Instructions
on reducing LBW data using IDL_LBW
Rules of the scavenger hunts:
1. Some questions are labeled (F: first timer) and (R: returnee).
First time attendees should focus on the "F" questions; returnees on the "R" ones.
Everyone should answer the questions without designations.
In all cases,
return attendees shall not
reveal the answers to first timers until sufficient effort is
expended. (And, any bribes must be shared 50:50 will the Scavenger Hunt creators.)
2. 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.
UAT14.01 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 2013. Be sure to familiarize
yourself with the various links above in order to figure out the answers to these questions.
1.0 During the 2014 UAT workshop, we are conducting observations under what Arecibo observing program? What was the
program designation in November 2013? 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 steps. What are they? (Be sure to note when the
telescope moves and when the calibration information is obtained; that will give you five distinct steps).
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. What is the unit of flux density called a Jansky?
g. (R) What is the continuum flux density of the radio source M 87?
h. What is the "radiometer equation"?
1.2 Setting up the Spectrometer
The ALFALFA followup LBW observing setup uses the "interim correlator" as the spectrometer. For the LBW observations,
the center frequency of each spectrum is centered at a particular frequency, generally close to the recessional velocity
of the expected HI signal. We refer to a "pixel" in frequency as a correlator "channel".
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 correlator setup we adopt produces four separate spectra each second. What do these four spectra
correspond to and in what ways are they the same/different?
d. How can you determine the separation between channels?
e. With our correlator setup, what is the velocity coverage of a low resolution
spectrum centered on 1000 km/s? Of a high resolution spectrum?
f. (R) 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.......)
g. What famous politician once said "However beautiful the strategy, you should
occasionally look at the results."? 1.3 Intro to LBW spectra: A quicklook at some favorite targets observed in 2013
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 2013 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 checklbw in the section titled
Check the spectrum produced for each ON-OFF pair. Note that in this Scavenger Hunt,
the filenames we use will be slightly different
from those given in the checklist instructions, because here we have chosen specific spectra
for you to look at, including some older spectra which are no longer found in the /share/olcor directory
(after a project gets finished, its data gets moved to a different location).
Not all machines have /proj mounted! We will need to check beforehand!
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 correlator data routines by typing @corinit
Compile our LBW reduction routines by typing @lbwinit
Execute the command checklbw 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.
a. Use the file /proj/a2752/corfile.23mar13.a2752.35
There are two features in this spectrum. What's the one at lower frequency?
The HI emission from the galaxy shows a classic two-horned profile with a sharp drop
in flux at the systemic velocity of the galaxy. What causes the drop in the center?
(Be sure you understand what two physical properties of the galaxy
determine the appearance of the HI emission profile.)
What do you notice about the location of the emission in the high resolution spectrum?
What do we mean by signal-to-noise ratio?
Make an "eyeball guess-timate" of the center velocity and
the velocity width (at 50% of the peak) for this galaxy.
b. Use the file /proj/a2752/corfile.20mar13.a2752.1
There are two features in the spectrum also. What is the one at 1420 MHz?
Why does the flux density of the feature at 1420 MHz switch from positive to negative
values as the frequency increases?
In this case, is it better to analyze the low resolution or the high resolution spectrum?
What does smoothing do to the signal-to-noise ratio? To the velocity resolution?
Make an "eyeball guess-timate" of the peak flux density, the center velocity and
the velocity width (at 50% of the peak) for this galaxy.
This galaxy does not show a two-horned profile. Why is that not surprising?
1.4 Reducing the low resolution LBW data
Now let's fully reduce some of the spectra taken by the awesome UAT in November 2013 to the point where we
can extract the numbers we need to do science . Be sure to refer to the general
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.
We presume you have already started up IDL in the right place. See the preceding step for instructions.
Find the data file you want to reduce:
ALFALFA team observers take careful notes on each observing run, allowing us to find the data afterwards. You can
find everything you need via the A2010 observer's page.
a. Find the observing log for November 30, 2013. Who were the observers that night?
b. Use the log file to figure out the name of the first file recorded during the
observations of that night. Assess your approach to the data reduction:
Read through the detailed instructions on how
to reduce LBW data. Follow them to answer the following questions about the spectral data file you just identified (i.e., the first observation made during the 13.11.30 observing session). You will also use these instructions more generally later to look at some "special" sources. Remember that
some sources are detected and some are not, so you need to use various different routines that we have developed for our LBW data.
c. Load in the file using readlbw. What is the source name, RA and Dec?
d. What do you notice about the vertical scale? How can you explain the offset from zero?
e. Does the profile look gaussian/single peaked or is it a more classic "double horned" profile?
f. Suppose you observe a very massive galaxy in HI. Under what circumstances might its
HI emission profile appear single peaked?
g. Compare the low resolution and the high resolution spectrum. Which one do you think
will give you the better measurements of the integrated HI line flux density, its velocity and velocity width? What criteria do
you use to make your decision? Fitting a baseline and measuring the RMS
Use the spectrum (high res or low res) you've selected in answering part g. above.
h. Start the routine baselinelbw.
What criteria should you use in selecting the regions over which the "baseline" will
be calculated?
i. What order polynomial appears to be the best?
What is the rationale used to decide which order to use?
j. What is Hanning smoothing?
k. What is the RMS (i.e. what is the measurement we are making, not just the number
given for this spectrum)? 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 measurelbw.
l. 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?
m. What is W50? W20?
Save your work
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.
n. Follow the instructions to use writesrclbw and save. And make a plot! o. 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 g. 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 /proj/a2669/corfile.23mar12.a2669.56
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. 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: Sun Jan 12 12:10:54 EST by martha
