GBT Performance

1
GBT Performance

• Dana S. Balser

2
Telescope Structure
Focus
Pointing
Surface
3
Pointing Requirements
Condon (2003)
4
Focus Requirements
Srikanth (1990) Condon (2003)
5
Surface Requirements
Ruze (1966) Condon (2003)
6
Repeatable Errors Pointing

• Telescope misalignments
• Azimuth track tilt
• Horizontal collimation
• Elevation axle collimation
• Encoder offsets

• Gravity assume linear elastic structure

7
Repeatable Errors Focus

• Gravity assume linear elastic structure

8
Repeatable Errors Surface

• Gravity
• Homologous design (almost)
• Photogrammetry (50.3 deg) ? Zero-point
• Finite Element Model (FEM)
• Best Fit Parabola (BFP)

Active Surface 2004 Panels
2209 Actuators
9
Point Source Calibrators
PCALS 4.0 7108 sources Two-dimensional rms error
lt 0.2 arcsec
3 mm pointing calibrators
Condon Yin (2005)
10
Pointing Model
Balser et al. (2002)
11
Focus Model
Axial Focus
Ghigo et al. (2001)
12
Finite Element Model Gain Elevation Curve
K-band (20 GHZ)
13
Finite Element Model Sidelobes
14
Non-repeatable Errors Thermal
15
Non-repeatable Errors Wind
16
Thermal Pointing and Focus Corrections
Constantikes (2003)
17
Data Collection and Quality
All-sky Observations Single Source Track Up-Down
at Night ? Gravity NCP Source ? Temperature
Gaussian Fits (Az, El, Focus) Polarization (LCP
RCP) Direction (Forward Backward)
Jack Scan
18
Thermal Model Focus
Constantikes (2003)
19
Thermal Model Azimuth
Constantikes (2003)
20
Thermal Model Elevation
Constantikes (2003)
21
Performance Blind Pointing
22
Pointing Data Quality Polarization
23
Pointing Data Quality Direction
24
Cross-Elevation Uncertainty
25
Elevation Uncertainty
26
Focus Data Quality Polarization
27
Focus Uncertainty
28
Performance Offset Pointing
29
Offset Pointing Uncertainty
30
Offset Focus Uncertainty
31
Performance Tracking
Half-power in Azimuth
Half-power in Elevation
32
Power Spectrum
Servo resonance 0.28 Hz
33
Servo Error
34
Performance Summary
Benign Conditions (1) Exclude 1000 ? 1800
(2) Wind lt 3.0 m/s
Blind Pointing (1 point/focus) Offset
Pointing (90 min) Continuous Tracking
(30 min)
35
Surface Improvements Holography
Traditional Holography (Phase Coherent Holography)
Maddalena
Out-of-Focus Holography (Phase Retrieval
Holography)
Nikolic et al. (2006)
36
Zernike Polynomials Radial Terms
Zernike (1934) Wyant Creath (1992)
37
Zernike Polynomials Images
Wyant Creath (1992)
38
OOF Holography Gravity
Nikolic et al. (2006)
39
Gain-Elevation Curve
FEM only
FEM OOF
Nikolic et al. (2006)
40
OOF Holography Thermal Effects