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With the wideband receivers it is possible to observe several lines simultaneously. Indeed extra lines will be there anyway in many sources so it's advantageous to tune them deliberately. At some frequencies, atmospheric absorption lines may complicate the tuning. For successful observations, some care is required for proper tuning and spectrometer setup and some additional steps are necessary during data reduction. To distinguish between lines in the two sidebands, it is worthwhile to take a few scans with frequency offsets.

Choose a primary line. Usually this is your principal interest but it can be any convenient line. Then use the LO command to locate this primary line in the IF bandpass. Take care to avoid atmospheric features or other lines in the image sideband (consult the line catalogs) or known artifacts in the spectrometer system.

UIP> VERIFY Name /LINE
UIP> LO Name /SIDE Sideband /IF Frequency [ /RECEIVER Rx_name ]

Name is the catalog name of your primary line. Frequency, in GHz, is the IF location of the primary line. Normally this location should be within the receiver IF passband (and a warning will issued if it is not). In special cases, such as line surveys, it may be useful to locate the primary line outside the receiver passband. The RECEIVER option is often not necessary.

Ignoring Doppler corrections, the LO frequency will be Name ± Frequency, depending on the sideband.

The IF processor for FFTS2 has known artifacts at 5, 7, 9, and 11 GHz. Avoid them.

The wide band spectrometers, AOS5 (4 GHz) and FFTS2 (8 GHz), usually need no adjustment.

The high resolution but narrow band FFTS1N/FFST1W (500 MHz/1 GHz) must be positioned within the IF bandpass to cover the line(s) of interest. Use the SPECTROMETER command with an IF Offset.

UIP> SPECTROMETER /FFTS1W Offset

Offset, in GHz, is the IF offset. Normally Offset = Frequency , the IF location of the primary line.

This page generates a receiver tuning diagram.

The underlying macros for the GILDAS ASTRO program are installed on the CSO computers.

1. cso_lo.astro defines the receiver tuning:
   ASTRO> @cso_lo name frequency sideband if_offset rx_bw
2. cso_ffts1.astro defines the FFTS1 configuration:
   ASTRO> @cso_ffts1 mode if_offset
3. cso_plot.astro plots the result.
   ASTRO> @cso_plot
   • cso_lines.astro supplements the standard line catalog. Used by cso_plot.astro

• The standard GILDAS line catalog does not include lines in the 460 GHz or 850 GHz bands.
• Ignore the E-ATM_2009_PATH error.
• These macros require the dec12 or later version of GILDAS.

Observe ¹²CO(2-1), ¹³CO(2-1), and C¹⁸O(2-1) simultaneously with the sidecab 230 GHz receiver and FFTS1. Position FFTS1W (1 GHz mode) to cover all three lines. ¹²CO(2-1) will appear in the center of FFTS1W with ¹³CO(2-1) and C¹⁸O(2-1) equidistant on either side. FFTS2 will cover the entire 4–8 GHz IF band.

To tune the receiver and configure the spectrometer:

UIP> VERIFY 12CO2-1 /LINE
Found in “/opt/uip/cat/default_catalog.line_cat”: Name : 12CO2-1
Frequency : 230.537970 GHz
Sideband : lower (default)
UIP> LO 12CO2-1 /SIDE U /IF 5.28
UIP> SPECTROMETER /FFTS2 /FFTS1W 5.28

The nominal LO frequency is 225.258 GHz. FFTS1 is positioned to cover 4.78 - 5.78 GHz in the IF. The magic IF offset, 5.28 GHz, is chosen so the three lines are equally spaced. [For distant sources, tune the receiver to 230.537970/(1+z) GHz and replace 5.28 GHz with 5.28/(1+z) GHz to keep the lines equally spaced. This works for z ≤ 0.17.]

To make this diagram:

ASTRO> observatory cso
ASTRO> time
ASTRO> let water 2 ! mm
ASTRO> @cso_lo 12co2-1 230.538 u 5.28 4
ASTRO> @cso_ffts1 w 5.28
ASTRO> @cso_plot

Observe ¹³CO(3-2) in USB and ¹³CO(3-2) in LSB simultaneously with Barney and FFTS1. Position FFTS1W (1 GHz mode) to cover both lines. AOS5 and FFTS2 will both cover 4–8 GHz in the IF.

To tune the receiver and configure the spectrometer:

UIP> VERIFY 12CO3-2 /LINE
Found in “/opt/uip/cat/default_catalog.line_cat”: Name : 12CO3-2
Frequency : 345.795990 GHz
Sideband : upper (default)
UIP> LO 12CO3-2 /RECEIVER RX345X /SIDE U /IF 7.5 /LOCK SMA
UIP> SPECTROMETER /AOS5 /FFTS2 /FFTS1W 7.5

The nominal LO frequency is 338.296 GHz. FFTS1W is positioned to cover 7.0 - 8.0 GHz in the IF. In practice, there is some leeway in choosing the IF offset. The example IF offset of 7.5 GHz spaces the lines by 208 MHz to avoid overlap. IF offsets from about 7.3 GHz to 7.9 GHz will work but avoid 7.6 GHz because the lines will overlap there.

To make this diagram:

ASTRO> observatory cso
ASTRO> time
ASTRO> let water 1.5 ! mm
ASTRO> @cso_lo 12co3-2 345.7960 u 7.5 4
ASTRO> @cso_ffts1 w 7.5
ASTRO> @cso_plot

Observe ¹²CO(4-3) with the sidecab 460 GHz receiver. In this case, atmospheric absorption in the image sideband will degrade the system temperature. The best option is to put ¹²CO(4-3) in the LSB with an IF offset of 4.33 GHz, centered between two atmospheric lines in the USB.

UIP> VERIFY 12CO4-3 /LINE
UIP> LO 12CO4-3 /SIDE L /IF 4.33
UIP> SPECTROMETER /FFTS2 /FFTS1W 4.5

Observe CI(1-0) with the sidecab 460 GHz receiver. As with ¹²CO(4-3), atmospheric absorption in the image sideband will degrade the system temperature. The best option is to put CI1-0 in USB with an IF offset of 6.2 GHz, where the atmosphere transmission in the LSB has a local maximum.

UIP> VERIFY CI1-0 /LINE
UIP> LO CI1-0 /SIDE U /IF 6.2
UIP> SPECTROMETER /FFTS2 /FFTS1W 6.2

In the data, the frequency and velocity scales will only be correct for the primary line. Analysis of that line may proceed as usual. For other lines, three operations may be necessary:

1. Adjust the reference frequency and velocity offset,
2. Interchange the sidebands, and
3. Shift the velocity scale.

Although these operations are possible by manipulating the header information in CLASS, they are somewhat tricky. These three procedures may help. They are written for Example 1 above, simultaneous observations of three CO isotopomers. Please study them and, if necessary, modify them to suit your needs.

1. fix_reference.class
2. swap_sidebands.class
3. shift_reference.class

For an illustrated data analysis recipe, see Heterodyne Data Analysis, example.

• Simultaneous Observation of ¹²CO(2-1), ¹³CO(2-1), and C¹⁸O(2-1), Hiroshige Yoshida 2011-06-22
• more instructions and scripts (local access)