Sloan Digital Sky Survey Telescope Technical Note 19970514
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.
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
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.
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
Volume (m^3)
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).
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.
10405
9458
11384
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
Effective distance from axis (m)
Moment of inertia (kg m^2)
Rotating floor frame
2.6
20847
Floor panels (est)
363
2453
Baffle forks
454
2
1814
Hanging equipment
2.5
11338
Photometric camera, cart, etc.
635
3968
Baffle support assy w/out counterweights
2.53
10741
Baffle counterweights
1532
9806
Wind baffle
907
3628
Flat field screen
227
Wind baffle assy
10693
2.48
65502
Telescope
19567
1.32
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.
6712
2.95
13332
3.69
12350
4.97
5608
2.71
78422
1.33
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.
1.55
4031
1.51
3475
4344
2.42
25464
Optics support structure
4956
1.45
Wind baffle / OSS assy
9300
1.96
35869
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.
