Try to finish this hunt by 5:15pm on Tuesday afternoon

Useful links:
  • Using Arecibo for ALFALFA
  • A2010 observer's page
  • receiver and backend setup for ALFALFA followup with L-band wide
  • observing modes for ALFALFA followup with L-band wide
  • L-band wide receiver page
  • Phil's IDL routines for spectral line data taken with the interim correlator

    Rules of the scavenger hunts: You may consult any source anywhere but please be sure to indicate where you got your information. And watch out for bad websites!

    UAT12.01 LBW Scavenger Hunt: Introduction to ALFALFA Followup Observing with L-band Wide

    This scavenger hunt will provide an introduction to the point-and-track observations with L-band wide that will be undertaken in Spring 2012. This is (clearly) a work in progress.

    If you have not already done the exercise to examine the example dark objects in α.40 in preparation for the submission of the observing proposal, we suggest you should do it now.   Thanks to those who participated in the writing of the proposal, especially Catherine, Lucas and Wyatt!!!

    LBW.0   ALFALFA followup: Why bother?...
    Now that we have completed most of the ALFALFA drift scan observations, we are initiating a second phase of the survey project to conducted targeted observations of interesting ALFALFA detections or possible detections. We'll start these observations during the January 2012 UAT workshop (as if our annual trips to Puerto Rico weren't already exciting enough!).

    a.   How long does it take a point source to drift across an ALFA beam?   Hmmm... seems like we've thought about this one somewhere else..

    b.   We say that the "effective" integration time for ALFALFA is about 40 seconds per beam. How do we get the answer "40 seconds", i.e. what considerations apply?

    c.   The AGES program maps small areas (~10-20 sqd each) using ALFA with an effective integration time of 300 seconds. For the same spectal resolution, how much more sensitive (lower rms) is AGES than ALFALFA?   Hint: use simple scaling!

    d.   We might imagine that if we observe for a really long time, the noise in the spectrum would continue to decrease. In practice, this is not the case. Why not?

    e.   What rms noise (in mJy) will we expect our 3-minute ON-OFF observations to give us, at a velocity resolution of 10 km/s? How does that compare to ALFALFA?

    f.   The ALFALFA bandpass covers 1335-1435 MHz. What spectral line has a rest frequency of about 1424 MHz? If we wanted (deliberately) to observe emission from that line close to its rest frequency, where would we look?

    g.   Suppose we observe a source with a frequency that, if the line is HI, is -1515 km/s. Why don't we think this is an HI source? If it is an OH megamaser, what is its redshift?

    LBW.1   Position switching with LBW
    During the UAT12 workshop, we will begin the ALFALFA followup observing program, using a technique of total-power position switching, also known as "ON-OFF observing".

    a.   Why do we need an "OFF" source observation?

    b.   ALFALFA also uses "position switching" of a sort; how does that work?

    c.   What is the slew speed of the telecope in azimuth?

    d.   What is the slew speed of the telescope in zenith angle?

    e.   What observational consideration(s) led us to hold off a serious campaign of followup observations until now?

    f.   What famous politician once said "However beautiful the strategy, you should occasionally look at the results.". g.   When we look very close to the rest frequency of the HI line (1420.4058 MHz), we sometimes see the final spectrum show both positive and negative flux. Why is that?

    LBW.2  Setting the spectrometer: options, options, options
    For the A2010 drift scan observations, we use the WAPP spectrometer, but for LBW we use another device called the "interim correlator".

    a.   How does the resolution of the spectra obtain for the LBW observations in "high velocity" mode compare to the spectra we record for the A2010 drift scan ALFA observations?

    b.   What is the total frequency range that our observations cover when the spectrometer is set for "low velocity" mode?

    c.   What is 9-level sampling, and why is it advantageous?

    d.   For the HI line, what velocity corresponds to the central channel of Board 1 when we are in low velocity mode?

    LBW.3   Introduction to TOPCAT: the Tool for OPerations on Catalogues And Tables, an interactive java graphical program which has been developed by astronomers at the Virtual Astronomical Observatory.

    a.   Do you have access to the UAT groups Google sites page.   Hint: Find it at: If you do, go there and log in. If not, send email to Becky or David's gmail account. David's is They can add you to the site. When you reach the site, look the bar on the left. Under **Programs** you will see UAT TOPCAT Docs, click there. If you have not visited this site recently (or at all), briefly review the documents there; refer to them as needed as we proceed.

    b.   Start up TOPCAT on your machine.   If you have not installed TOPCAT on your laptop or local machine, follow the steps in **Installing and Getting Started** for your system. Java web start is generally the easiest, or Mac disk images, but TOPCAT is also easy to install on Windows machines.

    c.   In the latter part of the **Installing and getting started** on the UAT wiki, read section on the 2.1 **Loading data from a CSV file**. What does "CSV" stands for?

    d.   Go to the public ALFALFA catalog release page. If you have not already committed it to perfect memory, peruse the α.40 paper. You will find detailed information on the content and format of the three data files in the header of the ASCII versions of the data files. What does "ASCII" stand for?

    e.   Right-click (or whatever) to download the .csv file associated with Table 1 of the α.40 paper to you computer using the **CSV** link. Be sure you know where it is located on your disk. Then, using TOPCAT, load the .csv file into a table. How do you let TOPCAT know that it is a .csv file?

    f.   Review the column headers in the Table within TOPCAT. They should match the descriptions in the header of its ASCII file. At what resolution is the rms calculated?

    g.   Make a plot of distance (x-axis) versus log Msun for the alpha.40 database. What is log Msun?

    h.   Why are the points distributed so that they spread out in the vertical direction at nearer distances but are more confided in log Msun as the distance increases?

    g.   There appears to be a gap in the distribution near the right at a distance of about 220 Mpc. What causes that gap?

    h.   There is a less obvious gap at a distance of about 80 Mpc. What causes that gap?

    i.   There appears to be a vertical line of points at a distance of about 17 Mpc. What causes the apparent line?

    There are lots of other things you can do with TOPCAT. (For example, you can ask Wyatt how to match multiple catalogs.) We encourage you to learn how to exploit its capability!

    This page created by and for the members of the ALFALFA Survey Undergraduate team

    Sun Jan 15 17:32:22 AST 2012 by Martha