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Out of Focus OOF Holography for the GBT

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Week ending Apr 13: install OOF software, check data formats, have a means of ... Week ending Apr 20: Bojan Nikolic to assist in obtaining data and refining GBT ... – PowerPoint PPT presentation

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Title: Out of Focus OOF Holography for the GBT


1
Out of Focus (OOF) Holography for the GBT
  • Claire Chandler

2
Why OOF when you can do with-phase holography?
  • There are many advantages to traditional
    holography, but also some disadvantages
  • Needs extra instrumentation
  • Reference antenna needs to be close by so that
    atmospheric phase fluctuations are not a problem
  • S/N ratio required limits sources to
    geostationary satellites, which are at limited
    elevation ranges for the GBT (35-45)
  • Disadvantages of phase-retrieval holography?
  • Traditional methods require very high S/N

3
A new technique for OOF
  • Hills, Richer, Nikolic (Cavendish Astrophysics,
    Cambridge) have proposed a new technique for
    phase-retrieval holography. It differs from
    traditional phase-retrieval holography in three
    ways
  • It describes the antenna surface in terms of
    Zernike polynomials and solves for their
    coefficients, thus reducing the number of free
    parameters
  • It uses modern minimization algorithms to fit for
    the coefficients
  • It recognizes that the amount of defocusing is
    crucial to lowering the S/N requirements for the
    beam maps

4
Early OOF results for the JCMT and GBT (1)
  • The technique involves making both in-focus beam
    maps and maps with the secondary mirror defocused
    by known amounts. For the JCMT this has been
    carried out with focus offsets of 1mm
    (lobs850mm)
  • Images of 3C279 with focus -1mm, 0mm, 1mm

5
Early OOF results for the JCMT and GBT (2)
  • Aperture function of the JCMT inferred from OOF
    3C279 maps
  • Left, middle phase from two different
    observations on the same night (black to white
    p)
  • Right amplitude (results from Nikolic, Richer,
    Hills 2002)

6
Early OOF results for the JCMT and GBT (3)
  • One set of OOF maps using 12 GHz CH3OH masers
    have been used to measure the GBT surface
  • Left phase middle amplitude right an example
    OOF map

7
Some mathematics (from Richard Hills)
  • Consider the combination of a perfect parabolic
    antenna with aperture function A0, and phase
    errors Q(k).
  • If Q small, A A0(1 iQ), and the far-field
    electric field pattern is
  • E FT A0(1 iQ)
  • E0 iE0 Ä FT
    (Q) E0 iF
  • (defining F E0 Ä FT (Q) F contains
    all the information about Q)
  • Power pattern of the antenna is then
  • P E02 F2 2Á(E0)Â(F) -
    Â(E0)Á(F)
  • Small defocus Þ last term is negligible, and Q is
    derived from fitting for F2
  • Large defocus Þ end term dominates and different
    defocus values weight Â(F) and Á(F) differently
    to obtain independent information about F

8
S/N requirements for OOF holography
  • The S/N required for OOF maps depends on the
    highest order of the Zernike polynomial to be
    fitted
  • Some more work probably needs to be done to
    determine how S/N affects the accuracy of the
    fitted polynomial coefficients, but the JCMT
    results derived so far for terms up to radial
    order 7 (36 terms) used OOF maps with S/N 200

9
Sources of error for OOF on the GBT (1)
  • Thermal (system) noise (including a contribution
    from the astronomical source in some cases)
  • Pointing errors (current 3² rms limits possible
    surface accuracy from OOF to 180mm rms)
  • 1/f noise in receiver gain stability need to
    have
  • DG/G lt 10-3
  • over time it takes to make a map
  • Subreflector errors estimates of these can come
    from fitting the OOF maps, but there are some
    correlations between pointing and subreflector
    offsets
  • Changes in surface shape over the time it takes
    to make a map

10
Sources of error for OOF on the GBT (2)
  • To limit elevation range, make maps at 3 focus
    positions in less than an hour
  • Aim for S/N 103
  • Thermal noise will not dominate as long as the
    target source is stronger than 1Jy (continuum)
  • Receiver stability may be an issue

From Norrod (2003)
11
Project plan
  • Week ending Apr 13 install OOF software, check
    data formats, have a means of applying derived
    corrections to the GBT surface obtain test
    images using both continuum and spectral line
    sources
  • Week ending Apr 20 Bojan Nikolic to assist in
    obtaining data and refining GBT model in OOF
    package analyze receiver stabilities
  • Week ending Apr 27 Test software technique by
    applying a known deformation to the GBT surface
    and see if OOF holography can reproduce it
    reliably
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