P52-028: Polarization Monitoring of VHE gamma-ray BL Lacs
P.I.: Elina Lindfors
Instructions for ALFOSC/FAPOL R-band imaging linear polarimetry
The observations should be done using ALFOSC and FAPOL, linear imaging
polarimetry (lambda/2 retarder), R-band (NOT filter #76). For each source,
4 retarder positions should be taken.
See Observing Schedule below for which targets
to observe on which dates.
Instrument setup:
- The "Calcite" in the ALFOSC aperture wheel
- The lambda/2 retarder in FAPOL (for linear polarimetry)
- Standard R-filter (#76)
This setup should be default and always available.
Observing Schedule for P52 (2015-2016)
The sources referred to below are listed in the target and standard star
tables lower down on this page. Preliminary schedule:
- Oct 05, 2nd half, (sources 9-19, see visibility plot, NB! source 11)
- Oct 26, UT 23:00-02:00, (sources 9-14, 16-20 + standards)
- Nov 06, 2nd half, (sources 9-10,12-19)
- Nov 22, 1st half, (sources 9-14, 16-17, 19-23 + standards)
- Dec 05, 2nd half, (sources 12-15, 17-19, 21-23, 25)
- Dec 23, 2nd half (UT 00-04), (sources 12-14, 16-17, 19-20, 22-25 + standards)
- Jan 05, UT 23:00-03:00, (sources 12-14, 17-19, 21-23, 25)
- Jan 22, UT 23:00-03:00, (sources 16, 18-26 + standards)
- Feb 04, UT 23:00-03:00, (sources 15, 17, 19-26)
- Feb 24, UT 22:00-02:00, (sources 15, 17-26 + standards)
- Mar 16, UT 22:00-02:00 (sources 15, 17, 19,22-26,
1,2,3)
- Apr 01, UT 20:30-01:00, (sources 18-26 + standards)
For each month the second night will include the standards, i.e. tot
observation time is 2.5 hours. For the first night of the month it
is 2.3 hours.
The visibility of the sources is quite scattered and you may not be
able to take them all in one go. The above suggested part of the night
should be fine for most of the targets.
The standards may be taken in twilight.
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
Sky flats
The normal full-size skyflats in the R-band (NOT filter #76) are
adequate, i.e. no need for special flats taken with the polarimetry setup.
Observing instructions:
There are no seeing or sky quality limits. Any conditions
are fine.
Total time is expected to be 1.3 hours when standards are included and 1.1
hours else.
- If not already done, type tcs.setup-tel-alfosc
- In a sequencer window type cd ~/scripts/52-028/
- In this directory type mkcatalog. This puts all objects and
standards in the TCS catalogue.
- Check schedule below for which objects (targets and standards) to
observe tonight.
- For each object (target or standard) find the exposure time to use.
- Run the script elina-pol.script [-noreset] objectname exptime.
The script finds objectname in the TCS catalogue, goes to target,
displays the finding chart, starts auto-guiding, waits for acknowledge,
sets up ALFOSC for R-band polarimetry (if not already), and starts a
sequence of exposures through 4 retarder plate positions. If the
-noreset flag is used, it will keep the instrument setup (useful
if you do several targets in a row), otherwise it will reset everything.
Note : the script defocuses by 200 units for the bright standard HD204827.
- If target acquisition failed, the script failed, and/or the object
does not fit with the finding chart, interrupt with Ctrl-C, offset the
telescope correspondingly, and continue in manual mode (see bottom of
this page).
- Run the script without the -noreset flag for the last target,
OR when finished, go back to default ALFOSC setup with the
commands carriage OUT, resetxy, allopen and
focus-offset.
Targets, standards, finding charts, visibility plot of today
Below is the target list, coordinates (in 2000.0), the R-band magnitudes and suggested
exposure times (of single exposures) and finding charts. The exposure times have been
calculated assuming the given magnitudes, seeing 2.5", airmass 1.2, extinction 0.7 and
sky brightness G and aiming at S/N>200 (to be on the safe side). In good conditions,
slightly shorter exposures can be used, the aimed accuracy in the polarisation is ~0.3%,
i.e. S/N~167 is enough per exposure.
The finding charts are all with North up and East left. The target is marked with a plus
sign. Please, ignore any blue encircled stars.
The finding chart size is 7' x 7' arcminutes for those without blue circles, and 12' x 12'
for those with blue encircled field stars.
The visibility plots for today are shown below for the targets and the standards separately.
| Standard |
J2000 coordinates |
Magnitude
V |
Exptime
(s) |
Finding chart |
| G191B2B |
05 05 30.62 +52 49 54.0 |
11.78 |
15 |
finding chart (zeropol std) |
| BD+64106 |
00 57 36.71 +64 51 26.5 |
10.34 |
15 |
finding chart (highpol std) |
Manual observing instructions
If the observing script elina-pol.script does not work correctly, here is a
listing of how to do the observations manually:
- If not already done, tcs.setup-tel-alfosc
- In a sequencer window type cd ~obs/scripts/52-028/
- In this directory type mkcatalog to enter all targets into current TCS catalogue
- Enter FAPOL lambda/2 plate into the beam with: alfosc.carriage IN
- Setup ALFOSC for R-band polarimetry with: alfosc.wheels -s Calcite -f 76 -forceFASU
- Make sure the focus is adjusted: alfosc.focus-offset
- Set the remote file storage path with: alfosc.rempath /data/service/52/028/
- Switch on remote saving: alfosc.remsave_on
- Check the schedule for which targets to observe, and for each target do the following:
- Display the finding chart with the command: xv target.png where target is the name of the target.
- Point to the target and start auto-guiding: tcs.guide-object target
- Set the window size to default for imaging polarimetry: alfosc.polwin
- Start the sequence of exposures at 4 angles of the lambda/2 plate with the script:
alfosc.linpolexpose 4 target exptime 1
where you have to fill in target and exptime accordingly.
Note that you will see two horizontally displaced (by 15") images of the same field as that
shown in the finding chart.
When finished with all targets and standards, reset ALFOSC CCD and optics with
carriage OUT, resetxy and allopen, focus-offset, and set remsave_off.
Invalid Proposal |
