Review this sheet while we demonstrate the observing on Monday afternoon. There aren't any questions for you to answer, but if you have questions about the observing, you should ask us!!!



UAT13.D: Demo of observing for A2707 on Monday afternoon


On Monday afternoon, we have been allocated a short observing session to demonstrate and discuss the LBW observing technique. This sheet is intended to help you understand the observing technique and procedure. Use it to follow along with the observing, to learn your way around the observing documentation, and to understand how we keep track of our many (many!) observing sessions by many (many!) different observers.


Useful links:
  • L-band wide observing
  • A2010 observer's page


    D.0   Setting up CIMA for LBW observing

    Follow along what Greg and Luke do to start up the observing using the ALFALFA observers' LBW checklist.


    D.1   Position switching for ALFALFA: Follow along as we observe!

    The LBW program consists of a series of ON-OFF pairs of observations of each HI target as specified in a "command file" which has been generated in advance by Martha, the czar of ALFALFA observing. Normally, each pair consists of a 180 second observation on the target ("ON-source") followed by a 180 second observation of an "OFF-source" position which follows the (presumably) blank sky (over the same frequency range) over an identical track in telescope coordinates (azimuth, zenith angle). The ON- and OFF- source exposures are separated by 60 seconds. Once the pair sequence for a given target is executed, the telescope slews to the next source. You may want to review the answers to SH #1, particularly 1.10 where we discussed position switching.

    Look over the command file that we are using for the UAT demo session here. Be sure you understand what it does.

    In setting up the command file for each observing run, Martha uses a program which estimates the start time of each target's observation, taking into account the time it takes to do a pair and to slew the telescope between sources. A few seconds of padding are inserted because it takes the telescope time to "settle" on source, to balance the power, etc. The program isn't perfectly accurate (and isn't intended to be... so CAVEAT EMPTOR!) and of course, we never know exactly when our observing sequence will begin, but it allows us to see whether we are far off from "the plan". Here is a file which keeps track of the approximate sequence in the demo run this afternoon.


    D.2   The key to a long-term program: Bookkeeping, bookkeeping, bookkeeping!

    Between the ALFALFA drift scan survey and the followup LBW observations, we have had almost 850 observing runs in the last 8 years and more than 200 people have been involved in actually conducting the observations (including attendees of previous UAT workshops). A big survey effort like ALFALFA requires a systematic approach to keeping track of the observing, and we do that by having the observers keep a log record, in the same format, of what happened during the observing. You can find all of the logs of past observing runs, since Spring 2005, here.

    To keep track of this, a template log file is generated for each observing session; here is the (currently incomplete) one for this afternoon's demonstration run, and here is one from March 21, 2012. The idea is that the observers fill in the blanks, keep track of the observations, take a quick look at the data and make notes of anything usual.


    D.3   Instant gratification: Let's look at the data!

    NOTE: We don't normally observing the afternoon; the spectra may not be up to our usual standards!

    At the end of each target's ON-OFF pair sequence, we can take a quick look at the resulting difference spectra to see if there is a signal. We do this in a quick way by executing some simple scripts, written in IDL by Greg. You can read more about how to do this on the LBW observing checklist. We will learn more about this tomorrow in SH#2. We hope that SH#1 has given you a chance to learn a bit about the spectra we obtain. For now, let's just take a quick look at a couple of the observations we plan to do this afternoon:


    To look at a night's data, we have to start up IDL and use the commands Greg has written (see the checklist. Then, to see the final (ON-OFF/OFF) spectrum of the first 2 boards of the first object, we use a command like
  • checklbw, file='/share/olcor/corfile.DDmmmYY.a2707.N'


    Since it is always a good idea to make sure that the system is set up and working properly, we deliberately picked a first source (assuming we start on time) which we already know has a detectable HI line signal. In the current sesssion's log file, we see this entry:
    SRCNAME             RAJ     DECJ                     AGNnr          LST   ZA   File#  ONScan     Status
H215040.4+283617  215040.3 +283527  j  3500 H v   # A310858 calib

    You all realize that this sheet was made up before the observing run, and since we are observing in the afternoon for demonstration purposes, the spectra may not be ideal. In the interest of testing the system as well as explaining to you what we might expect to see, we chose a source which we have observed before. In fact, Ron O. and friends observed this target on November 19th; see the log file for 12.11.19. The log record of the observation for that session reads:
    SRCNAME             RAJ     DECJ                     AGNnr         LST   ZA    File#  ONScan     Status
H215040.4+283617  215040.3 +283527  j  3500 H v   # A310858 calib  2148  10.32  1     232463762  Slight Detection @1404 MHz
    We can take a look at those data (just as Ron and friends did). To view the final spectrum, we can just type:
          checklbw, file='/share/olcor/corfile.19nov12.a2707.1'

    And, this is what we get:
    (click on the image to see a bigger version)


    or, to zoom in on the y-scale
          checklbw, file='/share/olcor/corfile.19nov12.a2707.1',yrange=[-55,-35]


    gets us:
    (click on the image to see a bigger version)



    We hope that this short demonstration helps to give you a better sense of what the observing and spectra data are about. And, if you have questions, ask someone: We are here for you!


    Last modified: Mon Jan 7 15:29:37 EST 2013 by martha