Observing with PEPSI PFU



This document describes how to observe with PEPSI Permanent Fiber Unit (PFU) at Large Binocular Telescope (LBT).

Observations are performed with following steps:
I Accessing PEPSI GUI
II Connecting with PFU
III Selecting and sending targets to LBT
IV Guiding
V Observing

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

I Accessing PEPSI GUI

  1. Either
    • use the PEPSI 4K monitor in the LBT control room, or
    • observe remotely
      • from Linux machine
        • take VNC connection by typing in terminal vncviewer -via ajarvinen@ssh.lbto.org -shared
        • environment (telemetry) status can be seen at
        • passwd is given on need to know basis
        • in the end, close the connection from the upper corner ‘x’
      • from Mac
        • one has to first make a VPN connection  vpn.lbto.org
        • username and passwd are given on need to know basis
        • and then take a VNC (with TigerVNCViewer) connection to server alpha.pepsi.lbto.org:1 (or :2) 
        • in the end, close the connection with Fn+F8, exit viewer and disconnect VPN
    VNC view of the user interface (:1)

    VNC view of telemetry (:2)
  2. 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 PFU

  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 PFU radio button is selected:
  2. A ‘PEPSI@LBT‘ window should appear:


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!
          2. check that ‘PEPSI@LBT’ has the same target as you chose, or resend:
          3. check that ‘PEPSI Spectrograph Control Interface’ changed (target name, selected cross-dispersers, exposure time, etc.) according to selected target:
              • Note that the changes happen only after CCD readout 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
    2. don’t have any targets or you want to change targets
      1. click ‘add another object’ in ‘Observing programmes’ window:
      2. a new window having targets pops up:
      3. choose the target and send it to the observing block:
      4. 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.
      • Note that ‘Preset‘ can be done in four different modes
        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
    • Difference between Left, Right, and Sync?


IV Guiding

  1. Click ‘Guider’ in both Left and Right to open guiding window to keep the target on the pinhole. Engage?
    • Offset buttons offsets the telescope in small steps so that the image of the direct-reflected image of the target moves on the guider CCD according to the directions of the buttons.
    • Offset step size specifies the offsets in arcsec.
    • The faster way to move the target into the pinhole is to click the target with the right button of the mouse (it does not matter if it seems that cursor is focused on the box instead of the star).  (Also with LBT? Has to be done for both SX and DX?)
    • Focusing? How, how often? During summer 2019 focus in Left is around 30000 and in Right 30500.
    • Normally, the gain value in both ‘SXPFU‘ and ‘DXPFU‘ window should be 0.9. Is there a standard value? FAP value 3 should be good.
    • Adjust binning according to brightness of the target. Any additional info?
  2. You may also want to check the hot spot coordinates (direct reflected image position). If you see the image of the star on the pinhole spilling over asymmetrically, correct hot spot coordinates. As reference, summer 2019 values were x=97 and y=218 for SXPFU and x=150 and y=172 for DXPFU.
  3. When you are happy with the plots in ‘SXPFU‘ and ‘DXPFU‘ windows, start guiding by ‘Start’:
      • The upper most plots show the cross profiles of the target in both X and Y direction and they should overlap and converge to zero. The second plots from top show the guider offset and thirds FWHM of the star. The bottom plots show how much flux the guiding camera gets. This gives a good indication of passing clouds.
    • Guiding in the next target can be started already while previous one is read out.
    • In the morning, make sure that you ‘Stop’ guiding.


V 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.


VI 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:

VII Estimating S/N

  1. Select an area with mouse from highest continuum values to lowest (don’t be distracted by high fabry-perot peaks):
  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‘:


VIII 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.


IX Telemetry

    • It is essential to check the telemetry from time to time:
      • Add plot
    • 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.

X 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.