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3mm spectral-line observing with Mopra

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3mm spectral-line observing with Mopra. Tony Wong. ATNF & UNSW ... Interpolate to timestamps of spectra in aips reader (or better yet, using ... – PowerPoint PPT presentation

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Title: 3mm spectral-line observing with Mopra


1
3mm spectral-line observing with Mopra
  • Tony Wong
  • ATNF UNSW

Mopra Induction Weekend - 28 May 2005
2
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

3
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.
4
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!
5
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.
6
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.

7
Position switching
  • Strong and variable atmosphere at 3mm requires
    frequent bandpass calibration.

8
Position switching
  • Strong and variable atmosphere at 3mm requires
    frequent bandpass calibration.

9
Position switching
  • The quotient spectrum is in units of TA.

10
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.

11
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
12
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.

13
OTF (Raster) mapping
one scan
14
Scanning in alternate directions
15
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.

16
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.

17
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)!

18
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).

19
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

20
SiO maser beam mapping
2003 Oct Beff 0.4
21
SiO maser beam mapping
2004 Jun Beff 0.5
22
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

23
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.

24
Horsehead Nebula in Orion
12CO
13CO
6 x 6 fields
25
Horsehead Nebula in Orion
12CO
13CO
6 x 6 fields
26
Cha I region - N2H
Cha-MMS1
27
Cha I region - N2H
28
Cha I region - N2H
29
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.

30
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.
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