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LDSS3 Long Slit Spectroscopy

Daytime Preparation:

  • Confirm with the Instrument Specialist that LDSS3 has been setup with the masks, filters and any other "Special Requirements" noted in your Instrument Setup Request Form that was submitted a month in advance of your LDSS3 observing run.
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  • The instrument should already have been setup with the GUI running on one of the observer workstations. If the LDSS3 GUI is not running (or, if it stops at any point) simply open an xterm and type "ldss3". With the GUI running, take a few minutes to familiarize yourself with the controls and exercise the components (filter changes, aperture mask changes, grism changes). You should confirm that the filters you require are installed and that the instrument is properly focused. While the Instrument Specialist should have verified the focus as part of the daily setup, it is always a good idea to at least verify that the focus values are near the nominal value (standard filters have a focus range of ~900-1000 at about 10C) and that as you change the filters the focus values are properly updated. Note that focus changes tend to be executed more reliably
    when you INCREASE focus numbers than when you decrease them. [Note: the focus values vary as a function of temperature, but the variation is not normally measureable over a range of only a few degrees C that is typical for an observing night - In the future, the focus offsets for temperature will be automated in the instrument control, however, for now, the Instrument Specialists have measured a linear fit to this variation and can offset the focus value to the appropriate value for night time operating temperatures.]
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  • In order to take some afternoon exposures of slit masks or daytime calibrations, you will need to have the Instrument Specialist prepare the telescope. The mirror covers will need to be opened and you should confirm that the tertiary is properly facing the West Nasmyth instrument port. Next you may start the Flat Field Screen gui by typing "ffs" in an xterm. Once the gui appears (and stops flashing 'Wait'), click the mouse on the "IN" button to put the screen into the telescope beam. In order to prepare for target acquisition and setup during the night, you will need to take images of each of your long slit masks. Make sure that if you have the subraster set to use the ldss3.sub region (this subraster covers the exposed portion of the CCD and should be used in all cases), your subraster is writing in FULL mode as this is required for all of the IRAF field acquisition software. Set the binnning and speed to match what you will use for science observations. For mask images there will probably be sufficient ambient light in the dome that you can simply take a 10 second exposure to get a useable image of the slit mask.

    Repeat these instructions for each slit mask you will use during the night/run. Once you are finished, turn off any lamps you may have used, click the mouse on the "OUT" button to take the flat field screen out of the telescope beam, and you can ask the Instrument Specialist to close the mirror covers. To take a sequence of bias images, in the LDSS3 GUI, set Loops to the number of exposures you wish to take, ExpType to "Bias", and the Binning X and Binning Y values to whichever values you plan to use at night, and then click on the "Start" button to start the loop sequence.

     

  • The afternoon is also a good time to start IRAF on the observer workstation and load the LDSS3 packages by typing "ldss3" at the 'ecl>' prompt. Take some time to familiarize yourself with the LDSS3 packages, in particular "lstitch" is useful for all observing modes as a means to stitch together the two FITS files that result from a standard readout of the dual-amp CCD. The resultant stitched image will display the full CCD in the orientation matching that of the LDSS3 quick look tool.

    Now would be a good time to go ahead and run lstitch on each of your slit mask images. Then display the resultant image and measure and note the pixel coordinate of the slit position that you will want to use for your target(s) during the night. Record these values as they are required input for the 'ltoslit' routine that you should use for initial target acquisition.

    Note: If you experience a large difference between the two amplifier when displaying the stitched image, change the bias region in the parameter file to a smaller line range (i.e. from [2033:2160,800:3490] to [2033:2160,800:3000])

     

  • If you have not already done so in preparation for your observing run, you should be sure to prepare your Observing Catalog File
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Nighttime Operations:

  • Have the TO slew to the object that you wish to observe from your Catalog File. As soon as the telescope has slewed to the position of the object and the rotator has reached the desired rotator angle, run "ltoslit" with the X,Y position of the slit recorded from the afternoon. Once at the offset position, the TO can set up on a guide star and an off-axis S-H star.
  • You should now be ready to image and align your first target. In the LDSS3 GUI, set the Filter to the desired position, make sure the aperture and grism positions are "OPEN", set ExpTime to 10-30 seconds depending on target magnitude and Loops to 1, and then click on the "Start" button to begin the exposure. Next take an image of the field with longslit installed (this is required to allow a precise measurement of the slit position when installed at the rotation angle of your observations). When the exposure is finished you will need to run the 'lcobject' routine to center the target on your desired slit position.

    Type "epar lcobject" in IRAF. Set the "image" parameter to the name of the image of the field and the slit image parameter to your slit image. Then enter the CCD coordinates of the slit (these can be your recorded values as measured on the stitched image in the afternoon - you will be able to compensate for the effect of instrument flexure between daytime and nighttime slit positions in the following steps) in the "xslit" and "yslit" parameters and enter an appropriate box size for extraction from the stitched image in the 'length' parameter (this is the size - in pixels - of the box to extract around the X,Y slit position) This means that when the images are displayed in the next step you will be aiming for the center of the extraction box. As an example, a reasonable extraction box size is 500 - your goal will be pixel [250,250]

    When you run "lcobject", you will be queried if you want to skip setting the slit image - accept the default "no" and proceed to mark the slit position in the extracted and displayed image. Next you will be asked if this is the first acquisition on target (this is important for deciding where the image subsection will be extracted from a stitched image - if you reply "yes" it will extract at the center of the field where the telescope should be pointed to your target only if you did not opt to run 'ltoslit' prior to 'lcobject'; if you reply "no" it will extract the same area of the field image as the area extracted for the slit image - this works for the method described here and any minor tweaks to the alignment in subsequent iterations of 'lcobject'). The image section of the field will be displayed and you will be asked to mark the object which you wish to center in the slit. If you hit the "space bar", the coordinates of the object center will be calculated using marginal distributions. This is appropriate for stars or galaxies with compact nuclei. For diffuse objects, the "m" key can be used to manually mark the position which should be centered in the slit. When you've finished, the program will output the required offsets to put the object on the desired slit position.

    You will now be asked if the calculated offsets should be sent to the TCS. Answer "yes" and you will be asked if you want to do a "coordinated offset". Answer YES to this question. Let the night assistant know you will do the offset.

    Our experience is that for offsets of a few arc seconds, the object will be at the appropriate place without further iterations. If the offsets are big, then you should perform another lcobject command (and you should also check the values that you input to the previous 'ltoslit' command), but this time you don't need to measure the center of the slit.

    Select the appropriate longslit aperture and grism, check that you have the appropriate gain setting and subraster (if desired) and start integrating.

  • Following spectral observations when you want to make comparison lamp and flat field observations, you will first need to notify the TO so that the guiding and S-H corrections are stopped. You can then move the screen in, turn on the He-Ne-Ar lamps, and take the arc exposure. For the red grism it is recommended to take at least 3 flats at the object position (to eliminate the fringing). Use the Qf lamp.

 


Updated on 18 February  2008   rcovarrubias at lco.cl

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