ALFALFA L-band Wide Followup Observing Modes
Current as of Mar 9th, 2012
As discussed in the ALFALFA followup background page,
the spectrometer we use with LBW 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.
Spring 2012
In this observing
period, we proposed to conduct LBW followup observations of four sets of ALFALFA sources:
- high quality ALFALFA detections without optical counterparts and not associated with
known tidal debris fields: the "dark galaxy"candidates;
- ALFALFA sources without optical counterparts which are candidate OH
megamasers because of their coincidence with optical sources at the appropriate
redshift;
- low signal-to-noise, low velocity (nearby, cz < 1000 km/s), narrow line width ALFALFA sources
without optical counterparts: the extreme gas-dominated dwarf galaxy candidates;
and
- statistical samples of the low signal-to-noise ratio detections possibly
associated with optical galaxies.
There are lots of objects in categories (3) and (4) but fewer in (1) and (2).
Because the extreme gas-dominated dwarf galaxy candidates are all at very low velocity,
and because there are some in all the other categories likewise at low velocity
(including cz < 0 km/s), the density of low velocity sources on the sky is quite high
but the range of frequencies needed to be covered is fairly small. On the other hand,
there is a large spread in the frequencies of sources in the other categories.
Given all the considerations, we have decided to adopt for the Spring 2012 LBW followup
observations (program A2669), two different observing setups which depend on the
velocity (and hence the frequency) of the target HI source/candidate. Both setups will
produce spectra with 2048 spectral pixels ("channels") covering 25 MHz, but where we
center them and how we decide what frequency to center them on will be different, depending
on the expected velocity (doppler shift) of the target source/candidate.
- Low velocity mode (cz < 2000 km/s):
For sources/candidates which lie at velocities so that the observed frequency
should lie in the range 1410-1430 MHz (OHM candidates might be at apparent
HI blueshifts!), we will center a 25 MHz bandwidth at 1420 MHz, giving coverage
from 1407.5-1432.5 MHz. With this setting, the spectra will be sampled at
9-levels (good dynamic range in the presence of RFI) and 2048 channels (twice
the resolution of ALFALFA's 4096 channels over 100 MHz).
We will not doppler track, but rather conduct the observations
at fixed frequencies (topocentric frame, with the "velocity" set to 0 km/s).
With this setup, it may be possible to use more than on OFF for each ON source
(if we don't move the telescope much) and we will be able to look for low-level
RFI in the band.
Boards 1 and 2 of the correlator will record the spectra (25 MHz, centered at
1420.0 MHz) from the two polarizations
respectively. We will center Boards 3 and 4 at 1405 MHz, also covering
25 MHz, with Pol A on Board 3 and Pol B on Board 4. These spectral will let us look for
signals over the frequency range from 1392.5 to 1417.5 MHz. Remember:
we don't really need the spectra recorded by Boards 3 and 4, but just in case
there is a signal lurking in there, we'll see it!
What this setup means is that the candidate sources observing in this mode will shift
in the spectra, depending on the frequencies corresponding to their doppler shifts.
The configuration file called a2669_lowvel.conf will set up the interim correlator
(the "backend") for this configuration. All observations made with this configuration
will cover the exact same frequency range, since no doppler corrections are applied.
The corresponding source catalog file is a2669_lowv.cat.
- High velocity mode (cz > 2000 km/s):
For sources/candidates which lie at velocities greater than cz~2200 km/, the observed
frequency should lie below 1410 MHz, but because they are spread throughout the
ALFALFA bandpass down to ~1340 MHz, we'd have to have a bunch of different setups
to cover the whole range. That makes things too complicated, so we instead will
work in velocity space and center the spectra taken for each individual target
at the frequency range appropriate for its expected HI doppler shift. So that
sources don't always fall exactly at the band center (sometimes funny things can
happen to correlator channels at exactly the half-way mark), we'll round off the HI
velocity to the nearest hundred km/s.
As before, Boards 1 and 2 will 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.
In this mode, we normally don't really need Boards 3 and 4. In January, we set them to
be identical to Boards 1 and 2. In March however, we decided to use the second set
to produce a higher resolution spectrum, covering only 6.25 MHz with 2048 channels.
The IF/LO setting is identical to B1 and B2. Usually, we won't need Beams 3 and 4
but having higher resolution may be useful for a few of the narrowest sources. This
mode will be called "high velocity high resolution" mode, and the configuration
file will be a2669_highvelres.conf. The corresponding source catalog
file is a2669_highv.cat.
Note also that we will, on occasion, also observe sources at cz < 2000 km/s in
this mode, if, for example, we want the high resolution, or if we need to fill
a gap in the schedule.
This page maintained by members of the
ALFALFA Observing Team.
Last modified: Fri Mar 9 14:46:43 EST 2012 by martha