Common ALFALFA L-band Wide Followup Spectrometer Setups

Current as of Jan 5, 2015


Modes using the WAPPs in search mode

Beginning in January 2015, we are looking at galaxies whose redshifts are unknown (actually in spring 2015, some of the objects we are observing have known redshifts, but we are using them to test the criteria by which we can predict that a galaxy will have HI and lie in the distance range from 30-100 Mpc). Since we don't know the redshift but hope to detect the HI in the galaxy, we want to (1) look over a broad range of redshift but at the same time (2) maintain relatively high spectral resolution, sensitivity and dynamic range, and (3) avoid the FAA radar at 1350 MHz. Our compromise solution is to use the WAPP spectrometer which is more powerful in terms of the number of spectra it can collect at 09-level sampling than the interim correlator which we have used before (and is discussed below). Some of us remember that we used the WAPPs for the ALFALFA drift scan survey.

In this setup, we will be recording 8 spectra simultanously. Pairs of spectra will cover both polarizations collected by the LBW-receiver, set over 4 staggered frequency ranges (bandpasses) of 25 MHz, each of which is sent to a separate WAPP "board". To accomplish the overlap of frequency bands, we set the "rest frequencies" of the four bandpasses to 1415.4058, 1395.4058, 1375.4018 and 1350.4058 MHz, corresponding to WAPP boards 1 to 4, respectively.

The true rest frequency of the HI line is, of course, 1420.4058 MHz.

Note that the frequency decreases from Board 1 to Board 4.


Modes using the Interim Correlator

  • Look here for the discussion of the setup used in Spring 2012

    As discussed in the ALFALFA followup background page, the interim correlator we use with LBW when we know the redshift of the target has more capability than we actually need for the followup observations. So, we are going to try to make best use of it! We also don't want to have too many different setups so that we don't waste time making too many changes, we can use the same calibrations, and we can monitor the system performance and the behavior of RFI.

    In our first LBW observing runs, we used two distinct setups; you can read about them here. Starting in March 2012, we adopted a single setup for all LBW followup runs in 2012-2014, regardless of the scientific aim of the program or the kind of sources we target. We call this spectrometer setup the "hybrid/dual bandwidth, doppler tracking" mode.


    Hybrid/dual bandwidth, doppler tracking mode:

    This correlator setup yields four separate spectra, in two pairs:
    • Two spectra, one for each of the two polarizations, of 2048 spectral channels covering a 25 MHz bandwidth, centered at the frequency corresponding to the heliocentric velocity specified in the observing catalog.
    • Two spectra, one for each of the two polarizations, of 2048 spectral channels covering a 6.25 MHz bandwidth, centered at the frequency corresponding to the heliocentric velocity specified in the observing catalog.

    The first set of spectra are referred to as the "low resolution" spectra; the second are the "high resolution" ones.

    In terms of the correlator setup, we use 4 "boards", one for each of the four spectra.

    Boards 1 and 2 record spectra at 9 level sampling from the two polarizations separately. Each spectrum will cover 25 MHz with 2048 spectral pixels. Spectra will be centered at the frequency corresponding to the HI doppler shift, rounded to the nearest 100 km/s; this shifting will be done by reading the velocity from the input target catalog file.

    Boards 3 and 4 are also sampled at 9 levels and centered at the same velocity but produce the higher resolution spectra by covering only 6.25 MHz also with 2048 channels.

    We use the high resolution spectrum when the source emission is very narrow in velocity and gaussian; then the higher resolution can help us measure the width more accurately, if the signal-to-noise ratio is adequate.




    This page maintained by members of the ALFALFA Observing Team.
    Last modified: Tue Jan 6 15:25:13 EST 2015 by martha