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P60-018: Physical properties of Earth's temporarily-captured satellites '

P.I.: Grigori Fedorets

This is a ToO override program to do either or both of the two observing modes:

  • 1.8'' slit spectroscopy with Gr#11
  • FAPOL lambda/2 polarimetric imaging (R filter)
of faint targets with non-sidereal tracking.

These observations can now (since Sep 2019) be prepared and triggered through our OB generator.

Spectroscopy setup:

  • Gr#11 (typically mounted in the ALFOSC filter wheel)
  • Slit_1.8''
  • Binning 2x2 for all spectra and calibrations

Polarimetry setup:

  • FAPOL lambda/2 (linpol)
  • Calcite plate
  • R-band filter (#76)

Daytime calibrations:

  • In the evening twilight the standard (non-polarimetric) R-band skyflats are sufficient. These are made using the alfosc.easyflat script.
  • In the morning after closing the dome and when the dome is dark start the script alfosc.alfosc-calibs before leaving.
Remember to fill the tracking log of this program.

Observing instructions

  • Go to directory: cd ~/obs/scripts/60-018/

    The target name is given in the trigger email.

    The standard star for spectroscopy (solar analog) and the standard stars for polarimetry (high-pol and zero-pol standards) should have OB generated scripts.

  • Target pointing and non-sidereal tracking:

    Explanation of how it works:
    The command tcs.append-object command adds the target coordinates and epoch to the TCS input catalog. The command points to the target and acquires a guide star in normal mode. Differential tracking and guiding is started with tcs.reposition-guide-probe, which takes the tracking rates as input parameters.
    This involves the following steps: auto-guiding is stopped, differential tracking rates are set, the guide box is moved to the optimal position on the TV screen, differential motion of the star box is started, the guide star is put into the box (by moving the guide probe only), and autoguiding is started in differential tracking mode. This works until the box reaches the edge of the screen.

    NB! Note that for a fast target it is important to have a minimum delay between the two latter commands. If pointing takes a long time (because the telescope or the rotator needed to move a lot), then it is worth while to point again to the target (with updated coordinates), otherwise the target moves away from the centre of the field before differential tracking/guiding is started.

    ''TimeToEdge'' (in seconds) is the time for the star box to reach the other edge of the autoguider screen. This is the time available before the star-box must be repositioned!
    It is output on the terminal by the reposition script, and you can also check it in real time on status page 24 (command sh 24 on the TCS UIF).
    The TCS sends an audible warning (and a yellow Talker note) when there is 120, 90 and 60 seconds left.

    What to do:

    1. To speed up pointing and re-pointing on the TCS type disable-TV-filter-optimization (to set it back, if you wish, use enable-TV-filter-optimization ).

    2. Run the asteroid ephemeris retrieval script tcs.asteph "object name"
      NB! Use full asteroid name instead of number! This is due to ambuigity in the naming conventions of asteroids, comets, and satellites.

    3. The script connects to JPL Horizons for the object name given and calculates pointing information for La Palma in the format the NOT TCS needs the parameters. It outputs four commands on the screen which you should cut and paste as commands into a sequencer window, one after the other (for example):
                tcs.append-object 2017_TC1-xxxx 20 02 53.11 +27 52 40.6 2000.0 0.0 0.0 20.06
                tcs.reposition-guide-probe -750.856 -353.934
    4. If you want the slit aligned along the direction of the target movement, then for this program using horizontal slits with ALFOSC you should add the flag "-f fieldrotation" to the command, where the fieldrotation angle is the same as the sky PA output from tcs.asteph, for example, if PA = -55 degrees, then use:
      -f -55 2017_TC1-xxxx
    5. If the target is not available in JPL Horizons (or if its server is down), you need to read from the ephemeris file the J2000 target coordinates and tracking rates (dRA/dt and dDEC/dt in ''/hour) for the current UT time, and issue the same commands as above.

      Note the exact format of the tcs.append-object and tcs.reposition-guide-probe as:

                tcs.append-object NAME RA DEC EPOCH PPM-RA PPM-DEC MAG     
                tcs.reposition-guide-probe dRA/dt dDEC/dt 
      where dRA/dt without the cos(DEC) correction is the required input to the TCS, since the TCS applies the cos(DEC) correction internally. If your ephemeris are given as (dRA/dt)*cos(DEC), which is the default output by JPL Horizons, then you need to divide by cos(DEC) to get dRA/dt.

  • Target observations:

    NB! Keep an eye on the star box on the autoguider screen!

    During all subsequent observations you must make sure that the star box does not reach the other edge of the screen.
    To help you with this, reposition-guide-probe outputs the "TimeToEdge", and the TCS UIF (sh 24) logs it in real time.

    For fast targets the star box may reach the other edge of the screen before your observations finish.
    In this case, you must reposition the star box BEFORE it reaches the edge. In a sequencer window type:

    tcs.reposition-guide-probe dRA/dt dDEC/dt

    This should not affect the telescope motion. (But for some guide star locations this may happen. Under investigation.) In most cases it is sufficiently accurate to use the same RA/DEC rates as when pointing.
    There is a limit to how many times you can reposition the guide probe (due to the finite guide area) before you need to repoint to the target.
    In this case, use updated coordinates and tracking rates!

    • Spectroscopy

      1. The following script does the interactive slit acquisition and takes one spectrum with Gr#11 and Slit_1.8'' using 2x2 binning.
        The input parameters are: object-name, exposure time for the spectrum, and exposure time for the acquisition image.

        spectroscopy.script object exptime acqtime
        spectroscopy.script 2017_TC1-xxxx 300 20 (example)

        In order to locate the target, which should be close to the centre of the CCD, you need a finding chart. In a browser on the Florence computer to your right, you can open either of the following links:

        Note that for the Lowell astfinder you can enter the target name, set the observatory to "950 La Palma" and FOV to 390'' for ALFOSC, plus you have the possibility to set the time steps of the trajectory, while for AladinLite you have to enter the pointing coordinates.

        If the target is fast enough or the acquisition time is long enough, stars will become elongated while the target should be round.
        Follow the instructions in the sequencer window to put the target on the slit.

      2. Typically a number pf exposures are requested (e.g. 5-10 times 300 seconds). If more spectra are needed, you can directly take another N spectra with the script:
        multispectrum.script object exptime N

        If you think you must redo the slit-acquisition (e.g. quickspec 1D spectrum shows declining flux), then run the complete script:
        spectroscopy.script object exptime acqtime

      3. If something goes wrong and/or guiding is lost, you can stop and save the ongoing exposure with the command
        alfosc.addtime -t to subtract out the remaining exposure time t. If inside a multispectrum script, you can use Ctrl-C.

    • Polarimetry

      The polarimetry observing script takes an image at each of 4 different lambda/2 angles and is run in the sequencer as follows:

      granvik-pol.script -noreset object exptime bin ncycles
      granvik-pol.script -noreset 2017_TC1-xxxx 90 2 15 (example)

      where the name of the object and the exptime are both given in the schedule, as well as which on-chip CCD binning and the number of cycles to run. In the example above bin=2, and ncycles=15.

      The -noreset flag is used to avoid taking out the polarimetry optics in case the script must be rerun a number of times, which is what we anticipate for these observations.

      The exptime should be chosen to be short enough that at least 4 exposures can be done before the star box reaches the edge of the screen.
      If more cycles are requested, you must re-position the star box when needed.

  • Solar analog spectroscopy:

    1. Run the OB generated script.

  • Polarimetry standards:

    Polarimetry standards are typically bright and can be done during twilight.

    1. Run the OB generated scripts for all the polarimetry standards.
No tracking data available