The ACSIS (Auto Correlation Spectral Imaging System) digital autocorrelation spectrometer is used as the backend for the spectral line receivers HARP, RxA and RxW. ACSIS can be configured with up to 2 (HARP) or 4 (RxA, RxW) spectral windows of 250, 440, 1000, or 1860 MHz, of which the 440 and 1860 spectral windows consist of 2 subbands each. The spectral resolution of ACSIS varies from 30 kHz to ~1 MHz, depending on the configuration used. Wider bandwidths than 1860 MHz are possible for receivers that support those.
To support fast mapping of large areas the data can be read out at a rate of 0.1s. This allows large areas to be scanned rapidly.
Available Bandwidth Modes
ACSIS has in total 32 down converter modules (DCMs) and correlator cards. A DCM “picks out” a 250 or 1000MHz wide subband from an IF input and feeds the signal to a correlator card. HARP only has 16 IF outputs while there are 32 DCMs and correlator cards. The “excess” of DCMs and correlator cards can be used in two ways.
- Chaining two correlator cards together doubling the number of channels in a 250 or 1000MHz subband. Only 16 DCMs are used in this case.
- Use two DCMs to extract two frequency subbands from each of the 16 IF inputs and feed each subband to a correlator card. This creates two spectral windows or spectral regions to follow the OT terminology. All 32 DCMS are used in this case.
- The OT support a special case of option two – by selecting a single spectral region with 440MHz or 1860MHz bandwidth the OT automatically set up two subbands that overlaps slightly. These bands can be merged later in the data reduction. New options are the 1600 and 1800MHz bandwidths, which have larger overlap between the 2 subbands (but the same spectral resolution) than the 1860MHz bandwidth. This is useful when the spectra contain very broad lines. Likewise there are 400 and 420MHz bandwidths available. Caution is advised when choosing to use this “wideband” mode due to baseline issues. Using wideband mode is only advised when absolutely necessary – as the Galactic Centre with its large velocity range, galaxies with significant (>600km/s) dispersion, or spectral line surveys.
- Note: the merging used to done by the ACSIS DR system – for further details see footnote*. When using 440 or 1860MHz bandwidth you might be better of not centering the line in the wide band – since that places the line in the overlap region.
An example of the second use is to observe 13CO 3-2 and C18O 3-2 using two spectral regions (subbands). A number of such setups are pre-prepared and can be selected as “spectral configurations” in the OT. They are listed at the bottom of this page.
An example of the third option is to select 1860MHz mode for a wide extragalactic source. It this case it is less error prone to let the OT do the setup than “manually” do the setup using the frequency editor. Further, it has the advantage that both spectral regions use the same rest frequency.
The spectral window and bandwidth possibilities for HARP are summarized in the table below.
||BW mode||Channel Spacing||Usable Bandwidth||Channels|
E.g. if two spectral windows are selected in the OT one with a bandwidth of 250MHz and the other with a bandwidth of 1000MHz, the result is one spectrum of usable bandwidth ~220MHz and channel separation 61.0kHz and one spectrum with usable bandwidth ~930MHz and channel separation 0.977MHz. Also not that if the 1860 or 440MHz option is selected only one spectral window is allowed – the 32 DCMs and correlator cards has already been used up! As can be seen from channel spacing in the table the 1860 and 440MHz mode are using 2 subbands. Conversely, if you select 2 subbands only the bandwidth options 250 and 1000MHz exists.
* The merging in the ACSIS DR occasionally created artificial features where the spectra were joined. The reason were to small overlaps and that the merge was weighted by the total power drop off. In principle Tsys weighting would be better but the estimated Tsys also gets very uncertain at the edge. Further, merging the spectra was an irreversible process. Thus, if there was a problem we could not go back and reprocess the data. Of these reasons we do now store the unmerged raw spectra and the merging is done in the post processing.
RxA & RxW
RxA or RxW are not able to use all the DCMs and correlator boards in ACSIS – see the HARP section. A maximum of 4 DCMs/correlators can be feed from the same IF in a usable way. Thus, you can select 1-4 spectral windows for each IF output from RxA or RxW with the BW mode summarized in the table below.
|Spectral windows||BW mode||Channel Spacing||Usable Bandwidth||Channels|
|3||A spectral window as in one of the four rows above|
|any other 250||0.061MHz||~220MHz||4096|
|any other 1000||0.977MHz||~930MHz||1024|
Note that it is possible to configure ACSIS with four 250MHz subbands with the centers separated by 220MHz to generate a ~880MHz wide spectrum. None of the current frontends has an IF bandwidth large enough to make it fruitful to implement the analog setup using 1000MHz subbands – the OT will not allow you anyway. Another possibility is to combined 3 250MHz subband to a ~660MHz spectral window and a separate 250/1000MHz spectral window. These possibilities are not explicitly supported by the OT or covered in table 2.
For the curious – with 3 spectral window and 4 DCMs/correlator cards the DCMs/correlator cards can only be distributed 1+1+2 over the three spectral windows. The spectral window with 2 DCMs/correlator cards can support higher resolution or higher bandwidth.
ACSIS Special Configurations
The table below describes the ACSIS special configurations which are available within the Het Setup component of the Observing Tool (OT). If the special configuration you require is not among the list below (or in the OT), you may create your own configuration using the Frequency Editor tool. However, we strongly urge you to send a request to email@example.com so that we can add the new configuration to the list.
Name Receiver Systems Mode Sideband Species transitions Bandwidths RxA_H2CO_250X3 A3 3 DSB LSB H2CO 3 0 3 - 2 0 2 250 H2CO 3 2 2 - 2 2 1 250 H2CO 3 2 1 - 2 2 0 250 RxA_13C18O_250X4 A3 4 DSB USB 13-CO 2 - 1 250 C-18-O 2 - 1 250 SO 5 6 - 4 5 250 SO 5 6 - 4 5 1000 RxA_SIOSO_250x2 A3 2 DSB LSB SiO 6 0 - 5 0 250 SO 4 3 - 3 4 250 RxA_SiO54_v12_250X2 A3 2 DSB LSB SiO 5 2 - 4 2 250 SiO 5 1 - 4 1 250 HARP_COH13CN_250 HARP 2 SSB LSB CO 3 - 2 250 H-13-CN 4 - 3 250 HARP_H2DN2H_250X2 HARP 2 SSB USB H2D+ 1 1 0 0 - 1 1 1 0 250 N2H+ 4 - 3 250 HARP_13C18O_250x2 HARP 2 SSB LSB C-18-O 3 - 2 250 13-CO 3 - 2 250 HARP_DCN_HNC_250x2 HARP 2 SSB best DCN 5 - 4 250 HNC 4 - 3 250 HARP_CO_H13CO_250x2 HARP 2 SSB best CO 3 - 2 250 H13CO 4 - 3 250 HARP_C170_C34S_250x2 HARP 2 SSB best C-17-O 3 - 2 250 C-34-S 7 - 6 250