Instructions
on reducing LBW data using IDL_LBW
Rules of the scavenger hunts:
(1): In all cases,
return attendees shall not
reveal the answers to first timers until sufficient effort is
expended. (2): Any bribes must be shared 50:50 with the Scavenger Hunt creators.
(3): You may consult any
source anywhere but please be sure to indicate where you got your information.
And watch out
for bad websites! Actually, if you find any mis-information, make a note
of it for the blog.
UAT15.01 Scavenger Hunt #1:
Using Arecibo and LBW for ALFALFA followup science
This scavenger hunt will explore some details of the L-band wide observations we are conducting during
the workshop by examining some of the similar data obtained by the awesome UAT in November 2014. Be sure to familiarize
yourself with the various links above in order to figure out the answers to these questions.
1.0 During the 2015 UAT workshop, we are conducting observations under what Arecibo observing program? What was the
program designation in November 2014? 1.1 Position Switching with LBW The LBW followup observations using a technique referred to as "total power position switching", also known
as "ON-OFF" observing.
a. Why do we need an "OFF-source" observation?
b. How is the position of the OFF-source specified? What assumptions are involved?
c. Our LBW observing technique consists of five distinct steps, four of which
produce separate raw datasets (which then get bundled into one file for us). One of the five steps is a telescope
movement when no data are recorded. What are the other four? Write them in the proper order below.
Step 1
Step 2
Step 3
Step 4
Step 5
d. What is the slew speed of the telescope in azimuth?
e. What is the slew speed of the telescope in zenith angle?
f. Why is the unit of flux density called a Jansky?
g. What is the continuum flux density of the radio source 3C 286 at 21cm? Where did
you find the answer?
h. What is the "radiometer equation"?
1.2 Setting up the Spectrometer a. What do we mean by "heliocentric" velocity?
b. To what velocity does a frequency of 1400 MHz correspond for the 21 cm HI line?
c. The 2015 correlator setup we adopt produces eights separate spectra each second. What do these eight spectra
correspond to and in what ways are they the same/different?
d. How can you determine the separation, in frequency, between channels for a given spectrum?
How does that convert to a separation in velocity? What's the complication in the 2nd case?
e. With the correlator setup we are using in 2015, what is the
total velocity coverage (all spectra)?
f. What is the current velocity of the Earth around the Sun in the
direction the telescope is pointed right now? Be sure to record the exact time/date when you provide this answer.
(Hint: Observers: we're sure you've seen that somewhere.......)
1.3 Intro to LBW spectra: A quicklook at some targets observed in 2014
While we are observing, it is possible to take a quick look at the data at the end of every ON-OFF pair.
(Actually, in practice, you cannot do this easily until the subsequent scan is started.). The observing team does this
during the observing run so that they can enter notes in the log file. We can also look at data taken previously using the same command.
Here, let's look at some of the spectra we acquired in Nov 2014 during the test of our search mode to give you
a feeling for what real observations look like.
Refer to the Observing checklist for LBW observing
to find the commands associated with the routine lbwquicklook in the section titled
Check the spectrum produced for each ON-OFF pair.
a. In this Scavenger Hunt, the spectra we use
were taken in Nov 2014 under a different project name
(see you answer to question 1.0). At that time, WAPP Board 3 was dead so, for most of the run,
we swapped around the IF/LO settings
for Boards 3 and 4. How can you figure out what was changed and when it took place?
What does such a change of setup do to the display presented in this exercise
as compared to what we will be seeing at the telescope during this run? Getting started in IDL
Here at the UAT workshop, please follow these instructions exactly:
Log into your assigned machine given the account and password we provide you.
Get to your working area by typing teamx where x is the letter assigned to your team.
Start idl by typing idl
Compile Phil's WAPP data routines by typing @wasinit
Compile our LBW reduction routines by typing @lbwinit
Execute the command lbwquicklook with its various options, as described in the
observing checklist to answer
the questions below.
In each of the following, you are given a file containing the ON-OFF pair of
a particular galaxy.
b. Use the file /share/pserverf.sda3/wappdata/wapp.20141127.a2899.0001.fits
Which WAPP board is each spectrum associated with? Give you answer in order of upper left, upper right, lower left, lower right.
This is the one of the first targets observed on the first night of our test of the WAPP search mode in Nov 2014. What do you notice
about the setup that is different from our discussion under a. above?
What famous politician once said "However beautiful the strategy, you should occasionally look at the results."?
What do we mean by signal-to-noise ratio?
The lower right spectrum shows one feature. At what frequency is it? What is it?
The upper left spectrum shows two features. One of them appears to oscillate between positive and negative flux density. What
does it arise from and why does it show this oscillation?
Make an "eyeball guess-timate" of the center velocity and
the velocity width (at 50% of the peak) for the other feature in the upper left spectrum
c. Use the file /share/pserverf.sda3/wappdata/wapp.20141127.a2899.0021.fits
What is different about the arrangement of the boards this time compared to that in b. above?
The lower right spectrum shows a feature extending to negative flux densities. At what frequency is it? What is it? Why does appear to have
a negative flux density?
Board 4 contains a feature extending to negative flux densities. What is the flux density at its most negative value?
At what frequency is it? What is it? Why does appear to have
a negative flux density?
Suppose we were to observe a galaxy whose recessional velocity we know to be 6300 km/s. In this hypothetical
observation, we observe a negative flux density feature
at about 1391 MHz. The mean flux density value of the spectrum across the whole bandpass is +235 mJy. How would you interpret the
negative flux density feature in this case?
Make an "eyeball guess-timate" of the peak flux density, the center velocity and
the velocity width (at 50% of the peak) for the feature in the upper right spectrum.
What else can you tell us about this target?
What else would you want to know about it?
d. Hypothetically,
ALFALFA team observers take careful notes on each observing run, allowing us to find the data afterwards. You should be able to
find everything you need via the A2010 observer's page.
Find the observing log for November 25, 2014.
Who were the observers that night?
Review the entry for the source A011509.8+334725. What it its AGC number?
What is the filename of the associated spectra?
Use lbwquicklook to have a quicklook at the spectra. What features do you see?
Which arises from the galaxy?
1.4 Reducing an LBW detection
Now let's work through the procedure to fully reduce one of the spectra of a galaxy detected in HI
by the awesome UAT in November 2014 to the point where we
can extract the numbers we need to do science . For this part of the exercise, we
will use data taken with the interim correlator, so you need to review that setup. Look at the section on
LBW observing modes.
a. Assess your approach to the data reduction:
Study the documentation supplied as the
LBW Data Reduction Instructions.
As before, some commands may be slightly
different because of the special circumstances of the workshop (i.e. the account we are using, the data we are examining).
Be sure you work in the right place.
For starters, you need to exit IDL at this point and restart it using the routines that work on "corfiles" not "wappfiles".
Start IDL again, but use @corinit rather than @wasinit. The interim correlator
spectral data files are found in /share/olcor and
you can figure out what file to grab by looking in the observing log (as always).
Using lbwfromcorfile, load in the low resolution spectrum
of the galaxy AGC 122941 observed
on 14.11.25. What are the RA and Dec of the source
as recorded in the spectrum header?
b. Fitting a baseline and measuring the RMS
Because of the ON-OFF technique, there is often some residual difference across the bandpass that we want to
subtract off. This is called the spectral baseline, and we want to remove it.
Use lbwbaseline to fit a baseline to the spectrum of
AGC 122941. What criteria should you use in selecting the regions over which the "baseline" will
be calculated?
What order polynomial appears to be the best?
What is the rationale used to decide which order to use?
What is Hanning smoothing? What is "boxcar" smoothing?
What do we mean by the "RMS" (i.e. what is the measurement we are making, not just the number
given for this spectrum)? What is the value you measure for this spectrum?
c. Measuring the HI line parameters
Remind yourself what the shape of the profile looks like (single or double peaked) and choose the appropriate branch
of the data reduction scheme as you proceed with lbwmeasure.
Measure the profile following the instructions. What values do you get
for the HI line flux density, the systemic heliocentric velocity and the widths W50 and W20?
What is W50? W20?
d. Save your work and make a plot.
Once you are happy that you have done a good job on reducing the source, you want to save the information into the IDL structure
and then write the file to disk so that it can be read in again.
Follow the instructions to use lbwtoascii and save.
And make a plot!
e. Repeat the baselining, measuring and saving steps on the other spectrum (high res or low res;
i.e., the one you did not select in above). How do the measurement results differ? Which do you believe is more robust? Did you make
the right decision before? 1.5 Dealing with non-detections
Not all of the sources we look at in the LBW observations will be confirmed. When the source is not confirmed, we want
to recordthe RMS noise in the basedlined spectrum so that we can derive an upper limit on the HI line flux density.
We can do that following the steps above.
a. Use the file /share/olcor/corfile.25nov14.a2899.24
to baseline and then measure the RMS of the high resolution spectrum. Save you result.
b. Suppose we want to know the upper limit of the HI mass of this source.
How would you go about making an estimate of that upper limit? What assumptions do you need to make? 1.6 Mysteries, discoveries and bad data
If everything were predictable, science would be boring! Explore the selection of spectra below using the tools you have at
hand to interpret what you see. In each case, we give you the filename containing the target to explore. Since these are
data taken with a project back in 2012, the files are in a different location not /share/oldcor/. Sometimes the
/proj directory is not mounted on a machine; you might have to hunt around for one that has it mounted. a. Use the file /proj/a2669/corfile.26mar12.a2669.11 What is going on here?
b. Use the file /proj/a2669/corfile.23mar12.a2669.7 What is going on here?
c. Use the file /proj/a2669/corfile.23mar12.a2669.5 What is going on here?
1.7 Movie and Arecibo trivia (Some of us think this is important too!) a. In what 1984 movie did the unsympathetic government agent say: " Do you seriously expect me to tell the
President than an alien has landed, assumed the identity of a dead housepainter from Madison Wisconsin and
is presently out tooling around the countryside in a hopped up orange and black 1977 Mustang?"
b. In a famous movie, where was Roger Thornhill when he said " I don't like the way Teddy Roosevelt is looking at me"?
c. What is odd about the rotation of Venus, and how and when was that determined?
d. In the movie "Some Like It Hot", who (what actor) utters the famous
closing words "Well, nobody's perfect."?
e. What is the temperature at which paper burns, according to
the famous futuristic 1967 film starring Julie Christie and Oskar Werner?
This page created by and for the members of the
ALFALFA Survey Undergraduate team
Last modified: Wed Jan 7 18:35:10 EST 2015 by martha
