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P52-006: Polarimetric monitoring of MAGIC blazars

P.I.: Stefano Covino

This ToO program has no override. Can only trigger during Tech and Nordic Service nights. Max 4 hours per observation.
Allocated: 2 triggers and a total of 12 hours.

Overview

When this ToO is triggered it can spend a total of 6 hours per target over period 52 doing imaging polarimetry with ALFOSC+FAPOL in either of the filters B or R, or both.

The trigger email will contain information about the target, and the PI will submit pointing scripts with finding charts for

  1. the target,
  2. one zero polarisation standard, and
  3. one high polarisation standard.
Note that the standards are usually bright and may be taken during twilight.

Possible targets:

Instrument setup:

  • The "Calcite" in the ALFOSC aperture wheel
  • The lambda/2 retarder in FAPOL (for linear polarimetry)
  • Standard B and R filters in ALFOSC (#74 and #76)
This is a setup that by default should be standard.

Afternoon Calibrations

In the afternoon, please take 10 biases with the polwin setup:
  • Set remote path: alfosc.rempath /data/service/calib/
  • and activate remote saving: alfosc.remsave_on
  • Reset the CCD to default values: alfosc.resetxy
  • Set the window to default for FAPOL imaging polarimetry: alfosc.polwin
  • Set the object name: object bias-polwin
  • Start taking 10 biases: mdark 0 10
  • De-activate remote saving: alfosc.remsave_off
  • Reset CCD window: alfosc.resetxy

Sky flats

The normal full-size skyflats in B and R (NOT filters #74 and #76) are adequate, i.e. no need for special flats taken with the polarimetry setup. These flats when taken with the script alfosc.easyflat will be automatically copied to the ftp directory pub/service/calib/ for external retrieval.

Observing instructions:

  1. If not already done, setup for ALFOSC: tcs.setup-tel-alfosc
    Make sure the telescope has been focused for ALFOSC, and that this focus is set.

  2. In a sequencer window type cd ~/scripts/52-006/

  3. In this directory find the pointing scripts for the target and both standards as mentioned in the trigger email from the PI. Run the script and make sure auto-guiding is ok.

  4. Find in the trigger email the filter(s) to be used (B or R or both). For each object (target and standards) find also the given exposure times to use.

  5. Enter FAPOL lambda/2 plate into the beam with: alfosc.carriage IN

  6. Setup ALFOSC for R-band polarimetry with: alfosc.wheels -s Calcite -f 76 -forceFASU
    or for B-band polarimetry with: alfosc.wheels -s Calcite -f 74 -forceFASU

  7. Make sure the focus is adjusted: alfosc.focus-offset

  8. Set the remote file storage path with: alfosc.rempath /data/service/52/006/

  9. Switch on remote saving: alfosc.remsave_on

  10. Set the window size to default for imaging polarimetry: alfosc.polwin

  11. Start the sequence of exposures at 4 angles of the lambda/2 plate with the script:

    alfosc.linpolexpose 4 target-name exptime N

    where you have to fill in target-name, exptime (in seconds) and the number of cycles N according to instructions in the trigger email.

    Note that you will see two horizontally displaced (by ~ 15'') images of the same field as that shown in the finding chart. The field-of-view is approximately 140''.

  12. Follow the trigger instructions regarding how many cycles to do in each filter, before changing the filter setup and the focus (see points 6 and 7 above).

  13. When finished with all observations, go back to default ALFOSC setup with the commands alfosc.remsave_off, resetxy and alfosc.allopen.

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