Frequency Switched KFPA Pipeline Use Case

Figure 1: JCMT 850 um infrared image of Ophiucus, showing regions images B, C and D imaged with the GBT by Friesen et al 2009.


The KFPA pipeline development is oriented towards producing images for a few standard observing modes. We take as a starting goal for the KFPA pipeline the ability to perform the single dish GBT mapping operations described in a paper by Friesen, Di Francesco, Shirley and Myers 2009, Ap. J. 697, 1457 Their paper describes observations of three star forming cores (B, C and D) in Ophiuchus. The paper describes combined VLA, ATCA and GBT observations. The processing steps for the GBT observations in this paper are the first set of requirements for the KFPA pipeline. We are not concerned with the interferometer imaging here.

In this document, we clarify the observing and data processing aspects of the paper which are relevant to the pipeline. We take as the first two goals for the pipeline system:

  1. Reproduction of the GBT images using the Frequency Switched (FS) mode described in the paper.
  2. Processing of the observations in Position Switched (PS) mode for comparison of the mapping results.

After completing the initial checkout of the KFPA, one of the first mapping targets for imaging and pipeline commissioning will be mapping the entire Ophiucus region shown in Figure 1.

GBT Observations

Certain observations must be performed for the GBT pipeline reduction to be successful. This section summarizes the observing requirement for frequency switched observations. We also note some optional actions performed if additional observations are made in conjunction with the observations. Friesen et al made their observations as part of GBT project 06A_065. Their observations were carried out over 12 epochs, from 2006 April 9 to 2006 November 3. An ascii summary of these observations is available at this link. The following text is directly from the Friesen et al (2009 Ap. J. 697, 1457) article:

The Ophiucus observations were done in frequency-switching mode, using the GBT K-band (upper) receiver as the front end, and the GBT spectrometer as the back end. This setup allowed the simultaneous observation of all lines in four 50 MHz-wide IFs, each with 8192 spectral channels, giving a frequency resolution of 6.104 kHz, or 0.077 km s−1 at 23.694 GHz. The data were taken using the GBT’s on-the-fly (OTF) mapping mode, using in-band frequency switching with a throw of 4 MHz. In OTF mode, a map is created by having the telescope scan across the target in right ascension (R.A.) at a fixed declination (decl.), or in decl. at a fixed R.A., writing data at a predetermined integration interval. The maps of Oph B1 and B2 were made while scanning only in R.A. at a fixed decl., while for subsequent targets (Oph B3, C, and F) the scanning mode was alternated to avoid artificial striping in the final data cubes. No striping, however, is apparent in the final B1 or B2 images. At the observing frequency of 23 GHz, the telescope beam was approximately 32" FWHM. Subsequent rows or columns were spaced by 13" in decl. or R.A. to ensure Nyquist sampling. Scan times were determined to ensure either one or two full maps of the observed region could be made between pointing observations. For all observations, pointing updates were performed on the point source calibrator 1622-254 every 45–60 minutes, with corrections approximately 2" to 3". The average telescope aperture efficiency ηA and main beam efficiency ηmb were 0.59±0.05 and 0.78 ± 0.06 respectively, determined through observations of 3C286 at the start of each shift. The absolute flux accuracy is thus ∼ 8%. The average elevation of Ophiuchus for all observations was approximately 26◦.

For their observations, the system temperatures (Tsys) varied between 48 K and 92 K over the observation dates with an average Tsys ∼ 62 K.
Figure 2: (a) Spectra of all species observed at the GBT at the integrated intensity C2S (21–10) peak in Oph B1. The NH3 (1,1) and (2,2) and C2S baselines are offset from 0 for clarity. (b) Spectra of all species observed at the GBT at the integrated intensity C2S (21–10) peak in Oph C. The NH3 (1,1) and (2,2) and C2S baselines are offset for clarity. Figure is from Friesen et al 2009. gbtOphSpectra.png

Tables 1 & 2: Observing frequencies and mapped regions for the Ophiucus project. Tables are from Friesen et al 2009.
Note that the GBT KFPA will use a different observing mode, 200 MHz signal band observations, not four 50 MHz bands centered on different lines.

As noted above, the observations were carried out in 12 separate sessions (epochs). Below the observing sequence for a single epoch is reviewed, and the relationship between these scans and the pipeline is identified. Table 3 shows a scan summary of the observing session AGBT06A_065_05, performed on 2006 April 28 from 5:30 to 8:30 UTC.

Start Stop    Source      Proc.  N Scans   RA  (J2000) Dec  Ints.   BW.   Backend  Date+Time        Pipeline Note
   1     2      3C286      OnOff      2  13h31m07  30d31'04     6    50.0 ACS 2006_04_28_05:42:26   Discarded
   3     6      3C286       Peak      4  13h31m08  30d30'33   591  1280.0 DCR 2006_04_28_05:44:30   Discarded
   7     7  3C286 FocusSubreflec      1  13h31m08  30d30'33   299  1280.0 DCR 2006_04_28_05:46:56   Discarded
   8     9      3C286      OnOff      2  13h31m07  30d31'07     6    50.0 ACS 2006_04_28_05:50:41   Discarded
  10    11      3C286      OnOff      2  13h31m07  30d31'08     6    50.0 ACS 2006_04_28_05:53:07   Reference Scan
  12    15   1625-254       Peak      4  16h25m47 -25d27'38   592  1280.0 DCR 2006_04_28_05:57:55   Discarded
  16    16  1625-254 FocusSubreflec   1  16h25m47 -25d27'38   298  1280.0 DCR 2006_04_28_06:00:17   Discarded
  17    18   1625-254      OnOff      2  16h25m39 -25d28'09     6    50.0 ACS 2006_04_28_06:03:39   Off for PS Test
  19    37      OphB2  RALongMap     19  16h27m29 -24d25'06   475    50.0 ACS 2006_04_28_06:06:57   Imaged
  38    56      OphB2  RALongMap     19  16h27m29 -24d25'06   475    50.0 ACS 2006_04_28_06:37:53   Imaged
  57    60   1625-254       Peak      4  16h25m47 -25d27'38   590  1280.0 DCR 2006_04_28_07:08:12   Discarded
  61    61  1625-254 FocusSubreflec   1  16h25m47 -25d27'38   298  1280.0 DCR 2006_04_28_07:10:43   Discarded
  62    63   1625-254      OnOff      2  16h25m39 -25d27'41     6    50.0 ACS 2006_04_28_07:14:17   Off for PS Test
  64    82      OphB2  RALongMap     19  16h27m29 -24d25'06   475    50.0 ACS 2006_04_28_07:17:45   Imaged
  83   101      OphB2  RALongMap     19  16h27m29 -24d25'06   475    50.0 ACS 2006_04_28_07:49:28   Imaged
 102   105   1625-254       Peak      4  16h25m47 -25d27'38   591  1280.0 DCR 2006_04_28_08:19:49   Discarded
 106   106  1625-254 FocusSubreflec   1  16h25m47 -25d27'38   299  1280.0 DCR 2006_04_28_08:22:17   Discarded
 107   108   1625-254      OnOff      2  16h25m39 -25d27'11     6    50.0 ACS 2006_04_28_08:25:44   Off for PS test
 109   127      OphB2  RALongMap     19  16h27m29 -24d25'06   475    50.0 ACS 2006_04_28_08:29:10   Imaged
 128   128     OphAN6      Track      1  16h26m32 -24d24'52    30    50.0 ACS 2006_04_28_08:59:04   Discarded
Table 3: Review of an observing session (epoch) and the scans used for pipeline processing. Note that many scans are discarded in the Pipeline process.
The pipeline scans can be brought into GBTIDL using the following command:
sdfits -scans=10,11,17:56,62:101,107:127 -backends=acs /home/archive/science-data/tape-0017/AGBT06A_065_05

Review of Table 3 brings up a number of important points concerning the observing schedule and associated processing.

  • The pipeline will not process continuum pointing and focus observations, which are processed by ASTRID
  • The pipeline will discard unidentified scans in the observing session.
  • Un-identified scans will not halt pipeline processing.
  • The pipeline should be capable of processing and applying "SCAL" observations
    • Normally the pipeline calibration will use previously "blessed" SCAL observations, were the noise diode effective temperatures are based on previously observed and checked observations of a radio source of known brightness. The SCAL observations are described pipeline SCAL page.
    • If SCAL observations are scheduled by the observer, they will be used in the data reduction process.
  • The SCAL processing is not a part of the PIPELINE project, but the SCAL project should produce a modular set of calibration
procedures that can be added to later versions of the PIPEPLINE.
  • In the example above, spectral line observations of the point/focus source could be used to monitor the variation in the weather. This is not the pipeline standard. The pipeline method for monitoring weather variations is described in the pipeline weather page.
  • Spectrometer Frequency Switched (FS) observations are actually pairs of (unswitched) spectral line observations with different center frequencies.
    • If consistent Off source spectra are available (as in this case) the observations can also be reduced in Position Switched mode, for comparison.
These observations will also be reduced in PS mode during the pipeline validation, to perform a consistency check between the two methods.

Calibration Steps

This document describes the sequence of standard calibration steps to be applied to pipeline data. This document does not describe each individual step in detail. The calibration documentation is the responsibility of a separate GBT development project. The reference for GBTIDL calibration is online: However this document needs some updating and revision.

One general point of difference between the current GBTIDL calibration and the pipeline calibration is the use of vector values for the calibration noise diode as a function of frequency. Also the pipeline will use/compute vectors for atmospheric opacity and system temperature as a function of frequency.

Reference Calculations

Tsys Calculations

(Signal - Reference')/Reference''

Imaging Steps

Friesen et al. (2009) summarize the imaging process, below:

Initial data reduction and calibration were done using the GBTIDL package. Zenith opacity values for each night were obtained using a local weather model, and the measured main beam efficiency was used to convert the data to units of main beam temperature, TMB. The two parts of the in-band frequency switched data were aligned and averaged, weighted by the inverse square of their individual Tsys. The data were then converted to AIPS SDFITS format using the GBT local utility idlToSdfits (written by Glen Langston). In AIPS, the data were combined and gridded using the DBCON and SDGRD procedures. Finally, the data cubes were written to FITS files using FITTP.

We next discuss in detail the requirements for the pipeline calibration.

Figure 3: GBT images of Ophiucus C, showing NH3 (1-1), NH3 (2-2), C2S and HC5N emission. Image from Friesen et al 2009. gbtNh3C2sHc5n.png

-- GlenLangston - 2009-08-09
Topic attachments
I Attachment Action Size Date WhoSorted ascending Comment
friesenOphiucus.txttxt friesenOphiucus.txt manage 46 K 2009-08-09 - 14:31 GlenLangston Friesen 06A_065 observing summary
gbtOphSpectra.pngpng gbtOphSpectra.png manage 25 K 2009-08-09 - 14:00 GlenLangston Oph B spectra
gbtOphTables.pngpng gbtOphTables.png manage 28 K 2009-08-09 - 14:21 GlenLangston gbt Ophiucus Obs tables
gbtOphiucusJcmt.pngpng gbtOphiucusJcmt.png manage 196 K 2009-08-09 - 13:41 GlenLangston jcmt ophiucus map
Topic revision: r11 - 2016-06-08, PatrickMurphy
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