Your observations will be more efficient and you will achieve better results if you have thoroughly prepared for your observing run before arriving at the telescope. Most of this work should be done at the time the proposal is written (see Chapter 1). For both spectral line and continuum observations, you should prepare the following before coming to the telescope:
Source List: A source list with epoch B1950 or J2000 RA and Dec or (lII, bII) Galactic coordinates. For spectral line observations you will also need the source velocities in the LSR, optical, or relativistic velocity reference frames (see Chapter 4). Keep in mind that the beam sizes for the 12m can be quite small (~20″) at the higher frequencies so the positions should be appropriately accurate. You can save time by composing your catalog prior to your observing run. See Chapter 4 for the source catalog format.
Line Rest Frequencies: You should have line rest frequencies which are accurate to at least 10 kHz. If emission lines are weak, test line frequencies should be included. For continuum measurements, choose your observing frequencies so that no strong spectral lines lie in either of the receiver sidebands.
Observing Mode: The available observing modes for spectral line observations are
while the available observing modes for continuum observations are
Reference Positions: The reference offset position, in angle or frequency, should also be considered carefully. If you are using beam switching, you should consider carefully the optimum beam separation. The default beam throws are ±2′ at 3 and 2mm wavelengths.
Spectrometer Configuration: For spectral line observations you need to determine how you will configure the filter banks and Millimeter Autocorrelator (MAC), including the resolution and the mode of operation. This decision hinges on the resolution and total bandwidth required. No firm rules exist, but the minimum resolution acceptable should probably give 3 - 5 channels across the line and the minimum bandwidth should have 10-20% of the band on each side of the line.
A general startup checklist for both spectral line and continuum observations is given below. Although the Observatory staff tries to provide a fully functional system and advice about calibration constants and procedures, the responsibility for the integrity of the data rests with the observer. This checklist will help insure that the system is configured properly and that variable quantities such as pointing and focus are properly set. Completion of this checklist may take an hour or more, but the time will be well-spent.
NOTE: Pointing and focus may change as the temperature of the dish (or parts of the dish) changes. Pointing and focus should be checked (at least) after nightfall and daybreak and more frequently if the dish is illuminated by the sun.
For spectral line observations, you should perform the following checks to insure that the receiver is tuned correctly, the spectrometer is properly configured, and the calibration scale is correct.
(a) If the program line is weak and no other strong lines are in the bandpass, tune first to a strong test line that is as close by in frequency as possible. “Strong” means any line that will produce a good signal-to-noise spectrum in a 5 - 10 minute integration, for example. Standard sources are listed in the ARO standard catalog (called “standard.cat”). If possible, use the same observing setup (same spectrometer mode and observing mode) as will be used for the program observations. If the observations are of a common species, such as CO, there is no need to tune to another line.
(b) Perform a calibration scan and check for bad channels in the filter banks. Report the bad channels to the operator, who will flag those channels in the control system software.
(c) Observe a test line in a strong source. Observers may wish to verify the sense of the velocity/frequency scale by shifting the rest frequency or center velocity by a small amount and seeing if the line moves in the correct direction for the sideband choice.
(d) Check that the line temperature calibration is correct. This can be done by observing a standard source, presuming that the test line has known strength. Measurements of many of the sources in the ARO standard catalog “standard.cat” have been made in the CO, 13CO, and C18O J=1->0 and J = 2->1 transitions. Plots of these spectra can be found at the telescope and on the ARO Home Page here.
Two other manuals, one for spectral line and one for continuum, describe the data reduction systems in use at the 12m. These are available at the telescope and upon request. The discussion below is intended only as a quick reference list to help the observer get started.
Raw data is accumulated into two data files; one which contains filter bank data and a second which contains Millimeter Autocorrelator (MAC) data. Both of these files are in the “sdd” or Single Dish Data format. Each of these files has an associated gains, or “gsdd” file which contains results for spectral line calibration scans. In the following we give a brief introduction to the analysis of the spectral line and continuum data accumulated into these data files.
Table 2.1: Subscan Codes For 12m Data
Before observations begin, the operator will set up a subdirectory containing data files. This subdirectory is private to each observing team, and is denoted by /home/obs/ini, where ini are the 3 letter initials of the lead observer. The data files in this subdirectory are also labeled with the same initials. Log into the system as username obs (the operator can tell you the current password for this account). You will then be prompted for your initials, discussed above. After the login process is complete, you will be in your /home/obs/ini subdirectory.
The spectral line system always records data from two 256 channel filter banks and up to 65536 channels from the Millimeter Autocorrelator (MAC). Each scan is composed of a number of subscans/indexdata!subscan which individually contain one filter bank or Millimeter Autocorrelator (MAC) measurement. Table 2.1 lists the subscan codes with their associated polarizations and backends.
To start the continuum analysis program from the Unix prompt, type
while to start the spectral line analysis program at the Unix prompt, type
Table 2.2 lists a number of condar and line commands and their function, while Table 2.3 lists a number of basic data analysis procedures.
Another data analysis packages available at the 12m, which can be used to analyze 12m data, is CLASS.
Conversion of sdd-format data to CLASS format is currently done on-line. All scans except continuum and OTF measurements are automatically converted to CLASS format and put in a file called class.12m in the observer's directory. One can also convert other sdd-format data files to CLASS format using a utility program called uni2class. To convert all data in the sdd file sdd.jgm_001, issue the following command from any Unix prompt on any of the mountain workstations:
uni2class /home/data/sys/sdd.sys_001 sdd.sys_001.class
for the filter bank data and
uni2class /home/data/sys/sdd_hc.sys_001 sdd_hc.sys_001.class
for the Millimeter Autocorrelator (MAC) data. The second parameter in uni2class is the output file name. The current features and limitations of this conversion are:
Table 2.2: Analysis Commands in UniPOPS
Table 2.3: Analysis Procedures in line
When you have finished your observations, an archive CDROM or DVD of the observers' files will be made. If you submit a Data Request Form, the staff will create a CDROM or DVD of your data and mail it to your home institution. These disks are in the ISO-9660 format, readable by any Linux, Solaris or Windows machines.
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