CFHT, Instruments, Spectroscopy, Gecko: The Coudé f/4 Spectrograph

Web Manual for Gecko - The Gecko Session

This section presents the Gecko Session and how to use its different forms to control the instrument and take exposures:

  1. Starting the Gecko Session
  2. Optimum use of the 3 screens
  3. The 'gs' script
  4. The Gecko Controls Window
    1. Gecko Configuration
    2. DetCom Camera Control
    3. Focus/Align Tool
    4. Grating Rotation Calculator
  5. How to use the Grapher
  6. Examples of calibration exposures viewed with SAO Image
  7. Gecko Observing Checklist

NOTE: you may click on some of the images to get a better quality or bigger image.

The Gecko spectrograph and CAFE unit are both remotely controlled from the Control Room. The Gecko Session runs on a 3-screen display. The login and password will be given to the observers by the Support Astronomer at the beginning of their observing run.

1. Starting the Gecko Session

The Gecko Session is started by entering the Login and Password. When the Gecko Session is ready, users see:

2. Optimum use of the 3 screens

To make an optimum use of the 3 screens available, we suggest using the middle screen for Gecko and Camera control, opening Terminal windows in the rightmost screen for the 'gs' script, and putting the SAO Image and Grapher in the leftmost screen. Before your first exposure, click on Image to open the current image (with file name current.fits) in SAO Image; click on Graph to get a plot of the (same) current image in the Grapher window; both SAO Image and Grapher should open in the leftmost screen.

3. The 'gs' script

To change the wavelength of observation, use the gs scripts in a terminal window, using the following syntax: gs r=XXXX, for example, gs r=4845. This script is prepared during the setup according to the wavelengths requested by the observers. The right filter or grism is moved into place; the detector is moved in the Z direction to do the focus; the grating angle is properly set. While the script is executed, feedback is provided in the terminal window.

4. The Gecko Controls Window

Click on the Gecko Controls button of the top Menu to open the Gecko Controls Window.

The Instrument Status picture will give you the current status of Gecko (and not the detector): Idle or Busy (when a Hartmann Mask is moving during a Focus Sequence, a filter is changed, a grism is moved into place, etc.)

Two forms are usually kept open during the whole night:

A few times during the night, you may use:

The last two forms, Engineering Use Only and View Setup Values are used by the staff only.

4.1 Gecko Configuration

The Gecko Configuration form should normally be opened at the start of an observing session and left open throughout in order to monitor and change the current spectrograph configuration. Each component of the spectrograph has associated with it a set of icons which graphically illustrate the current option selected. The settings on the form are updated dynamically whenever a process, such as beginning an exposure with the "Expose" button, changes the spectrograph configuration. The form therefore also acts as a status display for the spectrograph.

Settings can be changed by choosing an option from the list box of possible selections that appears when the cursor is placed on an option box with the left mouse button held down. Usually, all the necessary changes are made automatically when the 'gs' script is used to setup a wavelength, and the observers should not have to change anything by hand.

4.2 DetCom Camera Control

This window is used to set information that will go in the FITS header (PI name and run ID, Observer, Object Name and Comment if desired), select the exposure type (Object, Flat, Bias, Comparison, or Dark), desired exposure time in seconds and the number of times this exposure should be repeated (iterations). With the coudé f/4 spectrograph a "dark" exposure is actually a room dark. During the exposure the detector shutter is open while the slit shutter is closed.

Click on the Go button to start the exposure. To abort (stop but not read) an exposure, use the Abort button. To stop (and read) an exposure, use the Stop button.

If the Exposure Type is Object but a lamp is ON, an error message saying "slit_emshtr caused duck error E_Hazard" will appear, and no exposure will be taken.

4.3 Focus/Align Tool

The Focus Tool is used to check the focus at the beginning of the night or every time the wavelength has been changed. Select the comparison lamp and choose the ON option. To speed up things, you may use a smaller raster. In the Step 1 box, enter the number of seconds for the exposure, and the column number where the grapher cut should be made (this value is half the size of the raster in the X direction, e.g., 100 for a 200 by 4500 raster). After clicking on the Exposure & Graph button, two images will be taken with the Hartmann masks 1+3 open, and then 2+4; the masks are moved automatically, and their status is indicated will the Hartmann now icon. At the end of this automatic sequence, the Grapher is updated.

Select a line near the center of the detector, and measure the position of the centroids and FWHM. A good focus shows a difference of centroids smaller than 0.1 px, and a FWHM of about 2.8 px (lines are thinner at the blue end of the spectrum, and fatter at the red end). If the centroids differ in position by more than about 0.1 px, use the Step 2a box to enter their values, and click on Calc & Move det_z to calculate a better focus value and move the detector to that position. After the move has been performed, check the focus again by using the Step 1 box again.

If the exact Z position of the detector is known, it can be entered in the Step 2b box instead of using the Step 2a box, but this is rarely done.

Here are examples of an out-of-focus exposure and a focussed exposure.

4.4 Grating Rotation Calculator

The most common use of this form is to enter a desired wavelength, order, and setup constant, click on the radio button for the grating rotation, and then select "Re-calculate." The required grating rotation angle (in units selected in the list box) is then returned and the grating can then be rotated by clicking on the "Move Grating" button.

5. How to use the Grapher

The grapher is used to plot spectrum:

To see intensity as a function of the wavelength, chose a cut along the Y axis. The file name for the current exposure (last one) is current.fits; the file name for the previous exposure is previous.fits. The cut can be averaged (but not summed) over a few pixels.

To see the slices produced by the slicer, chose a cut along the X axis.

Up to 3 spectrum can be plotted at the same time. It is possible to zoom into a region, and get statistics on the lines.

To get the centroid and width of a line:

  1. Use Left-Click on the mouse to set 2 positions on each side of the line
  2. Use Right-Click on the mouse to do zoom area
  3. If necessary, do a Left-Click on the mouse again to refine 2 positions on each side of the line
  4. Use Right-Click on the mouse to do area stat (which gives centroid and FWHM). The information will appear in the lower-right window.
  5. Use Right-Click on the mouse to zoom1 and get the whole spectrum again.

If the printing option does not work, click on the desktop's background to get a menu, select the print summit option, and then click on the window you wish to print.

Click on Zero Bias to get the Y axis to go from 0 to the maximum value.

6. Examples of calibration exposures viewed with SAO Image

A flat field exposure

The individual slices can be seen.

A flat field exposure with a grism used

Note the other orders to the right and left of the central order.

A Th/Ar lamp exposure

7. Gecko Observing Checklist

  1. Setup your wavelength, using the 'gs' script.
  2. Check the focus of the spectrograph: check that the Exposure Meter shutter is closed, turn the Th/Ar lamp ON, choose a small raster, and use the Focus/Align Tool; reach for a difference of centroids smaller than 0.1 px
  3. When the spectrograph is focussed, put the Th/Ar lamp in AUTO mode, select a big raster
  4. If needed, take Flat Field exposures (make sure the E.M. shutter is closed)
  5. If needed, take Th/Ar exposures (make sure the E.M. shutter is closed)
  6. Give the coordinates of your object to the OA
  7. Click on the Move to Sky button to enable the CAFE TV camera
  8. Ask the OA to start the guiding
  9. Open the Exposure Meter Shutter
  10. Check the telescope focus, by maximizing the counts on the Exposure Meter
  11. Enter Object, Comments, integration time, number of exposures... and GO
  12. After the exposure, the Exposure Meter Shutter closes automatically; if it doesn't close it manually
  13. If needed, take Flat Field exposures and/or Th/Ar exposures; warn the OA that s/he will loose the guiding
A more detailed Memo for observers is available. See the Observer's Corner on the main Gecko page.

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