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Pointing and Focus - Observing Details

Pointing and focus observations are performed and analyzed using the the Astronomer's Integrated Desktop Astrid. Pointing observations consist of four scans through the specified source position: forward in azimuth; back in azimuth; forward in elevation; and back in elevation. Axial focus observations consist of a single scan where the telescope tracks the specifed source position while driving the subreflector through a specifed range. Listed below are five procedures that are available to perform these measurements.

• Peak: Performs a Peak observation. The parameters must be set by the user.

• Focus: Performs a Focus observation. The parameters must be set by the user.

• AutoPeak: Performs a Peak observation automatically with no input (e.g., scan length).

• AutoFocus: Performs a Focus observation automatically with no user input (e.g., scan length).

• AutoPeakFocus: Performs a Peak and then a Focus observation automatically with no user input (e.g., scan length).

Peak and Focus data are analyzed in real-time using the GBT Fits Monitor GFM within Astrid. Polynomial models are fit to the baselines regions and Gaussian models are fit to the Peak and Focus curves as appropriate. The fitted pointing and focus offsets can be applied either manually or automatically to the control system. The table below lists the recommended parameters for Peak and Focus measurements. These are the parameters used by AutoPeak, AutoFocus, and AutoPeakFocus

Recommended lengths and rates for performing peak and focus measurements

Notes:

1. Prime Focus: Peak Lengths are chosen to be 5 x FWHM with a scan time of 15 seconds to have good sampling across the beam. The Peak Rate is derived appropriately.
2. Gregorian Focus: Peak Rates are chosen to give 2 seconds across the FWHM, Peak Times to give a scan time of 30sec (to allow vibrations to settle), Peak Length is derived appropriately. Peak Times are conservative, we should investigate shorter scan lengths.
3. Prime Focus: Axial focus measurements are not recommended at PF since the gain changes only slightly over the entire focus range.
4. Gregorain Focus: The optimal focus length is 2 x FWHM, but to allow for varying baselines we currently recomment ~ 3 x focus FWHM, plus 40mm at each end to allow for the fact that focus measurement is done wrt to focus tracking curve, not last offset. The Focus Rate is then chosen to give a 60sec scan time. This is a tradeoff between completing the focus scan quickly, and allowing any potential scan-start anomalies to die away.
5. At L through X-band, the focus rates and lengths are conservative limits set by subreflector hardware (the absolute maximum would be 600mm/min and 600mm).
6. At Ku-band and higher frequencies the peak length is rather larger to accomodate the beam separation in azimuth for these multi-beam receivers.

For full details on choice of parameters, see PTCSPN 10

Subreflector notes

• Focus curve may be approximated by a Gaussian with FWHM approx 4 wavelengths.
• Maximum rate of travel of subreflector in Ys is 600mm/min
• Maximum absolute range of subreflector in Ys is -710mm to +305mm.
• Optimal focus tracking curve (model3 2003) is Y = xxx + xxx cos(E) + xxx sin (E)
• this corresponds to xxmm at 10deg E, xxmm at 85deg E, so the maximum positive throw at high E is approx xxmm.

-- RichardPrestage - 04 Feb 2004