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ALFOSC Cookbook

Last updated November 10 2014
Please refer to the online version for the latest update

Table of Contents

Preparing Observations

Observing scripts

Observing scripts can be generated automatically through the Observing Script Generator or crafted manually. If done through the web, the script will be uploaded and within a few minutes available in ~obs/scripts/proposal ID, where proposal ID refers to the proposal ID entered in the web form (e.g. 38-042). If the directory does no exist, it will be created. If you make your observing scripts manually, first make the proposal ID directory (using mkdir) and place them therein.

The scripts are executed from the sequencer window, from the directory where they are placed. Note that if you are saving scripts or files elsewhere in the ~obs account, the directory will be cleaned up after every run and it will not be possible to recover them.

Create a TCS Target Catalogue

A source catalogue can be made well in advance of your observations by using the web interface, NOT Source Catalogue Creation. Note that it is also possible to enter targets on-the-fly and create a catalogue directly on the TCS while observing (See section, Loading Target Catalogue).

Exposure Time Calculator

The Exposure Time Calculator can be used to estimate the required exposure times for ALFOSC, NOTCam, FIES, MOSCA and StanCam (both in imaging and spectroscopy mode, where it is available).

The CCD and controller

The detector is linear (better than ±0.5%) up to ~350-400 kADUs (full well= 459 kADU/pixel, 150ke-/pixel) and the digital saturation is 232ADU.
Typical bias level is about 10 kADU. See more details about the system from the detector and CCD-controller features pages.
A subtractable bias ripple exists in frames binned in the X direction.

Note, the shutter is 86 milli second faster than the actual requested time, i.e. requested one second integration keeps the shutter open for 0.914 seconds.
The minimum integration time is 0.3 seconds (with correction), this is the time for the sequence "move the shutter to open position - stop the shutter movement - move the shutter to close position".
Also, for the bias frames, i.e. the shutter have been kept closed the FITS-header KEYWORD EXPTIME will have value of 0.002.

If you wish to correct the "EXPTIME" FITS-header keyword you can use e.g. iraf/hedit-task
Correct the bias frame integration times to 0.00 seconds and apply the 86 millisecond correction to all the frames e.g.
iraf>hedit ALwa01*[0] exptime '(exptime!=0.002 ? exptime - 0.086 :0.0)' ver- sho+ add+
iraf>hedit ALwa01*[1] exptime '(exptime!=0.002 ? exptime - 0.086 :0.0)' ver- sho+ add+

Correct the integration times between 0.1 and 0.3 seconds to 0.3 seconds e.g.
iraf> hedit ALvf2600*[0]* exptime '(exptime < 0.35 && exptime > 0.0002 ? 0.3 : exptime)' ver- sho+ add+
iraf> hedit ALvf2600*[1]* exptime '(exptime < 0.35 && exptime > 0.0002 ? 0.3 : exptime)' ver- sho+ add+

Afternoon Preparations

Overview of everyday afternoon tasks

  • Update the TCS target catalog if necessary, and upload it to the TCS

  • Find out the TCS access code of the day on TCS status page 1

  • Shutdown the ALFOSC Sequencer (not from a "sequencer window" !) :
    [obs@tessa]$ shutdownobssys alfosc
  • Kill all remaining windows on the lisa display
  • Startup the ALFOSC Sequencer , in case of FIES override startup the FIES Sequencer
  • In all ALFOSC Sequencer windows change the working directory to the one assigned to your program, for instance
    [obs@tessa][ALFOSC]$ cd ~obs/scripts/38-025
  • Edit and test your scripts
  • Take calibration frames

Loading the TCS Target Catalogue

  • On the TCS, press the catalogue button CAT to view the object catalogue. If you want to erase the current catalogue that is displayed, type erase-catalogue on the TCS.
  • Load a source catalogue by typing read-catalogue catname.cat on the TCS, where catname is the name of the catalogue. A source catalogue can be created by using the web interface or the objects can be added directly to the current TCS catalogue by pressing the key, Enter Object (2) and then type according to the example below:

    Name RA DEC Epoch ProperMotionRA ProperMotionDec Magnitude

    SN1995N 14:49:28.27 -10:10:15.40 2000.0 0.00 0.00 0.00
    The proper motion should be given in arcseconds/year

  • Add a catalogue of blank fields, if needed, for obtaining twilight flats fields by typing read blanks on the TCS.
  • To select an object, press the catalogue button CAT on the TCS, and then use key number 1 or 7 to move to the next or previous object, or use the Prev/Next buttons to scroll up/down an entire page. The telescope will move to the selected target when pressing the key Preset to selected (4), or if observing on the parallactic angle, pressing the key Preset to par ang (6).

TCS access code

Any instrument computer which needs access to the TCS, for instance to move the telescope during slit acquisition or dithering, will ask for the current TCS access code at start-up. This code is updated every day (around lunch time). When you are asked to supply the TCS access code, do the following:

  • On the TCS VT510 terminal, make sure that the brightness is turned up
  • On TCS type: Show-Page and then 1 (or type sh 1 for short)
  • The TCS access code is shown on the top part of the screen as: AccessCode xxxxxxxxxx

 

Starting the ALFOSC observing system

  1. Switch on the lisa screens

  2. On the XDMCP Chooser click on the line
    'tessa.not.iac.es Willing to manage'

  3. Log in with username 'obs'. Password is the standard one.

  4. In a terminal window type
    [obs@tessa ~]$ startobssys alfosc
    A window will pop-up, asking for the TCS access code. Type it and press enter. If you type the wrong code a red error message appears and you must try again.
    All the ALFOSC software will now be started - the UIF, CCD3COMM and the FAPOL control. You will notice two terminals in blue/white colours with the title "ALFOSC Seq". These blue/white Sequencer windows (also called "seqterm"; use this command to get another sequencer terminal) are used to type-in the sequencer commands.
     
    The data will be saved in /data/alfosc and will have the format ALph160001.fits, where 'AL' is the image prefix for ALFOSC images, 'p' is for 2006, 'h' is for August, '16' is for the date 16 in the month and '0001' is a running number.

  5. The automatic ALFOSC observing log displays a listing of FITS headers for incoming data files. The System Talker lists incoming messages from the Sequencer. These two programs are displayed on the InfoSys monitor, and are automatically reset every afternoon. If you change between instruments you can change the obslog with the command e.g.:
    [obs@tessa][ALFOSC]$ alfosc.showlog

  6. The software interfaces for the instruments are very similar looking. Many commands are common for all instruments and are executed from the sequencer window. For instance, type object name to specify a name that will be the object keyword in the fits header. To make an exposure, type expose t where t is the time in seconds. See the list of the most commonly used sequencer commands. For a complete list of all sequencer commands, view the Sequencer Reference Manual.

  7. In all the SEQUENCER windows change the working directory to the one assigned to your program, for instance
    [obs@tessa][ALFOSC]$ cd ~/scripts/38-053
    as in this directory your ALFOSC observing scripts can be found, created, and executed.

  8. In the Talker you will find information about the correct start of the different software components or a corresponding error. If any errors about a program not starting are seen, please shut down the observing system and start it again.
  9. Use and enjoy the Sequencer.
     
    Click (twice) on the ScreenShots below to zoom in.
    ALFOSC obssystem layout ALFOSC obssystem layout

Obtaining calibration frames

For ALFOSC it is recommended to obtain at least one set of bias frames (nine or more) in the afternoon. The expected bias level is about 10 kADU/pixel , in case of e.g. 16kADU/pixel give a command "resetxy" or "xbin 1" ( see more known ALFOSC CCD3 problems and solutions ). In addition, some spectroscopic calibrations are best to obtain in the afternoon, such as windowed bias frames and (windowed) spectroscopic flats for blue grisms (bluer than Hα).

    Imaging

    Even though twilight flats should do a good job, it does not make any harm to take a set of dome flats as well for the broad band filters.
    For dome flats, place the "Dome FF Lamp" under the telescope, open the mirror covers, check the ccd-probe position, choose the desired filter and type e.g.

    [obs@tessa][ALFOSC]$ exp 15

    The expected integration times are about 150,30,20,15,15 seconds for BVRI, SDSS z respectively, if the telescope is pointing zenith. Note that the integration times are dependent on the telescope altitude and the location of the lamp.

    Spectroscopy

    Determine the CCD window to be used for spectroscopy. For example for horizontal spectra

    [obs@tessa][ALFOSC]$ resetxy
    [obs@tessa][ALFOSC]$ ysize 400
    [obs@tessa][ALFOSC]$ ybeg 851

    will set a rectangular window of 400 pixels wide (0.19 arcsec/pixel). The star/object will typically be centered at CCD pixel Y=1050 for horizontal spectra, and at X=1050 for vertical spectra.

    To obtain 9 bias frames use the commands

    [obs@tessa][ALFOSC]$ object bias
    [obs@tessa][ALFOSC]$ mdark 0 9

    and to obtain a 3-second HeNe arc lamp and five 10-second spectral flats use, for example

    [obs@tessa][ALFOSC]$ wheels   -g "#4"   -s Slit_1.0
    [obs@tessa][ALFOSC]$ calibexp   -o arc   HeNe   3
    [obs@tessa][ALFOSC]$ calibexp   -n 5   -o flat   Halogen   10

    A list of valid slit-names can be found here. Typical exposure times for wavelength calibration and flat field exposures can be found here.

    Note that the ALFOSC lamps will switch off automatically after 1200 seconds. When using the alfosc.calibexp script to obtain lamp spectra, the script makes sure to switch on the lamp before every exposure, and to switch off the lamp at script exit.

    Polarimetry

    For polarimetry it is recommended to take bias frames (with the same window as the science data) and especially for the WeDoWo dome flats. For WeDoWo take a dome flat fields (11 images for example) with slit/polarimetric mask, filter and polarimeter inserted with rot-pos 0 deg and second set with rot-pos 90 deg. if you keep the full xsize the benefit is having pre/overscan in the frame.
    For example dome flats with two rot-pos angles, reading full x and windowed y of the CCD, using filter #12.

    Switch-on dome lamps
    Open mirror covers
    [obs@tessa][ALFOSC]$ tcs-command "Rot-Pos 0"
    [obs@tessa][ALFOSC]$ resetxy
    [obs@tessa][ALFOSC]$ ysize 360
    [obs@tessa][ALFOSC]$ ybeg 820
    [obs@tessa][ALFOSC]$ wheels -g "'WeDoWo" -s Slit_10.0 -f 12
    [obs@tessa][ALFOSC]$ mexpose 10 11
    [obs@tessa][ALFOSC]$ tcs-command "Rot-Pos 90"
    [obs@tessa][ALFOSC]$ mexpose 10 11
    Close mirror covers

    FIES override

    In case of FIES override take the requested calibration frames
    you can change the electronic obslog between FIES and ALFOSC by

    [obs@tessa][ALFOSC]$alfosc.showlog
    [obs@tessa][ALFOSC]$fies.showlog

Start of the night

Overview of things to do at sunset

  • Power-on the telescope and open the dome following the procedure in the Cookbook

  • Once the telescope power is on, switch to ALFOSC. Run the script
    [obs@tessa][ALFOSC]$ tcs.setup-tel-alfosc
    in the ALFOSC Sequencer window, which sets up the telescope for observing with ALFOSC.

  • For spectroscopy, make sure that the "instrument-parallactic-angle" is set correctly: 0 for horizontal slits, 90 for vertical slits. For example:
    [obs@tessa][ALFOSC]$ tcs.instrument-parallactic-angle 90

  • Starting at sunset, run any sky-flat scripts that are needed.

  • When it is dark outside, switch on the two guiding monitors, and activate the guider at the TCS terminal or in a Sequencer window:
    [obs@tessa][ALFOSC]$ tcs.tv-on
    Note that tv-on needs to be issued every time the telescope has been parked at zenith.

  • Focus the telescope

Telescope Startup Procedure

Return to the dome at least 30 minutes before twilight and follow the instructions below.

Weather precautions

Humidity Wind Dusty Wind Temperature
> 90% - Close telescope

After 20min < 90% the telescope can be reopened.
12 m/s - Close sideports and lower hatch 10 m/s - Close sideports and lower hatch If the humidity is > 90% and the temperature is
< 0° C close telescope and do not open until all ice has melted on the outside of the dome.
15 m/s - Observe down wind 12 m/s - Close telescope
20 m/s - Close telescope

Telescope Control System (TCS)

  1. Turn up the brightness of the TCS screen (on right-hand side, below the screen).
  2. Look at the top-left of the TCS screen. If the item ST isincrementing, then the TCS is running, if not - then the TCS needs rebooting (rebooting the TCS should be done under guidance of a staff member). Type refresh if the screen looks strange.

Telescope power-on and Dome Opening

  1. Check the safety system in the computer room. Reset the system by turning the left key (marked as "First") and then the right key (marked as "Second"), one at a time (See TCS Manual Sect. 10.1 for procedure). If you hear a repeated clanking sound then the system has not reset, check the red lights above the keys to see if a safety stop or other item is set.
  2. Type on the TCS, power-on and confirm with y for yes. Look at the status page number 1 (type sh 1). On this page the status of the power units for the telescope motors can be viewed. When the TopUnitPower is on and the telescope is Idle, the telescope is ready to use. The power-on procedure takes about one minute in total.
  3. At sunset, if weather conditions are OK, open the upper hatch with the mirror covers closed. Press the key, START/STOP on the TCS and then the key Open Upper Hatch (7). Confirm by typing y for yes. It takes about 3.5 minutes to open the upper hatch.
  4. In the dome (While the upper hatch opens):
    • Open the side ports (only if weather conditions are OK). Do not use too much force and use the handle for lifting the ports if needed (Note that the side ports are easily damaged).
    • Check that nothing is in the way of the telescope motion (altitude: 6.5° - 90°).
  5. Open the lower hatch (only if weather conditions are OK). On the TCS, press the keys Start/Stop and Open Lower Hatch (8). Confirm with a y. It takes about two minutes to open the lower hatch. The TCS will not allow opening of the lower hatchbefore the upper hatch has completed opening. In case you can onlyopen the upper hatch, the maximum zenith distance for observing is 55°).
  6. The mirror covers can be opened at the same time as the lower hatch. On the TCS, press the keys Start/Stop and Open Mirror Covers (9). Confirm with a y. It takes about 50 seconds to open the mirror covers. Please note that the power for hatches and mirror covers is completely independent of the telescope power and the electronics in the control room.

Set default telescope parameters

  1. This should be done in any sequencer terminal with
    This sets amongst other items, the default telescope focus and rotator angle, and auto-pos-full for fully automatic autoguiding when pointing to an object.

Camera Probe

The Camera Probe is used for the standby Camera (StanCam) and for FIES and should be put in park position for all other instruments. Put the Camera Probe in the correct position by typing the TCS command as indicated in Table 1. The correct Camera-Probe position will be set when executing the script setup-tel-alfosc

Sky Flats

If you are observing in imaging mode, obtain sky flats in the evening twilight with the filters you are going to use. Typical flat fields have count level about 100k ADU per pixel (bias level is about 10 kADU and linearity is good up to 350 kADUs). If you know before hand that you might be requested to perform observations for a Target of Opportunity (ToO), it could be convenient to obtain sky flat fields in the standard filters (U, B, V, R, i) already in the evening.

Below is a short guide of how to obtain twilight flat fields for ALFOSC.

  • If not already done, start with
    [obs@tessa]$ tcs.setup-tel-alfosc
  • Remember to set tv-off on the TCS before making sky flat fields (adequate for morning flat fields, the TV monitor is often already off in the evening).
  • Choose a proper blank field from the blanks catalogue. A useful blank field target in the eastern sky that is not too low (i.e. RA 1-4 hours more than current ST).
  • point the telescope to the blank field:e.g.
    tcs.goto-object Blank03+31 from the sequencer terminal or
    goto-object Blank03+31 from the TCS.
    • You can also read the full catalogue:
    • Load a catalogue containing empty fields by typing read blanks on the TCS. Access the catalogue by pressing the key CAT and select an empty field with the keys 1, 7, or Prev, Next. Preset the telescope by pressing the key Preset to selected (4).
  • It is recommended to be close to the focus for each filter. Type foc-pos value where value is the default focus for the instrument and can be found in Table 1. Then type foc-del value where value is the focus offset (focus delta) for the filter, see Table 2 (remember to change the value of the focus delta when you are changing filter).
  • Choose the desired filter and start the script easyflat N, where N is the amount of exposures (script is avaliable for ALFOSC). The script first takes a windowed image to find the exposure time. It then starts the number of exposures, aiming for a count level of 100 kADU. If the count level in the test image is too high or to low, the script retries after 20 seconds. The script makes a small offset between each exposure. Repeat for next filter.

    or

  • Flat fields can also be obtained manually. Make an exposure by typing expose t in the sequencer window where t is the exposure time in seconds. It is recommended to make a small offset between each image in order to be able to remove stars from the frames. To make an offset, choose the move mode Telescope RA/DEC of the TCS by pressing 5 on the numerical key pad until the right move mode is selected. Then make sure that the offset size is put to a suitable value by typing o-s x y on the TCS, where the value of x and y is in arcseconds (it is recommended to use an offset size of ten times the size of the FWHM of the seeing disk in order to easily filter out the stars). Make the actual offset while the CCD is reading out by pressing down AltFunc together with any of the arrow keys (make the offsets in the same direction). One keystroke corresponds to the value of the offset size.

AutoGuiding

It is recommended to have the telescope autoguide on an optical source close to your target object. The TCS is automatically selecting a star from the HST guide star catalogue. The guide probe is then automatically placed at the correct position to auto-guide on this star.

  1. After obtaining sky flats, switch on the two Guide TV monitors above the TCS monitor. When it is dark outside, type tv-on on the TCS to power on the TV guide camera. The TCS command tv-off switches the TV camera off.
  2. On the TCS, type auto-pos-full to choose the fully automatic guiding mode. Note that this is the default guiding mode as set in tcs.setup-tel-<inst>. In this mode, a guide star will be automatically found, centered on the screen and autoguiding will be switched on (ag-on).
    If the guiding does not start automatically but you see a guide star on the TV monitor, type auto-pos-retry.
  3. The TV filter can be adjusted to the intensity of the guide star by typing tv-filt name on the TCS, where the options for name is Open, Closed, Grey, Red, Yellow or Blue. Choose an appropriate TV filter based on the brightness of the guiding object and keep an eye on the guide monitors during your observations so you can change TV filter after weather conditions and brightness of the guide stars.
  4. The autoguider TV focus is also set by the script tcs.setup-tel-<inst>. The TV focus is depending on instrument and can be adjusted by typing tv-foc value on the TCS, where the value for default focus is found in Table 1.
    If you wish the TV-focus to follow the telescope focus type au-t-f (Automatic-TV-Focus, default setting) on the TCS, else to restrict to only manual TV-focus settings type ma-t-f (Manual-TV-Focus).
    In automatic mode, after every pointing of the telescope the TV-focus will be set to a default value that takes the telescope-focus position into account. If that value is not adequate, you can recalibrate the TV-focus on the spot: after adjusting the tv-focus type calibrate-tv-focus in the TCS interface.
  5. If you want to choose a new guide star, type get-guide-star 2 (or 3,4,5, etc.) to get a new guide star from the guide server at the current pointing, followed by auto-pos-retry.
  6. On the TCS, the monitor signal strength from the autoguider is shown as a number. A guiding value between 1 and 23 is useful. Put in a filter if the signal is above 24, otherwise the TV filter will be set to grey automatically. If the signal is close to 1 the autoguiding is not reliable (e.g. clouds and/or bad seeing). Set tv-filt open and check for clouds.
  7. Make sure that the smaller box, the sky box is placed outside the star box. If needed, move the sky box by selecting the function Sky Box with Change move mode (5) and move the box with the keystroke combination, Alt Function + arrow keys.

    Always turn-off autoguiding (ag-off) before moving the telescope.

Manual AutoGuiding

If guiding mode is set to auto-pos-off, the guiding has to be activated manually. For manual choosing of a guide star, start with preseting to target. Find a guide star on the screen and move the star box to the star by pressing the key change move mode (5) on the TCS several times until the option Star Box is shown. Move the box by using the keystroke combination, Alt Funtion + arrow keys. Alternatively, choose the move mode Guide Probe to move the star into the box. To set the step size of the Guide Probe, type probe-step-size x y on the TCS, where x and y are in arcseconds. When the star is placed in the starbox, type ag-on on the TCS to switch on autoguiding. To switch off the autoguiding, type ag-off.

Guiding on moving targets

If you are observing moving targets, view the page Tracking of Moving Targets, in the Appendix.

Focusing the Telescope with ALFOSC

The telescope focus depends on temperature and zenith distance. These relations have been measured to a high accuracy and the corresponding corrections are applied in real time by the TCS (note that you will not see these corrections in the focus value displayed on the TCS since this value is kept constant to avoid confusing the observer).

We encourage the observers to check the focus a couple of times during the night. Below we describe how to set and monitor the focus in general terms, and then specifically for our different instruments.

  • The easiest way to focus the telescope is to use script alfosc.focus
    • In the evenig when it is dark enough, type in the sequencer window e.g.
      alfosc.focus
      this will setup ALFOSC for focussing using alfosc.focuspyr-setup, move the telescope to the optimal field (-a-option), take a ten seconds integration, analyse the focuspyramid image, find the best focus, adjust the telescope focus, if needed, take a new integration, analyse the focuspyramid image,find the best focus, adjust the focus and take more integrations until the "best telescope focus" difference between subsequent frames is less than 13 focus units or the maximum number of images is reached.
      Note1, if seeing is bad you might have to increase the integration time (-t-option).
      Note2, if seeing is bad, sometimes no pyramids are found automatically. You can analyse the image using alfosc.focuspyr script or use the default ALFOSC focus and focus the telescope later when the seeing has improved.
    • if you need to focus during the night use
      alfosc.focus -t 12 -a nearest
      or if you pointing at a suitable field already:
      alfosc.focus -t 12 -a noacq
  • Remeber to take the focus-pyramid from the beam and reset the CCD windowing
  • If you wish you can use less automatic way to find the best telescope focus:
    • Preset the telescope to focus star field (e.g. read-cat focus.cat or read-cat focus_bright.cat ).
    • Use the image acquisition software recommended for the instrument you are using.
    • Change the telescope focus by typing foc-pos value on the TCS where value is the new focus value.
    • If you, for any reason, cannot focus the telescope, use the default focus specified in Table 1. You can also change the telescope focus by a few units below and above the current value to manually find the best focus. Type expose time in the sequencer window (where time is the exposure time in seconds) to obtain an image of the focus field. The exposure time should be set to 10 to 15 seconds in order to fully sample the seeing frequencies. Use the 'imexam' command to analyze the image in the DS9 window. Press the key r to view the radial profile and to find the fwhm of a star (the pixel scale for each instrument can be found on the instrument pages).

For focusing with ALFOSC, a focus pyramid is used, an optical device which splits the light beam into four segments, each of them producing an image of the star on the CCD. The distance between the four images is linearly dependent on the focus of the telescope. By measuring the distance between the four images of a star, the correct focus of the telescope is given independently of seeing variations. Note that the internal ALFOSC focus has a default value of 1940 and is found to be very stable. For ALFOSC polarimetry the recommended focusing procedure is a bit different from the one described below.

If you wish to observe with several filters, determine first the telescope focus with no filter in the beam, and use this as a reference focus value. Set the correct telescope focus value for no filter by typing foc-pos value (make sure that foc-del is set to zero while evaluating the focus). Then find the appropriate filter focus offset to be applied for each filter and set foc-del offset always before using that filter. If no offset is available, determine the focus once more according to the suggested steps below - this time with having the filter in the beam - and note down its offset.

Start Observing

When the telescope is focused it is time to start the observations. Visit the individual pages for instrument specific information: ALFOSC

Overview: observing an object at night

Field Rotator

In an alt-az telescope the field rotator is used to keep the orientation on the sky fixed while tracking. When the field rotator is in automatic mode, which is the default, the TCS command field-rotation value sets the rotator such that value is the angle between the sky and the instrument. The default value of the Field Rotator is different for each instrument and can be found in Table 1. The field-rotation value will be set to the default angle when executing the script setup-tel-alfosc

The rotator-position is the physical position of the rotator with respect to the telescope fork, and it has a limit at +- 200 degrees. Its current value is shown on the TCS status display together with the time-to-reach-limit in minutes. Before reaching the limit, it will send a warning and suggest you to turn the rotator 360 degrees at any convenient moment. This is done with the TCS command ro-tu-360 after switching off autoguiding with ag-off. Note that this is possible only in the ranges [-200,-160] and [160,200]. If the limit has been reached, the TCS will eventually, after repeated warnings, perform the full rotation. The exposure commands for ALFOSC, FIES, MOSCA and StanCam checks whether the rotator limit is reached within the exposure time, if this is larger than 300 seconds. For NOTCam, where typically multiple short exposures are made, this is not the case.

If you want to follow the same target continuously for many hours, and you wish to avoid having to turn the rotator, for instance because you want your target on exactly the same pixels all the time (transit light curves), you might consider using another field-rotation, based on the time-to-reach-limit values.

Note that if you do slit-spectroscopy only, setting the field angle will usually not be useful as you probably would like to observe on the parallactic angle, in which case the field angle will be different for each target. To observe in this mode, on the TCS simply type instrument-parallactic-angle value (or i-p-a value for short) at the start of your run. See Table 1 for correct value, depending on instrument. For presetting to target when observing in this mode, choose the preset option Preset to par ang (6) instead of the usual Preset to selected (4). Upon every preset the rotator position will be close to the value defined as above at start. If you want to know your current field angle, for instance to be able to compare an acquisition image with a finding chart, then type sh 1 and look at the values of RefFieldRot and Instr.Par.Angle.

Note that when swapping from ALFOSC spectroscopy to ALFOSC imaging you must remember to set the rotator to field-r +90 if you want North up and East to the left in the images.

Quick Observing Instructions ALFOSC

Imaging

For details see the ALFOSC pages.
Note, the detector is linear (better than ±0.5%) up to ~350-400 kADUs (full well= 459 kADU/pixel, 150ke-/pixel) and the digital saturation is 232ADU.
Typical bias level is about 10 kADU. See more details from the detector page.

It is recommended to offset the telescope between the integrations, if possible. Using the dithered images you can compensate bad pixels, correct cosmic ray hits and fringing for the red filter. Typical offset size is 10-15 arcseconds.

Add your target to the TCS-catalogue and preset the telescope

[obs@tessa][ALFOSC]$ tcs.append-object my_star   12 12 12   +23 23 23   2000 0 0 0
[obs@tessa][ALFOSC]$ tcs.goto-object my_star

or

[obs@tessa][ALFOSC]$ tcs.guide-object my_star

Setup the ALFOSC wheels and adjust the telescope focus, e.g. for filter #12

[obs@tessa][ALFOSC]$ alfosc.wheels -f 12
[obs@tessa][ALFOSC]$ alfosc.focus-offset

Finally start observing, e.g. single 300sec exposures at each pointing of a dither pattern with 12 arc sec offset

[obs@tessa][ALFOSC]$ alfosc.dither 5D 12 300

You can examine an image using the ALFOSC DS9 and analysis scripts

Spectroscopy

Add a target to the TCS catalog, point the telescope with parallactic angle, and start guiding

[obs@tessa][ALFOSC]$ tcs.append-object my_star   12 12 12   +23 23 23   2000 0 0 0
[obs@tessa][ALFOSC]$ tcs.guide-object-with-parallactic my_star

In case you want to get more than one target on the slit, make a short full-frame exposure and use slitrot to determine the required field rotation, change the rotator angle and restart guiding:

[obs@tessa][ALFOSC]$ alfosc.expose 10
[obs@tessa][ALFOSC]$ alfosc.slitrot
[obs@tessa][ALFOSC]$ tcs.ag-off
[obs@tessa][ALFOSC]$ tcs.field-rotation <rot-angle>
[obs@tessa][ALFOSC]$ tcs.get-guide-star
[obs@tessa][ALFOSC]$ tcs.auto-positioning-retry

Do target-to-slit acquisition. A list of valid slit-names can be found here.
Note that alfosc.acquisition invokes alfosc.wcs-align which toggles the DS9 display to either the North=Up orientation (easy to find your target with finding chart) or the normal detector X,Y orientation. You can also use the DS9 buttons (in the 'Frame' pull-down menu) to set or unset this.

[obs@tessa][ALFOSC]$ alfosc.acquisition -c -q Slit_1.0 <expotime>

As for doing a blind offset, this is just doing a telescope offset after having acquired the target from which to apply the blind offset.

Choose the desired CCD setup (windowing, binning), e.g.

[obs@tessa][ALFOSC]$ alfosc.resetxy
[obs@tessa][ALFOSC]$ alfosc.xsize 400
[obs@tessa][ALFOSC]$ alfosc.xbeg 851

Put in the grism, e.g. grism #4

[obs@tessa][ALFOSC]$ alfosc.wheels -s Slit_1.0 -g "#4"

Make optional lamp calibrations with the telescope pointing at the target, e.g. 3 Halogen flats and 1 Helium-Neon arc. Approximate exposure times for calibrations are listed for each grism in this table.

[obs@tessa][ALFOSC]$ alfosc.calibexp -lmi -n 3 -o "Halo at my_star" Halogen <expotime>
[obs@tessa][ALFOSC]$ alfosc.calibexp -o "HeNe at my_star" HeNe <expotime>

Start the on-target integration, e.g.

[obs@tessa][ALFOSC]$ alfosc.object "My Star"
[obs@tessa][ALFOSC]$ alfosc.expose 1200

Once the exposure is finished the spectrum will be on-line extracted by alfosc.quickspec. To configure the behaviour of quickspec use

[obs@tessa][ALFOSC]$ alfosc.quickspec-config

An example of an image of the slit, with the star in the slit, and of what a target spectrum looks like, are given below.
 

Polarimetry

For details see Polarimetry with ALFOSC.
Note, the detector is linear (better than ±0.5%) up to ~350-400 kADUs (full well= 459 kADU/pixel, 150ke-/pixel) and the digital saturation is 232ADU.
Typical bias level is about 10 kADU. See more details from the detector page.

First add your target to the TCS-catalogue and preset the telescope. Make sure that the field rotation is as desired, typically "90".

[obs@tessa][ALFOSC]$ tcs.append-object my_star   12 12 12   +23 23 23   2000 0 0 0
[obs@tessa][ALFOSC]$ tcs.guide-object my_star

    Imaging polarimetry with FAPOL

Note that the internal ALFOSC focus will be automatically offset by -870 units when any of the calcite plates/polarimetric slitlets are entered into the beam and the alfosc.focus-offset script is run. The default instrument focus for ALFOSC is 1940.

Setup all 5 ALFOSC wheels, adjust the focus and take a test image to identify the field. Offset the telescope using e.g. teloffset 20 -15, if needed. If your target is very bright or easy to identify you might skip this full frame image.

[obs@tessa][ALFOSC]$ alfosc.wheels -f 12
[obs@tessa][ALFOSC]$ alfosc.focus-offset
[obs@tessa][ALFOSC]$ alfosc.expose 20

If you are pointing to the right field, setup all 5 ALFOSC wheels for imaging polarimetry, move the retarder plate in the beam, adjust the telescope focus, window the CCD and take a test image to estimate the integration time. The test image through the full polarimetric setup is recommended, as high S/N is needed.

[obs@tessa][ALFOSC]$ alfosc.wheels -f 12 -s Calcite
[obs@tessa][ALFOSC]$ alfosc.carriage in
[obs@tessa][ALFOSC]$ alfosc.focus-offset
[obs@tessa][ALFOSC]$ alfosc.polwin
[obs@tessa][ALFOSC]$ alfosc.expose 20

Take your data, e.g. one set of 90 seconds exposures with retarder plate angles 0°, 22.5°, 45°, and 67.5°.

[obs@tessa][ALFOSC]$alfosc.linpolexpose 4 my_target 90 1

For example, below is a DSS image and FAPOL image of the same field.

For quick look analysis you can use qilpol for linear polarimetry or qicpol for circular polarimetry

[obs@tessa][ALFOSC]$alfosc.qilpol ALwa210127
[obs@tessa][ALFOSC]$alfosc.qicpol ALwb210127

    Spectro-polarimetry with FAPOL

Add a target to the TCS catalog, point the telescope with parallactic angle (NB! Note polarisation position angle correction!), and start guiding

[obs@tessa][ALFOSC]$ tcs.append-object my_star   12 12 12   +23 23 23   2000 0 0 0
[obs@tessa][ALFOSC]$ tcs.guide-object-with-parallactic my_star

Acquire your target on the polarimetry slitlet using the interactive alfosc.acquisition script which will: 1) put the retarder plate in the beam, 2) put in the Calcite for the acquisition image, and 3) use alfosc.focus-offset to correct both the internal ALFOSC focus and the telescope focus. Make sure you have no filter in the beam. NB! Select the left component of the target in the interactive acquisition script! When the star is centred in the slit, put in the grism.

[obs@tessa][ALFOSC]$ alfosc.acquisition -c -q Pol_1.8 exptime
[obs@tessa][ALFOSC]$ alfosc.wheels -s Pol_1.8 -g "#4"

Window the CCD to an appropriate size. Take your data, e.g. one set of 600 seconds exposures with 4 retarder plate angles (0°, 22.5°, 45°, and 67.5°), typically used for linear polarimetry with the lambda/2 retarder, as follows:

[obs@tessa][ALFOSC]$alfosc.linpolspec 4 my_target 600 1

Or, in the case of circular polarimetry with the lambda/4 retarder, take for instance 3 sets of 300 second exposures with 2 retarder plate angles (0°, and 90°), as follows:

[obs@tessa][ALFOSC]$alfosc.cirpolspec 2 my_target 300 3

Note that in order to use the ALFOSC calibration lamps, you first need to take the FAPOL carriage with the retarder plate out of the beam!

[obs@tessa][ALFOSC]$ alfosc.carriage out

Switch on the lamps of your choice and take your calibrations with the grism and slit in the beam, using the same CCD window as for your targets.

The the science frame and He arc lamp image should look like this:

    Imaging polarimetry with the WeDoWo

For WeDoWo imaging the 10 arcsec wide slit is used as a mask. Here is an example of a windowed (see below) domeflat:

Typical commands for WeDoWo imaging in the i-band (filter #12) could for example be to first take a acquisition image with the required filter in the beam:

[obs@tessa][ALFOSC]$ alfosc.wheels -f 12
[obs@tessa][ALFOSC]$ alfosc.focus-offset
[obs@tessa][ALFOSC]$ alfosc.acquisition -q -c Slit_10.0 2

When the target is on the slit, window the CCD, move the WeDoWo into the light path, and expose:

[obs@tessa][ALFOSC]$ alfosc.resetxy
[obs@tessa][ALFOSC]$ alfosc.ysize 360
[obs@tessa][ALFOSC]$ alfosc.ybeg 820
[obs@tessa][ALFOSC]$ alfosc.wheels -f 12 -s Slit_10.0 -g WeDoWo
[obs@tessa][ALFOSC]$ alfosc.focus-offset
[obs@tessa][ALFOSC]$ alfosc.expose 10

Typical night time calibrations include high and zero polarization standard stars.
If you want to have two targets on the slit use alfosc.slitrot

Sequencer scripts

A typical sequencer script could contain the above steps, comprising the target acquisition, instrument setup and exposure commands.

Scripts should be made 'executable' and the script name should not clash with names of other system command names. See details from Usage and examples of Sequencer scripts .

In scripts all the sequencer commands should be prefixed with alfosc.<scriptname> or tcs.<scriptname>.

End of the night

Overview of things to do at the end of the night

Closing Down Procedure

Closing the telescope

  1. Before obtaining morning flats fields, turn the guide TV off by typing tv-off on the TCS.
  2. Put the telescope to zenith by pressing the key Start/Stop and then Zenith (0). This will park the guide probe and it also sets rot-man. Wait for Rotator -90°.
  3. Park the building by pressing key 4, Park Building. This process can take up to 8 minutes. Building will stop at Az ≈ 119°, Alt ≈ 90°.
  4. Close the mirror covers by pressing key 3, Close Mirror Covers. This takes 42 seconds.
  5. Close the lower hatch by pressing key 2, Close Lower Hatch. This takes about 3 minutes.
  6. Close the upper hatch by pressing key 1, Close Upper Hatch. This takes about 5 minutes. If timeout occurs or hatch does not move, see TCS Manual p. 43.
    NB: When the upper hatch has closed the dome cooling will start and the staircase cooling will stop.
  7. Close all 4 side-ports in the dome. Please, do not use violence. If the ports are stuck, use the handle to lift them before closing.
  8. Power down the telescope. After all of the above have been completed, type power-off on the TCS, then type sh 1 to display the telescope power units. Wait until all the telescope power displays show "off".
  9. Turn down the TCS screen brightness. Use the knob on the right-hand side of the TCS monitor, near the power switch. Never turn the TCS monitor off.
  10. Turn off monitors. Turn off the two autoguider monitors and dome TV camera monitor.
  11. Turn on the control room webcam and the dome webcams. Make sure that the webcams in the control room as well as in the dome are turned on before you leave.
  12. Have a look in the dome at hatches and telescope, to check for any faults that might have occurred.
  13. Complete the WWW report forms. Fill in and submit Internet NOT End-of-Night Report. Be sure to include any faults that occurred and submit a fault report, so we can attend to problems promptly. If it is the last night of the observing run, also fill in and submit Internet NOT End-of-Run Report.
  14. Lockup the dome. Lock the outer dome door when you leave the building.
  15. Clean up the kitchen/lounge area in the service building and switch off all lights when you leave.
  16. Lockup the outer door in the service building before leaving for the Residencia.

Stopping the ALFOSC observing system

  1. In a terminal (not a sequencer) window type
    [obs@tessa ~]$ shutdownobssys alfosc
  2. Wait for all the control programs to close.

  3. Close remaining windows manually.

Troubleshooting

If you have ...

  • ... cannot reset the safety system:
  • ... no light on the CCD -- check:
    • Dome open, mirror cover open
    • Lower hatch not obstructing at low altitude
    • Guide probe out of light path
    • Camera probe in correct position
    • FASU shutter open (if using ALFOSC)
    • Correct filter in instrument
    • Correct telescope focus
    • If it is cloudy
    • Check all the above again. If there still is a problem, then phone duty staff.
    • If MOSCA or StanCam image has a count level of 0, this can actually mean saturation and you should reduce the amount of light falling on the detector.
    • StanCam shutter controller has power (if using StanCam)
  • ... wrong field on CCD -- check:
    • Coordinates of field
    • Epoch of coordinates
    • No typos in entering coordinates in TCS
    • Field rotation is correct
  • ... no guide star on guide probe screen -- check:
    • Dome open, mirror cover open
    • Guide probe in correct position
    • Field and rotator angle
    • TV switched on
    • TV filter open
    • TV focus correct
    • Try typing in the X and Y values of another Guide Star fromthe The 10 brightest currently available Guide Stars
    • If it is cloudy
  • ... lost tracking/guiding:
    • How close to the zenith are you? (Good guiding and tracking is guaranteed only 5 degrees or more from zenith).
  • ... changing tv filter does not work:
    • at TCS type in the command 'Initialize-TV-Filter'.
  • ... ctrl x', 'ctrl g', 'ctrl t' and 'ctrl v' commands on the TCS give incorrect functionality, e.g. telescope offset rather than star box movement.
    • wrong function key menu on the TCS keyboard press keypad key 5.
  • ... "building crash":
  • ... other TCS problems
    • Press the keypad key labelled Log and then keypad 2 to see the latest entries in the TCS log.
  • ... ds9 shutting down while doing acquisition
    • Using the mouse to set the contrast while image is loading?
      Using the mouse to set the contrast while this process takes place can cause DS9 to crash. Best procedure is to wait tuning the contrast level until the image has been fully loaded into DS9, and zoom / contrast levels set.
      A DS9 crash while in 'imexam' while leave the postprocessing system in a non-workable state. Rather than shutting down the whole observing system to recover, it is in most cases sufficient to run the sequencer command 'killpostprocess'.
  • ... "sequencer/obssys" does not respond or behaves badly:
    • Standard procedure in such circumstances is to restart the observing system using 'shutdownobssys' (from a terminal window) followed by 'startobssys'.

If you encounter a fault or a problem with the instrument, the computers or the telescopeduring the night, then fill in a Fault Report.

Troubleshooting ALFOSC

  • ... ALFOSC UIF does not start with an error "The ALFOSC UIF did not start. Restart the observing system.":
    • Power cycle the motor controller and try again
    • Check that the network cables are connected and try again
  • ... FAPOL retarder plate is out of beam, going red, although FAPOL was already out "Timed out waiting for FAPOL carriage to arrive at position Out":
    • Initialize the polarization unit. If this fails try to
    • Restart obssys. If this fails try to
    • Power cycle FASU controller and restart obssys
  • Appendix

    Tables for Instrument Settings

    instrument field-rot i-p-a camera probe foc-pos tv-foc orientation c-w-h
    ALFOSC +90 0 (horizontal slits)±90 (vertical slits) c-p-p 22610 480 North ↑ East ← Horizontal slits: E-W (P.A. = ±90) 0.75
    FIES 0 N/A c-p-s 22850 530 North ↓ East ← N/A
    NOTCam -90 -90 c-p-p 22770 495 North → East ↑ 0.95
    MOSCA -90 N/A c-p-p 22420 450 North ↑ East ← 0.55
    StanCam 0 N/A c-p-c 22360 430 North ↑ East ← N/A
    SOFIN 0 -90 c-p-p 23290 490 N/A 0.9
    TurPol 0 N/A c-p-p 24240 650 North ↑ East → 0.12

    Table 1a shows the values of the: field rotation(field-r), instrument-parallactic-angle (i-p-a), ccd probe, focusposition (foc-pos), TV focus (tv-foc), field orientation andcounter weight height (c-w-h) for the different instruments. On the TCS, type thecommand followed by the value, for instance field-r -90. The default focus value stated for NOTCam is for the K band in imaging mode using the WF camera (internal camera focus of 5650).

    From To foc-delta
    ALFOSC no filter StanCam R-filter (#10) -260
    MOSCA R-filter (#106) StanCam R-filter (#10) -155
    NOTCam Ks-filter (WF) StanCam R-filter (#10) -380

    Table 1b shows the default focus offsets between the instruments.

    Filter name NOT filter number foc-del
    Bessel U #7 -70
    Bessel B #74 -85
    Bessel V #75 -10
    Bessel R #76 -80
    Interference i #12 -5

    Table 2 shows the focus offsets for ALFOSC filters, U, B, V, R, i. More filter offsets are found here.


    Commonly Used Instrument Commands

    Command Comments
    CNTR-C kill ongoing command or script
    abort kill ongoing exposure

    object text Sets the value of the FITS keyword OBJECT to text
    autosave_on Turns on autosave. For off, type autosave_off
    rempath link Sets the remote saving path to specified link
    remsave_on Turns on remote saving. For off, type remsave_off
    bin n Sets the binning factor in both X and Y directions to n
    xbeg n Sets the X beginning to n (use ybeg for y).
    xsize n Sets the X size to n (use ysize for y).
    expose t Makes an exposure of t seconds. To abort the exposure press Ctrl-C and type abort in the sequencer window.
    mexpose t n Makes n exposures of t seconds
    dark t Makes an dark exposure of t seconds
    mdark t n Makes n dark exposure of t seconds
    teloffset n m Offsets the telescope in the CCD X,Y direction by the amount n in the X direction and m in the Y direction. n and m are given in arcseconds.
    resetxy Resets the CCD to unbinned full-size readout
    For more commands, view the Sequencer Reference Manual.

    Commonly Used TCS Commands

    Command Comments
    ag-on Turns autoguiding on
    ag-off Turns autoguiding off
    auto-positioning-full Turns the guide star auto positioning to full-automatic
    auto-positioning-retry Try again to bring the guide star in the guide box
    camera-probe-stancam
    camera-probe-ccd
    Moves the camera probe to ccd position (for StanCam)
    camera-probe-park Moves the camera probe to park position (for ALFOSC)
    camera-probe-fies
    camera-probe-split
    Moves the camera probe to split position (for FIES)
    field-rotation fieldangle Sets the field rotation to fieldangle
    focus-position n Moves the telescope to focus position n
    focus-delta n Applies a focus offset to the current telescope focus
    instrument-parallactic-angle n Defines the parallactic angle for following presets
    read-catalogue name.cat Reads the catalogue
    tv-on Turns on the guide TV
    tv-filter name Selects the guide TV filter where name can be either closed, grey, open, red, yellow or blue
    tv-foc pos Changes the guide TV focus to position pos


    append-object string appends an object to the TCS catalog
    goto-object Object moves telescope to "Object"
    find-object selects an object in the TCS catalog
    preset-with-parallactic-angle-to-selected moves telescope to the selected object, with parallactic angle
    For more commands, view the TCS SEQUENCER Command Reference.


    Tracking of Moving Targets

    There are two ways of tracking a moving object (planet, comet, asteroid, satellite) depending of the speed.

    Slowly Moving Targets

    For objects moving rather slowly, i.e. less than about 1 arcmin during an exposure, the tracking is handled by moving the guide box on the autoguider screen, while autoguiding, in order to compensate for the object motion. The box-motion rate is given in RA/DEC coordinates, i.e. dDEC/dt and dRA/dt (in arcsec per hour), and the cos(DEC) correction is taken care of internally by the TCS. (In other words, if your RA rate is given as dRA*cos(DEC), you have to divide by cos(DEC) before entering its value.) The limitation on target speed is set by the size of the autoguider screen (about 70").

    • The commands below assume that the TCS does not center the guide-star automatically when pointing to a new object. Hence, auto-positioning-off should be in effect.

    1. Calculate your object's coordinate motion in RA and DEC rates in advance (rates are in arcseconds per hour).
    2. Preset to your object, and get a guide star on the guide monitor.
    3. Position the star box in an appropriate place on the screen with Star-Box-Position Xpos Ypos, where the XY range is 16-495. You can try with a quick box move with much higher rate than calculated above, to find out in which direction the box moves.
      Use Box-At-Center if you want to put the star box in the center of the field.

    4. Then move the guide probe to put the guide star in the box: first set the step size by typing p-s-s 5 5 on the TCS, then select the function Guide Probe Y-/Y+/X+/X- using Change move mod (5), and use either 'Alt Function'+'arrow' keys, or 'Ctrl'+'t,f,v,g' to move the probe. Start autoguiding with the TCS command ag-on when the star is placed in the box.
    5. Start then tracking of the moving object by the TCS command Box-Motion-Start RA-rate DEC-rate (or in short b-m-sta RA-rate DEC-rate). When the box reaches the opposite edge of the monitor, it stops moving. The telescope set-rate is automatically set by this command.
    6. Make the exposure, and then stop tracking by Box-Motion-Stop (or in short b-m-sto). This also resets the telescope set-rate.

    Rapidly Moving Targets

    For more rapidly moving targets, use the set-rate command on the TCS to continuously move the telescope from a pre-calculated change in position. RA-rate and DEC-rate as above. In this mode autoguiding is not used.

    1. Preset to your object.
    2. Set the rate of change by set-rate RA-rate DEC-rate (rate in arcsec per hour). The maximum rate is 60,000 arcsec/hour (≈ 17 arcsec/s).
    3. Make the exposure.
    4. Reset by typing set-rate 0 0 on the TCS.

    For Experts Only

    If the target has an unknown or rapidly changing motion, you might want to adjust the rate of change in RA and DEC while tracking. Use the set-rate command on the TCS and use command recall with the up/down keys and edit the set-rate.

    1. Preset to object.
    2. Guess the starting set-rate RA-rate DEC-rate values.
    3. Do as described above with command recall.
    4. Do set-rate 0 0 or just preset to another object.

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