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Observing Modes of the PACS Instrument


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Observing Modes and AOTs

  • Dual-Band Photometry
    • Single-Band Photometry
      • Line Spectroscopy Mode
        • Range Spectroscopy Mode
          • Parallel Modes
            • Calibration Measurements/Observations

Dual-Band Photometry
In this mode, both bolometer detector arrays are operating simultaneously and observing toward the same position. The cryocooler needs to be in its recycled state. The longwavelength array is imaging a field of view of 3.5' 1.75' in the 130-210 microns band, while the short-wavelength array is imaging essentially the same field of view in either the 60-85 microns or the 85-130 microns band. Both, the short wavelength array (32 x 64 pixels) and the long wavelength array (16 x 32 pixels), will fully sample the diffraction disks.
This mode will be the standard photometry mode for observations with PACS as the prime instrument and could scientifically be very profitable for parallel mode observations with SPIRE, provided data rate and thermal/power budget will allow this.
The main instrument-specific parameters an observer has to define in this mode are the wavelength band to be used in the short-wavelength section, integration time (per pointing position) and the chopper throw. Having a choice of chopper throw allows the observer to use on-array chopping for compact sources, thereby gaining a factor of 2 in effective integration time, or off-array chopping up to the maximum amplitude for extended sources. If the bandwidth of the bolometer detectors do not reach the goal, line scanning observations may require a "freeze frame" mode. In this case, the chopper will have to compensate the continuous slew of the satellite for ~1/4 (TBC) of a second with an accuracy of 1/2 (goal:1/4) of a blue photometer pixel, in order to prevent image blurring along the slew direction, and then jump by a TBD integer multiple of a blue photometer pixel. The resulting wave form is a saw tooth modulation.
Single-Band Photometry
In this mode, only one bolometer array is operating. The data rate in this mode is only a fraction (20% for the red and 80% for the blue channel) of the dual-band mode. This could be a fall-back mode for parallel observations with SPIRE in case the data rate budget would not allow dual-band operation.
Parameters needed to define an observation are, as in dual-band observing mode, integration time and chopper throw plus the choice of the wavelength band and eventually the "freeze frame" mode for line scanning.
Line Spectroscopy Mode
This mode is called such because the instrument instantaneously produces for each of the 5 5 pixels a (short) spectrum covering ~1500 km/s with a resolving power of ~1700. This spectral coverage will be ample to detect lines even in extragalactic sources with sufficient baseline if the radial velocity of the source and the rest wavelength of the line are known. Depending on the requested wavelength/grating order, only one of the two detector arrays is used at a time. Valid data will be recorded also on the other array, its wavelength however cannot be selected by the observer since it depends on the chosen wavelength for the target line. Due to the required higher sampling rate (256 Hz) compared to the photometry modes, the raw data rate is larger and will be ~3.6 Mbit/s. In principle, the instrument could be set up for detecting a specific line by comicronsanding the corresponding grating tip angle and selecting the proper array. For improved flat-fielding, particularly for long integrations, the grating will be scanned by a number of discrete steps around a specified centre position such that drifts in detector responsivity between individual pixels are eliminated. The scanning will be synchronised with the chopper which is foreseen to be used for the majority of observations. For stronger and very extended sources, scanning of the grating can be used alone to subtract the telescope background (frequency-switching). This option will allow unchopped mapping of larger areas with improved efficiency. The spectral coverage can be arbitrarily increased by increasing the step size and/or the number of steps of the grating scan, at the expense of correspondingly increased integration time for a given target sensitivity. However, scans must not exceed the wavelength range covered in one diffraction order of the grating, i.e. they have to stay within the ranges 57-72 microns, 72-105 microns or 105-210 microns, respectively. Instrument parameters needed from the observer are the centre wavelength(s), the scan width (the default being 0), the chopper throw, the integration time and whether frequency switching is required.
Range Spectroscopy Mode
Instead of observing individual spectral lines only, this mode will provide the possibility to observe entire wavelength ranges for each of the 5 5 pixels. These ranges will be specified by a start wavelength and an end wavelength, the grating will be comicronsanded then to step through the respective angles, synchronised with the chopper. Since the objective of this mode is to cover larger spectral ranges, both arrays will be used at a time, although in many cases the prime scientific interest might be focussed only on one range. Full spectral scans covering 57-210microns will be possible in this mode. However, like for the Line Spectroscopy mode, individual scans must not exceed the wavelength range covered in one diffraction order of the grating, i.e. they have to stay within the ranges given in section 6.3. The specified wavelength ranges will be scanned by default at full spectral resolution, however it will be possible to quickly obtain low resolution spectra by stepping the grating at angles corresponding to the size of the instantaneous spectral coverage of one spatial pixel (the 116 detector pixel column per spatial pixel covers ~1500 km/s corresponding to a spectral resolution of ~200 ). Instrument parameters needed from the observer are start- and end-wavelengths, resolution mode, the chopper throw and the integration times for each range.
Parallel Modes
As parallel modes, both dual-band photometry and single-band photometry modes are foreseen to be used. At least the latter one, with SPIRE in parallel, is considered a prime observing mode vital for the HERSCHEL mission. Science and housekeeping data are produced at a rate compatible with the CDMU, as defined in the RD-3. The total telemetry bandwidth will be shared among the two active instruments to a still TBD amount.
Calibration Measurements/Observations
For calibration measurements and observations, PACS will be operated in various instrumental settings not occurring in any other mode. Development and optimisation of AOTs, exploration of optimum detector settings (ex.: heater and bias levels) and tests of satellite pointing modes in combination with PACS observations are just some major instances where the instrument will be run in these modes. These modes will potentially require dedicated instrument data configurations. Data rates and on-board compression/reduction strategies will be highly individual per mode. Throughout all mission phases, the definition and actual use of these instrument modes will be restricted to the ICC only.

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Instrument Operating Modes


Credits

The contents of this page are based on the PACS observer's Manual produced by Bruno Altieri, Roland Vavrek and the PACS Instrument Control Centre and also on the Herschel Pointing modes document.

For more information, see Observer Manual

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This file was last modified on 21 May 2007.