweights	1678	1.55	4031
Baffle counterweights	1532	1.51	3475
Wind baffle	907	3.69	12350
Flat field screen	227	4.97	5608
Wind baffle assy	4344	2.42	25464
Optics support structure	4956	1.45	10405
Wind baffle / OSS assy	9300	1.96	35869

Procedures

The telescope drive assemblies are pushed against their respective drive disks by radial links. The contact force must be large enough to transfer the necessary drive torque to the telescope via friction but must be less than the force that would permanently deform the disk or roller. Increasing the force beyond that necessary to drive the telescope will increase the drive friction and should be avoided. In the case of the azimuth axis, the radial link must limit the contact force during seismic accelerations. Also, the links must provide an extremely stiff link between the azimuth drive housings and the telescope pier to allow high control system bandwidths and resist wind-induced tracking error.

The azimuth drives are preloaded against the azimuth drive disk through a series of springs, a set of soft springs, and a set of hard springs (drawing E326004). One other spring is important and that is the material of the frame.

The hard springs are Belleville washers (Associated Spring part number B2500-175). The retaining cap for these washers should be put snugly in place and then compressed an additional 0.261 inches. This produces a preload on the hard springs of about 12000 lb.

The soft springs are Belleville washers (Associated Spring part number B1875-127). Their purpose is to provide the preload for initial assembly. Only two of the four assemblies contain soft springs. They are compressed by extending the radial turnbuckles. During initial assembly, they should be compressed until the Belleville Plunger bottoms out against the Belleville Housing. This produces 2600 lb of force on the azimuth drive housings and will nearly flatten the soft washers.

Finally the radial turnbuckles should be extended until some motion of the indicator rod is detected. Count the number of turns of the radial turnbuckles that are necessary. Then the radial turnbuckles should be retracted by half the counted turns. This should produce a force on the azimuth drive housings of 6000 lb. The Belleville Housing will still be bottomed out against the Side Member Support. This provides a very stiff coupling to the Side Member Supports. During seismic accelerations, the hard springs compress until the earthquake bumbers are encountered by the drive disk. This limits the force on the azimuth drive housings to 20000 lbs or so.

The altitude preload is adjusted through springs driving pushing the motor housing against the altitude drive disks (drawing E326007 ). These springs are Danly Die Springs (9-4028-21) which have a spring constant 638 lb/inch and a free length of 7 inches. Currently these springs are compressed to approximately 5.75 inches, producing approximately 800 lb of preload.

Date created: 05/14/97
Last modified: 05/23/97
Walter A. Siegmund