CFH12K Observer's Guide

  Table of contents [v2.0, January 2001]

1: Scope of this document

    This document is intended to provide the CFH12K observer, the support astronomer and the observing assistant a complete overview on how to correctly and efficiently observe with the CFHT wide-field CCD camera CFH12K. It ranges from formal descriptions of each command, step-by-step description of critical procedures, to tips and tricks.

    For the observers, it will be a clear description of what they will be facing when using CFH12K (this document includes text and graphics). For the support astronomers, it is a literal version of what an introduction to a visiting astronomer should be (CFH12K related topics only). For the observing assistant, it will be a reference to help the observers in the course of the observing process if support is needed.

2: Introduction

    This document provides a complete overview for use of the CFH12K camera on the sky in regular observing mode. CFH12K can be controlled from a Graphical User Interface (GUI), a Command Line Interface (CLI), or from scripts (SC, any shell).

    The central window of the CFH12K session is the Director window which acts both as a CLI and as a status window (this text based window usually stands in the lower right corner of the central image acquisition computer screen).

    The main GUI window (CFH12K User Interface) serves as both camera status display and control interface. This GUI reproduces a reduced set of information available from the Director window; elements such as progress bars and detector temperature only appear in the Director window which serves as the main status window.

    The different parameters defining a given exposure can be set up and exposures can be launched (and stopped if necessary) from either the CLI or the GUI. An interface for efficiently taking a set of twilight flat-fields with constant flux level is provided in the main GUI window. Complex interactions between the camera and the telescope (dithering patterns) are pre-programmed and available through the GUI or scripts. The focus form (CFH12K Focus Interface) launches focus sequences and can be used to control the z (focus) value of the prime focus bonnette. The file format form (CFH12K FITS File Format) allows the user to select the image FITS format for saving data to disk.

3: Using forms, the command line interface and scripts

4: System status

    The three fields at the top of the main GUI window provide the name of the current exposure being taken, the file format and the current status of the camera. The filenames for the CFH12K FITS images include: the current CFHT odometer number (a unique 6 digit number) plus a one letter extension (o=Object, f=Flat, d=Dark, b=Bias, x=Focus). For example: 550076o

    If the Multi-Extension FITS format is selected (FITS=MEF), a unique FITS file is created with a ".fits" extension. If the MEF mode is off (FITS=SPLIT), the 12 individual CCD images will be saved in independent standard FITS files along with the primary header (converting to MEF mode is easy), in an individual sub-Directory. A binned by 8 image of the full mosaic respecting the relative output positions is created in both image format modes as a normal independent FITS file (the structure of this file is independent of the FITS file format selected). Its name is the root of the image name with an added "-B8". For the previous example: 550076o-B8.fits. This binned 8x8 (mean) image is automatically displayed in SAOimage (no choice yet) at the end of each exposure and is intended only as a quick-look tool to let the observer check if something gross is affecting his data (bright objects, moon gradient, guide probe, etc...).

    See below "Selecting & converting image FITS file format" for more information on image format.

    The right field in the GUI gives the current state of the camera, which can be: IDLE, CLEANING, CLEANED, EXPOSING, EXPOSED, READING, POWER OFF.

    Note that since the odometer is accessed by all CFHT instruments, there is no guarantee you will get a sequential set of image numbers during your night as some other instruments might be running for setup and testing during the observing period (specially during daytime).

5: Setting up FITS headers

    The "OBSERVER", "OBJECT" and "COMMENT" FITS header fields can be changed for future exposure(s). The text will be truncated if it exceeds 70 characters. Note that some scripts (such as the dithering scripts) impose the "COMMENT" field but the "OBSERVER" and "OBJECT" fields are set by the observer or the scripts he built himself. Any character is allowed in these fields.

    The command line interface syntax for this procedure is:
    CLI= fits observer CFH12K aficionado
    CLI= fits object NGC 3486 B band
    CLI= fits comment This is the comment

    The Principal Investigator name and Run Identifier must be entered by the Support Astronomer at the beginning of the observing period based on the information available in the observing schedule. The semester must be appended to the Run Identifier. It is coded with 3 digits: the last two numbers of the year (01 for 2001), and the semester: A (Feb-Jul) or B (Aug-Jan)
    CLI= header _piname CFHT
    CLI= header _runid D0601A

6: Setting up and taking exposures

7: Taking twilight flat-fields automatically

    Twilight flat-fields are a crucial part of the CFH12K data set. In the B and V bands they allow efficient flat-fielding without the need for night sky superflats. In the R, Z and I band, they are essential to remove the multiplicative component from the night sky data to derive the additive fringe patterns.

    Twilight flat-fields can be easily acquired at dusk and dawn with the following tool; hence it is strongly advised to always take them even if the observer thinks he has enough of them. However, when cirrus are present in the sky, the twilight flat-fields don't work too well (but are far better than nothing).

    The twilight flat-fields sequencer allows the user to efficiently acquire the maximum number of twilight flat-fields within a limited amount of time. As soon as the proper exposure time is determined by the user with the flux command for a given filter, the sequence can be started. Upon sky brightness evolution laws established for CFH12K for each of the different filters, the subsequent exposure times will be adjusted so that the flux level on all the images of the sequence will be uniform.

    The CFH12K readout time is one minute (including pre-exposure setup, detector cleanup and full mosaic readout), the write to disk time is not included since it is a background task: a new exposure can be started while the previous is still being written to disk.

    Note that the sky brightness evolution laws are for nautical twilight only (astronomical twilight is defined as the sun being 18 degrees below the horizon, nautical twilight as the sun being 12 degrees below the horizon). The total duration of twilight varies slightly from summer to winter but remains very close to 81 minutes (start/end of astronomical twilights time to sunrise/sunset time). To find out when to start the twilight flat-field sequence, refer to the sky calendar at the following address for the current day of observation: and find the fields "twi. end" and "twi. beg" in the Sun columns.

    It is possible to take exposures through various filters for about 30 minutes, hence with a 25-minute brightness limit from start of nautical twilight, this sets a 55 minute limit from the astronomical twilight to start/stop acquiring flat-fields.

    At dusk:
    -> subtract 55 minutes from "twi. end" in the sky calendar to find the time where you can start acquiring twilight flat-fields (sky still pretty bright, good for the narrow band filters, Z & I filters).
    -> subtract 25 minutes from "twi. end" to find the time where you should stop acquiring twilight flat-fields (the brightness law changes slope then and the sky is "dark"). It is then time to go to the field of interest and start a focus sequence.

    At dawn:
    -> add 25 minutes to "twi. beg" to find the time where you can start acquiring twilight flat-fields (sky still pretty dark, good for the B & V filters).
    -> add 55 minutes to "twi. beg" to find the time where you should stop acquiring twilight flat-fields (the sky is too bright).

    Example: September 20, 2000:
    "twi. end"=19:33 -> start flat-fields at 18:38 and stop them at 19:08.
    "twi. beg"=04:57 -> start flat-fields at 05:22 and stop them at 05:52.

    If the reported sky flux in the image does not seem appropriate, the observer can break the sequence and restart it with a revised exposure time.

    Note that you can conduct scientific observations without much impact for 20 minutes before the end of astronomical twilight at dusk, and after the beginning of astronomical twilight at dawn as listed in the Sky Calendar table. This still leaves, for example, about 5 minutes to return the telescope to zenith and be ready for the dawn twilight flat-fields.

    The exposure time in the sequence will increase during dusk flat-fields, and decrease during dawn flat-fields. At the end of the sequence, the user can pick a new filter if the the sky is not yet too dark or too bright and start again by adjusting the initial exposure time first with the flux function. Since the daytime and early twilight sky is darker towards the red wavelengths, always start at dusk with longer wavelength filters (i.e. Z, I, R in that order) and continue with shorter wavelength filters (i.e. V, B in that order) and vice-versa at dawn (B, V then R, I, Z in that order). The narrow band filter twilight flat-fields (Ha, HaOFF, TiO, CN) should be taken when the sky is very bright (still, the sun has to be below the horizon since CFH12K on CFHT is very sensitive). It is possible to take up to 7 exposures for each of two filters at either dusk or dawn.

    A measurement of the sky level on CCD08 and CCD04 (check the flux command description for the rationale on using these two CCDs) is made on each single exposure and provides a direct feedback to the user as the sequence evolves. Checking these values, the observer can decide to break and restart the sequence with a new exposure time if the model does not fit anymore with the current sky brightness evolution. The feedback on the Director window is:

      Sky level on CCD08 =  9714 ADUs (offset = 800 ADUs subtracted)
      Sky level on CCD04 = 11104 ADUs (offset = 540 ADUs subtracted)

    For the bands I & Z where the night sky will produce interference patterns on the CCDs (fringes), it is crucial to take twilight flat-fields when the sky is still very bright such that the contribution from the OH lines (which cause the fringing) is still largely dominated by the twilight sky continuum.

    Typical flat-field flux levels suitable for the different filters are: B=5,000 ADUs, V=10,000 ADUs, R=15,000 ADUs, I=20,000 ADUs. The CFH12K detectors are linear to within 1% up to 55,000 ADUs. Exposure times less than 2 seconds are not advised because the shutter ballistic could affect the large scale structures of the flat-field. Read the important note in the flux command description regarding the large discrepancies in the sky level depending which CCD is looked at.

    In the early phase of dawn twilights, the flux command is not appropriate since the exposure time would be too long. It is advised to refer to the time the flat-fields were taken at dusk after the Sun set and tune an exposure time in a symmetrical way vs. the time the Sun will rise. For example, if the Sun set at 6:20PM and you took a 120 seconds V frame at 6:50PM, you can predict that with a Sun rising at 6:00AM, you can take a 120 seconds exposure at 5:30 AM. After the very first exposure, you have information on the sky level of that exposure and you can then abort or break the sequence if needed to tune the exposure time up or down.

    Example of tf12k with real data taken on the night of the 28th of December 2000: The sun set at 18:02, twilight ended at 19:11, started at 5:37 and the sun rose at 6:46. The following sequence in B was started at 18:28 (after an R sequence started at 18:14): it shows how tf12k scaled the exposure time both at dusk and dawn. The right column gives the flux on CCD08 in ADU. The first dawn flat-field was taken at 6:08, 40 minutes before the sun rose, to match the flux from the last dusk exposure taken at 18:41, 40 minutes after the sun had set.

      Date		       Exp.Time        CCD08
      Dec 28 18:28:32 2000    14.0    	4303
      Dec 28 18:30:17 2000    19.0    	4320
      Dec 28 18:32:06 2000    26.0    	4130
      Dec 28 18:33:53 2000    36.0    	3983
      Dec 28 18:35:56 2000    51.0    	3950
      Dec 28 18:38:25 2000    77.0    	3917
      Dec 28 18:41:44 2000   129.0    	3869
      Dec 29 06:08:53 2000   140.0    	4303
      Dec 29 06:11:25 2000    82.0    	4397
      Dec 29 06:13:28 2000    54.0    	4431
      Dec 29 06:15:19 2000    38.0    	4540
      Dec 29 06:16:56 2000    27.0    	4442
      Dec 29 06:18:33 2000    20.0    	4513
      Dec 29 06:20:10 2000    15.0    	4623

    After taking a sequence in a filter and moving to another filter, a rule of thumb is to start the sequence with the exposure time of the last exposure taken with the previous filter. This usually gives a good match of flux ratio with the recommended values for the various filters (as long as your last exposure was properly exposed, otherwise the flux ought to be used.

    There are different ways one can run the telescope when taking twilight flat-fields: leaving the telescope parked at zenith presents the disadvantage that numerous stripes due to drifting objects will be present along the X axis of the mosaic, making it harder to analyze the sky background. If the telescope tracks, one will get the objects at the same place on the different exposures. The solution is to have the telescope tracking and having the telescope return to zenith between each exposure during the readout, hence the field is different in each exposure. The twilight flat-fields sequencer has been programmed to automatically execute the slew back to zenith. When the sequence begins, a confirmation window will appear and ask you to remind the Observing Assistant to allow these slews.

    To keep the contribution from the objects in the field low, the z focus must be set to a value close to the nominal value for that particular filter. Large doughnuts due to bad focusing are harder to erase when combining the frames.

    Telescope activities (slews, z focus change) do not impact the camera readout, i.e. the signal is not affected. The flat-field sequence parameters are: the initial exposure time, the number of exposures to take during the sequence, and the filter. The system automatically determines if this is a "dusk" or "dawn" twilight sequence. A "Start sequence" button controls the sequence execution.
    CLI= tf12k 13 7 auto B

    This tool only works for the B, V, R, I and Ha filters so far.

    To run the sequence from the GUI:

    • Adjust the number of exposures you wish to take ("Number of exposures" field).
    • Have the exposure time properly set based on the feedback from the "flux" command.
    • Launch the sequence with the "Start sequence" button.
    • Monitor the flux on the exposures as they get written to disk.
    • Break the sequence if necessary to reduce or increase the new exposure time.

8: Focusing

9: Dithering patterns

10: Small telescope offsets

    The telescope can be offset in guiding mode from its current position using this tool. Enter the offset along the East-West and North-South axes in arcseconds and then click on the Offset Telescope button (a confirm window will pop up). Offsets can be as much as 400 arcseconds if the position of the guide star is fine (the guide probe has a limited field of free motion to avoid the vignetting of the CFH12K mosaic and the Observing Assistant should alert you if you've lost the guide star). This command along with the snap command is handy when centering the field on the camera.

    CLI= offset -10 4

11: Selecting & converting image FITS file format

    "FITS file format" Split/MEF FITS format selection

    Open the FITS file format selection window. If the Multi-Extension FITS format is selected (FITS=MEF), for each image a unique FITS file is created with a ".fits" extension. If the MEF mode is off (FITS=SPLIT), the 12 individual CCDs will be saved in independent standard FITS files along with the primary header in an individual subdirectory. In the main image directory, there will also be a binned by 8 image of the entire mosaic.

    Note that as shown in the drawing, the name of the CCDs does not match the extension name in the MEF file. Refer to the map on this page when looking at a particular detector within the mosaic.

    The binned by 8 image has a "-B8.fits" extension. This binned image organizes the CCDs as shown on the drawing with the North towards the bottom and the East towards the right [Reminder: the CFH12K camera can not be rotated]. The relative geometry of the mosaic readout outputs is taken into account so that the image represents the observed field properly. World coordinates are set up correctly to allow the observer to travel in the CFH12K field and locate his objects of interest. Note that the binned resolution is very low (1.6 arcseconds/pixels!) and small objects won't be visible, but this quick-look is intended to be a tool for field recognition, centering, and checking. The high binning factor has the great advantage of enhancing the faint large scale features that would be invisible on the non-binned image. Use this image to detect any contamination problem (nearby bright star light scattering, vignetting by the guide probe, etc.). The binned by 8 image is automatically displayed at the end of each new exposure (that image is named current.fits).

    In MEF mode, the ls command would show the following Directory contents:

      495401b.fits  495401b-B8.fits 495402b.fits  495402o-B8.fits
      495403b.fits  495403b-B8.fits 495699b.fits  495699b-B8.fits
      495700o.fits  495700o-B8.fits
    In SPLIT mode, the result would have been:
      495401b/            495402b/           495403b/  
      495401b-B8.fits     495402o-B8.fits    495403b-B8.fits
      495699b/            495700o/
      495699b-B8.fits     495700o-B8.fits
    With each image Directory containing 12 FITS file and the primary header:
      495700o.phu     495700o04.fits  495700o09.fits  
      495700o00.fits  495700o05.fits  495700o10.fits  
      495700o01.fits  495700o06.fits  495700o11.fits
      495700o02.fits  495700o07.fits  
      495700o03.fits  495700o08.fits 

    It is easy to go from one format to another with the use of the split2mef and mef2split scripts. For example:

      SPLIT to MEF: split2mef 495401b -> will create 495401b.fits (it will not remove the Directory 495401b and files in it though).

      MEF to SPLIT: mef2split 495401b -> will create the subDirectory 495401b with all the files inside (it will not remove the file 495401b.fits).

    You can also convert a whole set of images at once. They need first to be organized in a single sub-directory: use splits2mef and mefs2split scripts. For example:

      SPLIT to MEF: splits2mef /h/cfh12k/images/Night161200 -> will create MEF files from all the SPLIT files found in the Night161200 directory (it will not remove the initial files though so beware of the disk space used!).

      MEF to SPLIT: mefs2split /h/cfh12k/images/Night161200 -> will create the SPLIT images from all the MEF files in the Night161200 directory (it will not remove the initial files though so beware of the disk space used!).

    CLI= mef off
    SC= split2mef 495401b
    SC= mef2split 495402o
    SC= splits2mef DIRECTORY
    SC= mefs2split DIRECTORY

12: Complete set of commands for the command line interface & scripts

    Scripts can be executed from any window opened on the session host Xterm or Rxvt on mahina, or from the Director window using the "!" character prior the script name. They can be as simple as the following examples or more complex as some of the scripts that can be found in the "/h/cfh12k/scripts" directory. Scripts can call other scripts. The user should build his own scripts in a Directory separate from the "/h/cfh12k/bin" directory to avoid confusion (in the existing directory "/h/cfh12k/scripts" for example). In the following examples, the scripts can be executed from the Unix prompt with the following command:

      source scriptname (from a terminal)
      !source scriptname (from Director)
      (the script can also be made "executable" with the "chmod +x scriptname" command and then the "source" command prior the script name is not needed)
      chmod +x scriptname (after editing the script)
      scriptname (from a terminal)
      !scriptname (from Director)

    WARNING: if you want to abort a script, first interrupt it with Ctrl C in its running window and then abort the exposure if necessary. Never abort an exposure before killing the script, it will confuse the system (use either the GUI or the CLI interface to send the break, stop, or abort command).

    Scripts can combine the following commands: (commands "flux", "snap", "tf12k", "focus", "mef" are not appropriate for use within a script of course)

      clicmd go n
        -> take "n" exposures ("clicmd go" for a single exposure).
      clicmd etype t
        -> set "t" exposure type (o=object, f=flat, d=dark, b=bias).
      clicmd etime n
        -> set exposure time to "n" seconds.
      clicmd raster full
        -> set raster readout mode (full, full bin2, full bin4).
      clicmd filter 0 B
        -> select filter 0 (here with B filter at that position), positions are: 0,1,2,3. Refer to the current setup of the CFH12K in the main GUI window to indicate proper name of each filters (the menu will list them top down from 0 to 3).
      clicmd fits observer CFH12K aficionado
        -> set the FITS OBSERVER header for the next exposures.
      clicmd fits object NGC 3486 B band
        -> set the FITS OBJECT header for the next exposures.
      clicmd fits comment This is the comment
        -> set the FITS COMMENT header for the next exposures.
      offset a b
        -> offset the telescope by "a" arcseconds East and "b" arcseconds North.
      pfocus z
        -> set the z of the bonnette (-1.0 < z < +6.0).
      dp12k n (2,3,4,5,6,7,8)
        -> Take a set of n dithered exposures.

    Example 1:
    Setup templates and launch the dithering pattern script with 8 positions

      climcd etype o
      clicmd etime 600
      clicmd raster full
      clicmd fits observer CFH12K aficionado
      clicmd fits object NGC 3486
      dp12k 8
    Example 2:
    Take a calibration field through 4 filters, 1 exposure per field
      climcd etype o
      clicmd etime 10
      clicmd fits object SA110
      clicmd filter 0 B
      clicmd fits comment B filter - 10 seconds
      clicmd go 1
      clicmd filter 1 V
      clicmd fits comment V filter - 10 seconds
      clicmd go 1
      clicmd filter 2 R
      clicmd fits comment R filter - 10 seconds
      clicmd go 1
      clicmd filter 3 I
      clicmd fits comment I filter - 10 seconds
      clicmd go 1
    Example 3:
    Take 3 exposures with an offset between each one
      clicmd fits comment Ref position
      clicmd go
      offset 10 4
      clicmd fits comment Ref position + [10"E + 4"N]
      clicmd go
      offset -20 6
      clicmd fits comment Ref position + [10"W + 10"N]
      clicmd go

13: The engineering form

    WARNING: do not use the engineering form if you are not a member of the CFHT staff!

    Engineering functions:
    • The "Power Supply Control" section deals with the commands to power up and down the CFH12K camera when it needs to be installed in the prime focus cage (MAIN POWER ON/OFF). The "ACE ON/OFF" is the command to use when the shutter needs to be replaced (don't power off the whole camera just for that).
    • The "Software/Hardware Setup" section deals with the computer facilities. The host is the acquisition machine: metallica is the main host (PC using the PCI cards) and paniau is the spare host (Sparc20 using the IfA custom fiber interface box). The BayTech is the control system handling the AC power connections in the prime focus cage.
      The "Running Options" section deals with the default options of the acquisition system. By default they should all be turned ON (selected).
    • The "Observing Run" section deals with the Principal Investigator name and the Observing Run identificator for the current observations. Refer to this section on the rules regarding the selection of these fields.
    • The "Filters" section allows the CFHT staff to set the proper name for the filters that have just been installed in the filter wheel. It is crucial to have these fields properly set at all time (do it immediately upon return from the prime focus cage, it is always good to write the setup on paper when still present in the cage to avoid any confusion 10 minutes later when selecting the names vs. position on the form). Don't confuse these widgets with filter selection. To install a filter in the wheel at a given position, select the filter (using the main GUI, the filters are ordered from position 0 to 3 when you click on the selection field, or you can also access the information from the command line interface using the "filter" command) on the opposite position in the wheel to place the filter that needs to be changed in the service port.
      • To remove/install a filter in slot "0", select the filter in position 2.
      • To remove/install a filter in slot "1", select the filter in position 3.
      • To remove/install a filter in slot "2", select the filter in position 0.
      • To remove/install a filter in slot "3", select the filter in position 1.

Jean-Charles Cuillandre, John McDonald
Canada-France-Hawaii Telescope Corporation
   Comments on the CFH12K pages to Jean-Charles Cuillandre:
   CFH12K is the CFHT CCD wide-field imaging mosaic.