SHFI receivers

Preliminary specifications

The new Swedish Heterodyne Facility Instrument (SHFI) receiver unit is planned to be in operation at APEX during 2008. The first telescope tests are planned for February/March 2008. Hence, the specifications listed below should be considered as preliminary. Only one of the four receiver bands can used at a given time. It will consist of the four bands:

• Band 1
  It is an SIS heterodyne receiver operating in the band 211-270 GHz. It will operate in single sideband (SSB) mode using sideband separation mixers. The SSB rejection is > 10 dB over the entire band. The SSB receiver temperature, T_SSB, is around 80 K over most (80%) of the band. Close to the upper band limit, T_SSB is expected to increase by 50%. The IF range is 4-8 GHz.
• Band 2
  It is of similar type as Band 1 but it is operating in the band 275-370 GHz, hence replacing APEX-2A. The SSB rejection is > 10 dB and T_SSB is around 135 K (increasing by 50% at the upper band edge). The IF range is 4-8 GHz.
• Band 3
  The frequency operating range of this SIS heterodyne receiver is 375-500 GHz. In its final state it will also be operated in SSB mode with an SSB rejection > 10 dB. The typical T_SSB will be 180 K over 80% of band. A 50% increase in receiver temperature is expected toward the tuning limits. The IF range is 4-8 GHz.
• Band T2
  This heterodyne hot electron bolometer (HEB) receiver has a tuning range of 1250-1384 GHz. Its operating mode will be double sideband (DSB) operation with a separation of 6 GHz between the upper and lower sidebands. The IF range is 2-4 GHz but it will be upconverted to 5-7 GHz before injected into the normal IF system. The expected T_DSB is 1200 K over most of the band.

The specifications listed above may change depending on the outcome of the on-sky commissioning which is planned to start in February and March 2008. For Band T2 (and also for Band 3 to some extent) the final on-sky tests may be delayed further since the commissioning and science verification will require excellent weather conditions. Also, it is currently (January 2008) not clear if Band 3 will be delivered according to specifications (eg. full SSB mode).

To obtain an observing time estimate for all bands use this calculator. More information concerning observing time calculations can be found here.

Observing modes and patterns

Switch modes

• Position switching
  In position switching mode the telescope moves between the ON-position and a user-defined reference position. Since APEX can move fast, reasonably stable baselines can be achieved under normal weather conditions. The reference position can be given in relative coordinates (to the catalog source position) or in absolute coordinates, in the horizontal or equatorial system. Integration times at ON- and OFF-positions can be different, and a number of different ON-positions can be observed using one OFF-position.
• Beam switching
  By using the wobbling secondary it possible to switch rapidly between positions up to 5 arcmin apart on sky without moving the antenna.
• Future switch modes (not yet implemented)
  
  • Frequency switching

Observing patterns

• On-Off
  Observations of a single position in any of the available switch modes.
• Raster map
  In raster mapping the telescope moves through a grid defined by the user, and any of the available switch modes can be used. The extent of the raster map is specified and spacings between raster points are also specified. The coordinate system can be either equatorial (RA and Dec) or horizontal (Az and El). The number of observed points per reference point can be specified (with automatic adjusting of integration times in the reference point). Calibrations can be performed during a raster map.
• On-The-Fly mapping (OTF)
  The On-The-Fly mapping procedure allows to scan continuously along one direction in a rectangular map while writing out data after given time intervals, thus saving the overhead between subscans that occurs for rasters. Otherwise the parameters are identical to that of the raster. The scanning pattern used is normally 'zigzag', in which the first row (scanning axis) is scanned by continously increasing the x-coordinate (normally RA). The second row is scanned in the opposite direction, and so on. The readout spacings along the scanning axis should not exceed about one third of the beam to avoid beam smearing. Due to technical reasons currently an OTF integration time of exactly one second is recommended. Please note that the APEX 2a observing time calculator is not yet prepared for OTF observing time estimates.