3mm spectral-line observing with Mopra

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3mm spectral-line observing with Mopra

• Tony Wong
• ATNF UNSW

Mopra Induction Weekend - 28 May 2005
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Outline

• How bright is your source?
• Estimate expected brightness temperature
• What is the required integration time?
• Depends on Tmb, Tsys, bandwidth
• What are the required calibrations?
• OFF scans, Tsys, pointing, standards
• How do I process the data?
• Emphasis on OTF see also tutorials

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Flux density units Janskys

• Flux density is the power received per unit
  collecting area per unit frequency interval, and
  depends on luminosity and distance.

W m-2 Hz-1 1026 Jy
Generally used for discrete objects of definite
size like stars, galaxies, and quasars.
For spectral line sources, can integrate in
frequency (velocity) to get units of Jy km s-1.
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Brightness units Kelvins

• Brightness (intensity) is the flux density per
  solid angle, and thus depends also on the beam
  size of the telescope.

Generally used for extended objects such as
clouds, but may be used in all situations since
single-dish telescopes are calibrated in K. If
using Tmb from another telescope for a compact
source, must rescale to Mopras Tmb using ratio
of beam areas!
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Getting K from Jy

• ? At 3mm wavelength, a source of 1 Jy will
  produce Tmb of 0.1 K in the ?35 Mopra beam.

Applying a beam efficiency for Mopra of ?b0.4,
this gives TA ?bTmb 0.04 K 40 mK.
Note this is only valid when the source size lt
beam.
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Sensitivity

• Radiometer equation

• ton is integration time in seconds
• ?? is bandwidth per channel in Hz (s-1).
• Tsys is the effective (corrected for
  atmospheric absorption) system temperature.
• Additional factors due to correlator efficiency
  etc.

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Position switching

• Strong and variable atmosphere at 3mm requires
  frequent bandpass calibration.

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Position switching

• Strong and variable atmosphere at 3mm requires
  frequent bandpass calibration.

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Position switching

• The quotient spectrum is in units of TA.

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Position switching

• Sensitivity equation for position switching

• Usual practice is to set ton toff 1 minute
  then repeat.
• For Tsys300 K, ??1 MHz, need ton30 minutes to
  get ?rms down to 10 mK (TA).
• In general would like to detect a signal at the
  5? level.
• Extra ?2 improvement by averaging 2 polarisations.

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Position switching schedule

• unit
• closefile
• observer Ned
• project standardspec
• telescope Mopra 22m
• receiver SIS
• corrmode NORMAL
• nfreq 1
• freq1 110201.393
• config ac_64_1024_2
• bandw 64
• chans 1024
• source OrionKL
• raj 053514.5
• decj -052229.56
• vel 9
• obsunit cycles
• obsval 36
• average 12

srcoff Reference lngref -000800.0 latref
-010000.0 obstype TRACK go unit srcoff
signal lngsig 000000.0 latsig
000000.0 obstype TRACK go unit srcoff
signal obstype TRACK go unit srcoff
Reference obstype TRACK go
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Multiple ONs per OFF

• If spectra are taken quickly, possible to use the
  same OFF spectrum for several ON spectra.
• This strategy is employed in OTF mapping, where a
  single OFF spectrum is followed by 45 ON
  spectra.
• One can show (http//kp12m.as.arizona.edu/12_obs_m
  anual/appendix_F.htm) that for N ONs per OFF, the
  optimal ton/toff 1/?N.
• This assumes that (Ntontoff) lt a few minutes,
  the time over which the atmosphere can change
  significantly.

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OTF (Raster) mapping
one scan
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Scanning in alternate directions
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Advantages of OTF

1. Less time spent off source (assuming whole map is
   interesting!)
2. Averaging multiple maps can help smooth out
   systematics related to pointing and weather.
3. Antenna position recorded continuously during
   scanning, so tracking errors not a problem. Also
   reduces overheads involved in acquiring a
   demanded position.

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When to OTF Map?

• If covering a region of 3 x 3 or larger.
• If the line is bright (0.5-1 K Tmb).
• For smaller regions, a large fraction of time is
  spent turning the telescope around.
• For weak lines, a large number of maps must be
  taken and averaged (32 MB each!).
• Alternative to OTF do position switched
  observations on a grid (grid mapping). Can
  step outwards in spiral pattern.

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Standard OTF parameters

• Map size 5 x 5, 31 rows, 45 spectra per row.
• Scanning rate 3.5 per second (14 Nyquist cell).
• ton4 sec, toff 19 sec gt ?rms 0.57 K
  (Tsys400 K, ?? 75 kHz).
• After gridding, ?rms reduced by about half (0.28
  K), since several spectra contribute to each
  cell.
• Total time spent mapping about 75 minutes!
• Same sensitivity could be achieved at a single
  position in 1 minute (tontoff)!

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otfsched program

• Interactive perl script to generate Mopra
  schedule files.
• Scan in single or alternate directions, in RA or
  DEC, with 1 or 2 rows per OFF integration.
• OFF position can be relative to map centre or
  given as a fixed RA and DEC.
• Default is Nyquist sampling along row with 10
  spacing between rows.
• With 2s cycle time, a 5 square map takes about
  80 minutes (1500 spectra).

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otfsched.pl ltinput-filegt

• project m143
• config ac_64_1024_2
• nfreq 1
• freq1 115271.202
• bandw 64
• chans 1024
• obsfreq 110000
• observer tw
• epoch j2000 (equatorial use gal for
  GLON/GLAT)
• centre 110645.0 -772251
• pmotion 0 0
• width 5 (size in arcminutes)
• rowspoff 1
• scandir 1 (DEC scanning)
• istart 1 (starting corner, 1NW and going
  cclockwise)
• absref 112600 -772000 (absolute ref
  specified)
• calint 30
• source cha1a-codc
• velocity 3.0

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SiO maser beam mapping
2003 Oct Beff 0.4
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SiO maser beam mapping
2004 Jun Beff 0.5
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Data processing

• The problem coordinate information only recorded
  for the beginning of each scan.
• The solution query ACC, record RA, DEC, and a
  timestamp in (u,v,w) variables of RPFITS.
• Interpolate to timestamps of spectra in aips
  reader (or better yet, using mapread/mapfix
  programs).
• Livedata package does bandpass calibration (using
  previous OFF scan) and baseline subtraction.
• Gridzilla package takes spectra from SDFITS
  files, grids them into FITS cubes

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Useful software

• rpflog summarise an RPFITS file
• rpfread detailed information about each scan in
  an RPFITS file.
• Otflook.csh quick-look OTF processing, produces
  plots of spectra and dot plot.
• Otfmap.csh much the same as Otflook.csh, but
  also runs mapfix to produce a new RPFITS file
  with corrected position stamps.
• Filewatch.csh runs rpflog on files as they
  complete and appends to a daily observation log.
• Comment.csh add comments to the daily obslog.

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Horsehead Nebula in Orion
12CO
13CO
6 x 6 fields
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Horsehead Nebula in Orion
12CO
13CO
6 x 6 fields
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Cha I region - N2H
Cha-MMS1
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Cha I region - N2H
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Cha I region - N2H
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Other calibrations

• System temperature every half hour should
  provide 5 relative accuracy in good conditions.
  Best to do off source. Can be included in
  schedule file.
• Pointing every 45-90 minutes, on an SiO maser
  lt30 from source.
• Tsys (and probably pointing) change most as a
  function of elevation.
• Standard spectra once a day, to check antenna
  gain and sideband rejection. Orion KL has lots
  of lines.
• SiO beam map once a week, to check for
  deformations. Standard OTF schedules should be
  available consult ATNF staff.

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Final remarks

• Always start with SiO pointing to test the
  overall system.
• When possible, observe your standard first to
  test the tuning.
• Calibrate more often in poorer conditions.
• When OTF mapping, process the first map
  immediately through Livedata/Gridzilla.
• Keep good logs (preferably electronic) of
  everything you do, esp. pointing tuning.