This section presents the Gecko Session and how to use its different forms to control the instrument and take exposures:
NOTE: you may click on some of the images to get a better quality or
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.
The Gecko Session is started by entering the Login and Password. When the Gecko Session is ready, users see:
In this very informative window, you will find:
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
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.
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.
|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.
To perform observations on the sky after calibration exposures, click
on the "Move to Sky" button to make the CAFE wheel move to an open
The halogen flat field lamp can be used with different levels of intensity. The observers are reminded that even a halogen lamp may show spectral features if used at high intensity or if very hot. Therefore, flat field exposures should be checked to insure that no lines are present.
If the flat field lamp is set to the default "Auto" mode then when an
exposure is started from the "Expose" form with the "flat" exposure type
selected the lamp will automatically turn on before the exposure is
started. After the exposure the lamp is turned off. During spectrograph
setup or if, as is often the case, a series of flat field exposures is
required the lamp should be turned on manually by selecting the "On"
option. Obviously the lamp should be returned to the "Auto" mode when
A Th/Ar lamp is offered for spectral calibration.
If the comparison lamp is set to the default "Auto" setting then when
an exposure is started from the "Expose" form with the "comp" exposure
type selected the selected comparison lamp will automatically turn on
before the exposure is started. After the exposure the lamp is turned
off. During spectrograph setup or if a series of comparison arc
exposures is required the lamp should be turned on manually by selecting
the "On" option. Obviously the lamp should be returned to the "Auto"
mode when finished.
Use this selection box to toggle between an open or closed state for
the slit shutter located just behind the image slicer or spectrograph
slit. This is normally only used when taking "room darks."
It is possible to install three different exposure meter pellicles on
the exposure meter pellicle slide. The Exposure Meter is fed by
reflection from one of those three pellicles/filters located behind the
slit. The "clear" pellicle is used in most of the cases; the other
options offered are filters used to feed the E.M. and also to cut some
extra-orders that overlap the order of interest when using a grism in
the red. It is important to select the "clear" pellicle and not a filter
if no extra orders are present.
Three exposure meter filters may be installed on the exposure meter
filter slide, to get a system response peak close to the observed
wavelength. The desired filter is set when the 'gs' script is used, or
can be changed with this selection of the "Gecko Configuration" form.
To protect the exposure meter photomultiplier tube, always ensure
that the exposure meter shutter is closed by using this option to close
the shutter before you enter the spectrograph room or the slit room. In
normal operation the exposure meter shutter opens automatically just
before an "object" exposure type begins and then closes again at the end
of the specified exposure time. In most cases, however, you will want to
open the exposure meter shutter manually once you have positioned a
target on the slit or slicer entrance slot so that the signal reaching
the exposure meter can be maximized before the exposure is actually
The Filter Wheel section of the Gecko Configuration form is used to
choose an order sorting filter, or remove a filter from the optical path
to the detector. The 'gs' script moves the appropriate filter in
place. Otherwise, simply select the desired filter from the list of
filters available in this drop down box.
For blue wavelengths especially, the observer will need to use one of
the three variable grism units to isolate the order of interest. The
'gs' script moves the appropriate grism in place, if
necessary. Otherwise, choose the appropriate grism from this selection
box. If fine-tuning of the grism rotation is required to center the
spectrum on the detector refer to the "Grism Alignment" form.
Various parts of the echelle grating mosaic can be masked with the Hartmann Mask assembly. Remote control of the mask is possible with this item. When the focus is checked, the "1+3 OPEN" and "2+4 OPEN" configurations will be chosen automatically during the sequence. The software automatically ensures that the "ALL OPEN" option is selected and that all Hartmann masks are moved out of the beam before an object exposure is initiated.
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.
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.
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"
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:
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.
A flat field exposureThe individual slices can be seen.
A flat field exposure with a grism usedNote the other orders to the right and left of the central order.
A Th/Ar lamp exposure