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Frequency Axis Documentation

• Frequency Axis Documentation
  • Sdfits Frequency Axis
    • I. Gather Frequency Info from IF, LO1 and possibly ACS FITS files.
      • A. For Spectrometer (acs), get sampler channel spacing:
      • B. IF FITS file information gathered
      • C. LO1 FITS file information gathered
    • II. Calculate State Center Frequency for (sampler, integration, phase)
    • III. Frequency Axis Info written to SDFITS
  • GFM Frequency Axis Documentation (for Spectrometer)
    • I. Retrieve Center Channel
    • II. Retrieve State Center
      • LO1 frequency for integration and phase calculated
      • Sky Frequency Calculation
    • III. Frequency Axis calculated

Steps for determining Frequency Axis for a given backend's sampler, integration, and phase.

Below, FITS columns or keywords will use the following notation:

DeviceName.Keyword

or

DeviceName.ExtensionName.Keyword

Unless otherwise noted a FITS value (ex: SFF_SIDEBAND), will be the value for the particular Sampler in question.

Sampler = Bank + Port (ex: 'A1' = 'A' + 1)

Sdfits Frequency Axis

I. Gather Frequency Info from IF, LO1 and possibly ACS FITS files.

A. For Spectrometer (acs), get sampler channel spacing:

• For each sampler, find the channel spacing, or Frequency Interval, which is the bandpass (fStart to fEnd) divided by the number of lags.
• fStart and fEnd are found using this Sky Frequency Formula:
  • fStart = (SFF_SIDEBAND * PORT.FSTART) + (SFF_MULTIPLIER * average(LO1A.LO1TBL.LO1FREQ) ) + SFF_OFFSET ;
  • fEnd is derived from fStart and the bandwidth:

if PORT.BANDWIDTH in (50e6, 800e6):

fEnd = fStart - PORT.BANDWIDTH

else:

fEnd = fStart + PORT.BANDWIDTH

B. IF FITS file information gathered

• If the Spectrometer is used, the center IF for each sampler is found:

if PORT.BANDWIDTH in (50e6, 800e6):

acsCenterIF = PORT.FSTART - (PORT.BANDWIDTH * (CenterChannel/NumLags))

else:

acsCenterIF = PORT.FSTART + (PORT.BANDWIDTH * (CenterChannel/NumLags))

will (CenterChannel/NumLags) always be (1/2)?

• For each sampler, the following values are gathered from the IF FITS file:
  • SFF_MULTIPLIER
  • SFF_SIDEBAND
  • SFF_OFFSET
  • (and CENTER_IF for Spectral Processor)

C. LO1 FITS file information gathered

• LO1A.LO1TBL.LO1FREQ, LO1A.LO1TBL.VFRAME, LO1A.LO1TBL.LO1RVSYS are gathered and matched up appropriately to the integration mid-DMJD, using LO1A.LO1TBL.DMJD closest to, but not greater than, the mid DMJD of the integration.
• LO1A.STATE.FREQOFF values are matched up with the backend phases

II. Calculate State Center Frequency for (sampler, integration, phase)

• lo1Freq = LO1A.LO1TBL.LO1FREQ for this integration) + (LO1A.STATE.FREQOFF for this phase)
• State Center Frequency calculation depends on backend:
  • Spectral Processor State Center Freq = (SFF_SIDEBAND*IF.CENTER_IF)+(SFF_MULTIPLIER*lo1Freq)+IF.SFF_OFFSET
  • Spectrometer (acs) State Center Freq = ((SFF_SIDEBAND*acsCenterIF)+(SFF_MULTIPLIER*lo1Freq)+IF.SFF_OFFSET) + samplerChannelSpacing

where samplerChannelSpacing was calculated in I.A and acsCenterIF was calculated in I.B

III. Frequency Axis Info written to SDFITS

• CRPIX1 - center channel for sampler
• CDELT1 - frequency interval as described in I.A
• CRVAL1 - State Center Frequency as described above

GFM Frequency Axis Documentation (for Spectrometer)

GFM uses does not use the same ScanData class that the Sdfits tool uses to calculate its frequency information. The eventual goal is to refactor Sdfits to not use ScanData and for these two applications to share the same API. In addition, the API used by GFM is still not complete, and, in fact contains a bug.

I. Retrieve Center Channel

This recreates the frequency axis needed to determine the center channel. Fortunetly, the Input class caches these arrays, so they are not redundantly calculated.

II. Retrieve State Center

LO1 frequency for integration and phase calculated

lo1Freq = LO1A.LO1TBL.LO1FREQ for this integration) + (LO1A.STATE.FREQOFF for this phase)

This is done properly using the new API: the LO1 class finds the appropriate LO1 frequency to use for this integration and phase using methods identical to Sdfits in section I.C above, and adds them.

Sky Frequency Calculation

The same calculation as what is done in Sdfits in section II above is completed:

State Center Freq = ((SFF_SIDEBAND*BACKEND_CENTER_IF)+(SFF_MULTIPLIER*lo1Freq)+IF.SFF_OFFSET)

Where the IF FITS file values are read using the API properly (i.e., using the PortTable class).

III. Frequency Axis calculated

-- PaulMarganian - 02 Nov 2006