configured appropirately by the operators. Currently, the OOF model

configured appropriately by the operators. Currently, the OOF model

The long-range goal for further improving the telescope performance at high frequencies (f > 43 GHz) is to measure and reduce the remaining small scale surface error to 240 microns rms. Traditional holography is currently (June 2007) the primary technique being investigated.

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observing procedures?.

observing procedures.

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to measure and model the GBT primary surface. The contractor performed photogrammetry measurements at a fixed elevation (45 deg) to determine what are called the "zero offsets", the best initial setting of the surface. As the telescope tips in elevation gravity will deform the primary surface shape. A structural finite element (FE) model for the GBT is used to correct these deformations. The NRAO has performed traditional holography observations at a fixed elevation using satellite transmitters and out-of-focus (OOF) holography observations at different elevations using astronomical sources. The OOF holography observations are parameterized by a set of Zernike polynomials that can be "dialed" into the active surface control system.

to measure and model the GBT's primary surface. The contractor performed photogrammetry measurements at a fixed elevation near the rigging angle to determine what are called the "zero offsets", the best initial setting of the surface. As the telescope tips in elevation gravity will deform the primary surface shape. A structural finite element (FE) model for the GBT is used to correct for these deformations. The NRAO has performed traditional holography observations at a fixed elevation using satellite transmitters and out-of-focus (OOF) holography observations at different elevations using astronomical sources. The OOF holography observations are parameterized by a set of Zernike polynomials that can be "dialed" into the active surface control system.

• Frequency > 4 GHz (C-band and higher): the FE active surface model is turned on to make surface adjustments versus elevation.

• Frequency > 4 GHz (C-band and higher): the FE model is used to make surface adjustments versus elevation.

• Frequency > 26 GHz (Ka-band and higher): the OOF holography models are applied to the active surface by dialing in a set of Zernike

• Frequency > 26 GHz (Ka-band and higher): the OOF holography model is used to make further surface adjustments in addition to the FE model. This is performed by dialing in a series of Zernike

The active surface consists of 2209 computer controlled actuators that are used to adjust the GBT's primary surface. Several different methods have been used to measure the GBT's surface. The contractor performed photogrammetry measurements at a fixed elevation of 45 deg. These adjustments are used as "zero points" for further refinements. A finite element (FE) model has been developed for the GBT and provides corrections to the surface as a function of elevation. The surface has also been measured using both traditional holography and an out-of-focus (OOF) holography technique. The OOF holography observations are parameterized by a set of Zernike polynomials that can be "dialed" into the active surface. A series of Zernike polynomials models have been interpolated versus elevation.

We have developed an incremental or layered approach that is a function of observing frequency.

• Frequency < 4 GHz (S-band and lower): only the photogrammetry zero point lookup table is required. For prime focus (PF) obsevations the active surface power is turned off due to RFI. Because the process of powering the hardware on and off is laborious, the power supplies are not turned on or off unless necessary. So when going from K-band to L-band the power supplies will be ON, while when going from PF to L-band the power supplies will be OFF.

The GBT's primary surface consists of over two thousand panels that can be adjusted by computer controlled actuators. This system is called the active surface. Several different methods have been used to measure and model the GBT primary surface. The contractor performed photogrammetry measurements at a fixed elevation (45 deg) to determine what are called the "zero offsets", the best initial setting of the surface. As the telescope tips in elevation gravity will deform the primary surface shape. A structural finite element (FE) model for the GBT is used to correct these deformations. The NRAO has performed traditional holography observations at a fixed elevation using satellite transmitters and out-of-focus (OOF) holography observations at different elevations using astronomical sources. The OOF holography observations are parameterized by a set of Zernike polynomials that can be "dialed" into the active surface control system.

We have developed an incremental or layered approach to improve the primary surface.

• Frequency < 4 GHz (S-band and lower): only the photogrammetry zero offset lookup table is required. For prime focus (PF) obsevations the active surface power is turned off due to RFI. Because the process of powering the hardware on and off is laborious, the power supplies are not turned on or off unless necessary. So when going from K-band to L-band the power supplies will be ON, while when going from PF to L-band the power supplies will be OFF.

are turned on by dialing in various Zernike polynomial terms to the active surface in addition to the FE active surface model.

are applied to the active surface by dialing in a set of Zernike polynomial terms.

Currently, the OOF holography model is activated by using special procesures discussed in Observing Procedures?.

The photogrammetry zero offset model and the FE model will be configured appropirately by the operators. Currently, the OOF model is configured by the observer using specific observing procedures?.

• GHz (S-band and lower): only the photogrammetry zero point lookup table

• Frequency < 4 GHz (S-band and lower): only the photogrammetry zero point lookup table

• Frequency > 4 GHz (C-band and higher): the FE active surface model

• Frequency > 26 GHz (Ka-band and higher): the OOF holography models

The active surface consists of 2209 computer controlled actuators that are used to adjust the GBT's primary surface. Several different methods have been used to measure the GBT's surface. The contractor performed photogrammetry measurements at a fixed elevation of 45 deg. These adjustments are used as "zero points" for further refinements. A finite element (FE) model has been developed for the GBT and provides corrections to the surface as a function of elevation. The surface has also been measured using both traditional holography and an out-of-focus (OOF) holography technique. The OOF holography observations are parameterized by a set of Zernike polynomials that can be "dialed" into the active surface. A series of Zernike polynomials models have been interpolated versus elevation.

We have developed an incremental or layered approach that is a function of observing frequency.

• GHz (S-band and lower): only the photogrammetry zero point lookup table is required. For prime focus (PF) obsevations the active surface power is turned off due to RFI. Because the process of powering the hardware on and off is laborious, the power supplies are not turned on or off unless necessary. So when going from K-band to L-band the power supplies will be ON, while when going from PF to L-band the power supplies will be OFF.

Currently, the OOF holography model is activated by using special procesures discussed in Observing Procedures?.

%META:TOPICINFO{author="DanaBalser" date="1128538361" format="1.0" version="1.1"}% %META:TOPICPARENT{name="AstActiveSurface"}%

Active Surface - General Strategy

-- DanaBalser - 05 Oct 2005