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Latest update: 2004/05/27 - Frequency axis improvements are in place (sdfits v1.1). See "Frequency Axis Descriptors" below.

• Performance Updates
  • Antenna Positions
  • Van Vleck Correction
• Usage Updates
  • Minus Signs
  • Mode Namings
• Header Updates
  • Beam Information
  • Coordinate Systems
  • Frequency Axis Descriptors
  • Procedure/Sequence Information
  • Timing Accuracy
  • Tsys
  • Various
    • From GO FITS
    • From IF FITS
    • From LO1A FITS

Performance Updates

Antenna Positions

In the beta release of sdfits (now ended), the interaction with FQL was to query "per integration" for antenna positions. This was an unnecessarily slow client-side usage of FQL.

Now, the interaction with FQL is such that all antenna positions pertaining to a scan are queried for just once, a priori. Then that data is worked with "in memory" for integration-specific purposes.

This improvement can yield a substantial speed-up, depending on the number of integrations per scan. The more integrations, the more impressive the speed-up. Typical usage shows a speed-up of about 5-7x.

Also worth noting here is that the beta sdfits judged a scan having no antenna positions to be invalid. Now, this is not the case. Zeroes are written out in the absence of antenna positions.

Van Vleck Correction

An improvement was made to the underlying Van Vleck C++ code that yields about a 2x increase in the processing speed of spectrometer data over that of the beta.

Usage Updates

Minus Signs

Options are now specified in the "single-minus" style rather than the beta's "double-minus" style. This reflects more common practice, where double-minus options are used for one of two purposes: 1) as full-word alternative options to single-character, single-minus options, or 2) to disambiguate the meaning of a single-character option.

For example, the Linux command /bin/ls has an "all files" option that can be specified by -a or --all.

Since sdfits has no single-character options (i.e. only one option per purpose), the double-minus doesn't really make sense.

Mode Namings

Mode names have been changed from the beta to better reflect how data is processed. The mode abbreviations avg, cal, and raw are now used instead of the old avg, int, and state, respectively.

Header Updates

Beam Information

In the beta release, no beam information was being written except polarization (Stokes in CRVAL4).

Now, beam information has been added that identifies the physical feed (FEED), physical switching feed (SRFEED), and their beam offsets relative to the tracked beam (BEAMXOFF, BEAMEOFF). The actual calculation of each beam's position using these offsets is not performed. This fact could change, but for now that task is left to downstream software.

Note that beam switching issues remain unsolved, such as the actual feed associated with a given data row.

Coordinate Systems

Azimuth and Elevation coordinates are now written out. This is in addition to the SDFITS-standard longitude-like/latitude-like coordinates.

As with the beta, we are using CRVAL2 and CRVAL3 to serve the SDFITS requirement, and they hold coordinates in the observer-selected coordinate system (i.e. major/minor).

We experimented with adding RA/DEC and GLON/GLAT columns in this release. But this almost always resulted in duplication of antenna positions already present in the observer-selected major/minor columns, which leads to data bloating and possible discrepancies and/or confusion.

After much discussion, it seems best not to write out additional coordinate systems such as RA/DEC or GLON/GLAT, but to leave this relatively easy computation to downstream software. Azimuth and Elevation have been added since computing them afterwards is somewhat more difficult.

Frequency Axis Descriptors

Frequency axis descriptor information is now fully parameterized by sampler, integration, and phase (state). Now observation modes such as frequency-switched, on-the-fly mapping, and multi-IF work can be done using sdfits. Center sky frequencies are provided for both sig (CRVAL1) and ref (FOFFREF1) phases.

Procedure/Sequence Information

The beta release had no procedure/sequencing information. Now, the GO keywords PROCSEQN, PROCSIZE, LASTON, and LASTOFF have been added.

Timing Accuracy

The beta release only recorded times to an accuracy of 0.1 seconds. Now, sdfits records times accurate to 0.01 seconds.

Tsys

The beta release did not compute Tsys (and simply wrote 1.0 in all cases). Now, Tsys is now computed for "avg" and "cal" modes (Tsys doesn't make sense for "raw" mode; 1.0 is still written in this case).

Various

The following were not featured in the beta, but are now written out by sdfits:

From GO FITS

OBSID, VELOCITY.

From IF FITS

SIDEBAND.

From LO1A FITS

VFRAME and RVSYS.