Observing manual for PEPSIPOL


The more complete pdf version of the manual can be downloaded from here.


Below is described how to observe with PEPSI in polarimetric mode (POL) at Large Binocular Telescope (LBT). Note that polarimetric mode can be used only when the polarimeters are mounted. All pictures can be clicked to enlarge them for details.

The first part of the document describes what one should take into account when planning the observations.

Observations themselves are performed with following steps:
I Accessing PEPSI GUI
II Connecting with Polarimeter
III Selecting and sending targets to LBT
IV Guiding
V Keeping target in pinhole
VI Observing

The document also describes following items:
VII Viewing obtained spectra
VIII Estimating S/N
IX Calibration frames
X Telemetry
XI Troubleshooting

Planning the observations

PEPSI can be used in several observing modes:
PEPSI observation modes.
PEPSI observation modes.
Wavelength coverage:
  • The entire spectral range is from 383 to 912 nm but it can not be covered by a single exposure
  • PEPSI has two arms, blue and red, that cover spectral ranges 383-544 nm and 544-912 nm, respectively
  • Each arm has three cross-dispersers:
    • CD1 383-426 nm in blue arm
    • CD2 426-480 nm in blue arm
    • CD3 480-544 nm in blue arm
    • CD4 544-627 nm in red arm
    • CD5 627-741 nm in red arm
    • CD6 741-912 nm in red arm
  • Simultaneously, one can observe one wavelength region in blue and one in red, however, CDs 3 and 4 can not be used at the same time
Spectral resolution:
  • Polarimetric observations can be done using only one fiber, that is with one resolutions:
    • 200 μm gives R=130,000
Exposure times :
  • Exposure time depends on the target and used cross-disperser (use Exposure Time Calculator)
  • One can have different exposure times and numbers of exposures for cross-dispersers in blue and in red arm (examples below)
    • 30 min in blue, 20 min in red ⇐ red is idle for 10 minutes
    • 30 min in blue, 2×15 min in red ⇐ both are ready around the same time
    • 3×10 min in blue, 6×5 min in red ⇐ both are ready around the same time, blue likely idle for some minutes due to read-out times (~ 80 sec / read-out)
Additional setups:
  • The blue mirror is the default setting
  • If interested in good radial velocity stability, simultaneous fabry-perot-etalon (FPE) should be on
  • Filter is selected automatically according the brightness of the target. However, it can be altered on fly to adapt to any changing conditions (e.g. clouds).
  • PFU beam splitter is selected automatically according the brightness of the target and cross-disperser used
  • If PEPSI is used on both telescopes, filters, binning, etc. need to be changed for both sides 
Observing limits:
  • The lower limit is 26 degrees
Finding chart:
  • The field of view for guiding is 28″ so finding chart is of no use
Observing blocks:
  • At the moment observing blocks can be created only by using PEPSI GUI

I Accessing PEPSI GUI

There are 2 options for from where the observations can be done. NOTE that for Option 2 you must request VPN access  to LBTO beforehand – at least 10 days before the run. Write to ScienceOps@lbto.org to get it!

  • Option 1: use the PEPSI 4K monitor in the LBT control room
  • Option 2: observe remotely (4K monitor helps if you want to see everything at once.)
    • first make a VPN connection to vpn.lbto.org (using Cisco AnyConnect)
    • then either open a VNC program and take connection to server alpha.pepsi.lbto.org:1 (or :2)  or type in terminal vncviewer alpha.pepsi.lbto.org:1
    • in the end, close the VNC connection, exit viewer and disconnect VPN
VNC view of the user interface (:5901)
VNC view of telemetry (:5902)

The PEPSI user interface program should be kept running all the time.

    • If it is not running, click Activities in the left upper corner. You should get a menu having PEPSI logo. Click on that logo to open the program.
    • Or use a terminal to open it (if not, open a new terminal)
      • on pepsi@alpha type ‘pepsi’

II Connecting with Polarimeter

  1. Now you should have ‘PEPSI Spectrograph Control Interface’ open.
    • In case PEPSI has been used for solar observations (SDI selected) and you have text ‘Waiting for Sun‘ in the ‘PEPSI Spectrograph Control Interface‘, click ‘Abort‘ to be able to start night observations.
    • Click on LBT check button to connect and open ‘PEPSI@LBT‘:
    • Check that POL radio button is selected:
  2. A ‘PEPSI@LBT‘ window should appear:
  3. The spectrograph has to be engaged (‘Engage‘ button) into observing mode, so that the hatches are opened for authorized sides and ADC is following the target elevation even for a pointing star.

III Selecting and sending targets to LBT

  1. Click ‘Program’ button on ‘PEPSI Spectrograph Control Interface’:
  2. When you
    1. already have an observing program, a ‘Observing Programmes‘ window and a ‘Target Visibility Plot‘ window opens. If not, go to ‘Table‘ menu, choose ‘Open or Close Table‘ and select the correct one.
      • If GUI had to be restarted, check that the correct program is open (YYYYMMDD.obs)!
        1. select the target (highlighted with teal color) you want to observe and send it to LBT. This does not yet move the telescope!
          • Note that target can be sent already during CCD readout!
          • This sends 2000 coordinates with proper motions to the telescope
        2. check that ‘PEPSI@LBT’ has the same target as you chose, or resend:
          • Telescope coordinates in GUI show 2000 coordinates whereas object coordinates are the actual coordinates
        3. check that ‘PEPSI Spectrograph Control Interface’ changed (target name, selected cross-dispersers, exposure time, etc.) according to selected target:
          • Note that the changes for the next target happen only after CCD readout of the previous target has finished.
        4. if you need to change pre-defined exposure times, click ‘Edit fields’:
        5. a new window pops up and you can make the changes
          • Note that values in ‘PEPSI Spectrograph Control Interface’ do not change unless you send the target information again (see A.)
          • You can also change values in ‘PEPSI Spectrograph Control Interface’ directly, but then the visibility plot does not change and block will not remember the changes made in future
        6. In case your full QUV sequence gets interrupted, you do not have to repeat it all over again, but continue, for example taking only V exposures
    2. don’t have any targets or you want to change targets
      1. click ‘add another object’ in ‘Observing programmes’ window:
        • In case you want to remove a target so that it is not shown at all, go to that target, erase it with Alt+Del and then delete it with Ctrl+Del
      2. a new window having targets pops up:
        • NEVER delete targets from this list!
        • Targets can be added to the catalogue:
          • when one starts to write the name, it is shown if the target already is in the catalogue
            • if the target already was in the catalogue and it has a PID (program ID) that is not yours, PID should be changed so that the data goes to the right person
          • in case it indeed is a new target, make a query and then apply to that it goes to the catalogue 
          • targets can be imported from a text file
            • names should be in that case in format HD123 or “HD 123” 
            • text file can contain also coordinates
          • if the target is a transient or nova, coordinates can also be given
          • for non-sidereal targets like Io or Jupiter, a name and approximate coordinates should be given and telescope uses then a special mode to observed them (talk with your telescope operator)
      3. choose the target and send it to the observing block:
      4. Additional things to take into account when making observing programmes in polarimetric mode:
        • One can take either QU or V sequence, or have the full Stokes QUV sequence
        • Getting one of the parameters means two exposures (= two angles)
      5. continue as in III-2-a.
  3. after the telescope operator gives the permission, click ‘PRESET
    • PRESET‘ to next target can be done already during CCD readout
    • Sync SX/DX mode of preset is the normal case
    • In case one of the telescope eyes does not move to the target, it is possible to make ‘PRESET‘ separately only with one of them (Left SX or Right DX):
    • Note that ‘Preset‘ can be done in four different modes, but only the first one should be used unless requested otherwise by the telescope operator:
      1. Active – the normal mode, uses wavefront sensor and guiding
      2. Guide – no wavefront sensor is used, guiding yes
      3. Acquire – only points and centres on hot spot
      4. Track – only points

IV Guiding

Collimating, guiding, and also focusing is done by the telescope operator. Do not do anything unless asked.

  • Engageopens the hatches so that the telescope operator can put the star in the position.
  • Guideropens guiding  windows

V Keeping target in pinhole

  1. First, make sure that the telescope is not moving anymore (= progress bar disappears)
  2. When everything works really fine, the target lands on the pinhole
  3. Sometimes the target is very close by, but not really in the pinhole
  4. In such a case, click target (bright spot) on the right mouse button on top row images (doing it on lower row is disabled) to move the pinhole (black circle) into it
  5. You may start ‘guiding‘ in guiding window, that is making small corrections to keep target in the pinhole, but also ‘Start‘ in next step starts it automatically

VI Observing

Start observations by clicking ‘Start’ in ‘PEPSI Spectrograph Control Interface’:

  • If the exposure needs to be interrupted for any reason, use ‘Stop‘ in case you want to save the data obtained so far orAbort‘ in case the exposure time was not long enough for useful data.
  • If due to the, for example, clouds you think that exposure time was not long enough, it can be changed on fly in the ‘PEPSI Spectrograph Control Interface’ as long as CCD readout has not started.

VII Viewing obtained spectra

It is good to take a look at the spectra in order to see if you should change exposure times etc.

  1. Select the spectrum (highlighted with teal colour) you want to look at from ‘FITS Image Browser‘ with Enter or click the arrow icon to send it to ‘Spectrum viewer‘. One can also look at multiple spectra by selecting them and clicking double arrow icon.
  2. The active window in ‘Spectrum viewer‘ has a cyan frame. Select the area you want to look at closer by drawing a red box with, for example, a mouse:
  3. To see a summed spectrum from selected area
    1. Press ‘Enter‘, or
    2. Click the normal ‘Sigma‘ icon (one dimensional cross-cut in horizontal direction) on top of the viewer (the other sigma makes it in vertical direction)
  4. Now you have a spectrum plot where you can see the ADUs:

VIII Estimating S/N

  1. Select an area with mouse from highest continuum values to lowest:
  2. Click ‘Sun‘ icon on top of the spectrum viewer:
  3. You get a pop-up window there the S/N estimate is given, save that information into fits list by clicking ‘Save‘:

IX Calibration frames

After the observing night is over, it is time to take the calibration
frames. Check that you have stopped guiding and guider camera is on pause.

In the ‘PEPSI Spectrograph Control Interface’ window:

  1. Click ‘Calibs‘ button to get a pop-up window:
  2. Check that for wanted fiber(s) ThAr, FPE, and Traces are chosen:
  3. Click ‘START‘ button on Control Interface window:
  4. When calibration frames are ready, click ‘Calibs‘ again to close the calibration sequencer window and to be able to do anything else.

X Telemetry

  • It is essential to check the telemetry from time to time:
  • If something is wrong and you are not trained to fix the problem, contact a person who is!
  • The most important is the ‘Pressure‘ in PEPSI Chamber window. If ‘In’ value has red background, and one sees that pressure curves follow each others, everything is not fine.
  • Other important measure is ‘Dewar heater‘ in PEPSI CCD window. If it approaches 0, the dewar pressure rises and pumping is needed.

XI Troubleshooting

  • Sometimes the software program freezes or even crashes (STA Archon time out). If that happens, find the following terminal

and type Ctrl+c and start all over again.

If terminal gives ‘Lost connection with Camera’, one can go to ‘PEPSI Control Unit’ Maintenance, but in order to do that, you should know what you are doing. Moxa-Power-Ccd Controllers, wait 10 seconds. In case of pressure problem, use again Maintenance.