MII/LDSS3 Observing Procedures

MII/LDSS3 Observing Procedures
Date of last modification: July 28 2011, 4:40 pm Chilean time

Complete instructions on how to operate LDSS3 in longslit mode can be found here selecting 'LDSS3', then 'LDSS-3 Observing Cookbooks' and then 'Long-Slit Spectroscopy'.

1. Accessing Data Acquisition Computer (Guanaco)

Username: obsmag2a or obsmag2b (check with the staff)
Password: check with the staff
Open an xterm window
cd /home/obsmag2b/data (e.g.)
mkdir 20020914_15 (e.g.); in CCD Camera GUI "DataPath" should point to this directory (see below).

2. Start Control Windows in Guanaco

The control software is started by typing 'ldss3' in a Guanaco xterm (you will ususally not need to do this because the instrument specialist will have the system initialized for you).
This will bring up the LDSS GUI from which the aperture, filter and grim wheels are moved by holding the left mouse button on the circle to the right of the wheel's name.
The filters and grisms are always mounted in the instrument and their names should not be changed.
The CCD startup window is started by a script called "ldss3" (again, it will usually be ready for you to start working). The "observer" field there will be updated with your name. Clicking OK there, will start the CCD Camera GUI.
In the CCD Camera GUI make sure that the "DataPath" found in the "Options" pull-down menu points to the directory where you want the data to be stored.
Readout speed should be set to "slow".
In the LDSS GUI you can adjust the focus (enter lDsS3 as password) to the nominal value for the r' filter at the current temperature in order to fit the list fixed on the wall (if in doubt, check with the staff). You should first set it to 2100 and then go down to the desired value.
Check that the long4_s (0.75", blue coverage) and the LS15.2S (1.5", blue coverage) slits are in the aperture list.
If the Quick-Look Tool has not been started, go to the "Options" pull-down menu of the CamGUI and click there to start it.

3. Subrasater

The chip is read in a 2-amplifier mode, so each exposure will be stored in a 2 different files: ccd????c1.fits and ccd????c2.fits. In order to set the subraster in the CCD Camera GUI you have to enter, for each of the two files (c1 and c2) the Xc Yc (X and Y of the center) the width (W) and height (H) of the subraster. The subraster menu calls them: chip 1 and chip2.
- chip1 1017 2100 2032 1801
- chip2 1017 2100 2032 1801
- Then click apply and save.
This will set the subraster to 1:2032; 1200:3000.
You may prefer to edit a file, for example, ldss3_csp.sub as follows:
1 1
1 1017 2100 2032 1801
2 1017 2100 2032 1801
Currently there is a predefined subraster suitable for long slit spectroscopy. It is called 'spec.sub'. You can ask the instrument specialist to help you in loading it.
Leave the "save mode" to "full", which will fill with zeroes the areas outside the subraster.

4. IRAF ldss3 package

Open an IRAF window and go to the working directory (20020914_15)
Load the 'ldss3' package
Open ds9

5. Check the slits and calibration lamps

Take a short (5s) exposure of each of the two slits.
Run 'lstitch' on the image of each slit. As a parameter you have to enter the root name of the slit image (e.g. ccd0013; if the corresponding files are 'ccd0013c1.fits' and 'ccd0013c2.fits') and the task will produce a combined file ('ccd0013.fits' in this example).
Do a display on the stitched image, and use imexamine to measure the approximate position where the targets will be placed, avoiding the defective region near x=3900 y=1870 (remember these coordinates refer to the stitched image) of chip1:

By choosing, for example, slit coordinates close to (2667, 2127) the objects will be placed on the slit in a clean region between two gaps. Now in the IRAF window edit the parameters for the task 'ltoslit' and fill in the slit coordinates you measured. These coordinates will also be required in order to run 'lcobject'. The slit position will move slightly along the night, thus a new slit image should be taken before each observation (lcobject will stitch that image and let you mark the exact position you want for the object).

Bring up the flat field/calibrations GUI (if it is not already there) by typing "ffs" in an xterminal of Guanaco. There you can put the screen in and out by clicking in "go in" and "go out" and turn on/off the comparison and quartz lamps.

SLITS: We will use the 1" slit with blue coverage (LS10.2s) as narrow slit, and the 1.5" LS15.2s as wide slit. If the seeing is very good we may want to use the 0.75" slit (Long4_s) instead. Remember to take dome flats for each slit that has to be used.

Set the VPH_All grism on and the narrow slit and take a 5s exposure of the He-Ne-Ar lamps.

If you plan to use the VPH_Blue grism, put it on and take a 20s exposure of the He-Ne-Ar lamps.

Do the same with the VPH_Red grism plus OG590 filter using an exposure time of 1s (some of the lines will be saturated but the spectrum will be suitable for wavelength calibration).

Since the VPH_All grism is available, we only occasionally will want to use the VPH_Blue and VPH_Red.

HeNeAr lamps should be always observed with the narrow slit.

6. Calibrations

Use the Qf quartz lamp for all the dome flat exposures.
Dome flat VPH_All: Flat field screen 'in'. ExpTupe=flat,, loops=7 (during twilights). Check that the dome flats are not saturated; otherwise decerase the exposure times. ExpTime=10s (narrow 0.75" and 1.0" slits) and 6s (wide 1.5" slit). Better than that, if possible, is taking 3-4 flatfield exposures at the position of each supernova; this will allow the right flatfielding at the red end of the spectrum.
Dome flat VPH_Blue: Flat field screen should be 'in'; ExpType=flat, ExpTime=40s, aperture=0.75" slit, loops=7 (during twilights). Take another sequence with the 1.5" slit, with ExpTime=20s.
Dome flat VPH_Red + OG590 filter: Flat field screen should be in; ExpType=flat, ExpTime=8s (4s for the 1.5" slit), loops = 3; after each VPH_Red comparison lamp.
Take a 10 bias sequence: set ExpType= bias; this will automatically set the exposure time to 0s. This can be done during the afternoon.

7. Object list

Your telescope operator will need a catalog with the selected objects for the night (to which you can add new targets at any time).
You should edit it following the instrucions given here
In that file, fields #8 (instrument rotator offset angle) and #9 (instrument rotator offset mode) should be set to -62.0 and HRZ.
This will set the slit along the parallactic angle.

8. Skyflats

SLEW telescope to the East
In CCD Camera GUI set ExpTime to 5 (minimum), object to "Sky_Flat", Filter=i' Aperture=Open, Grism=Open
In CCD Camera GUI set ExpType to "flat"
Start exposure with START botton
After completing the exposure slew telescope by 20 arcsec (in any direction)
Take 5 frames per filter (i', r', g') with exposures longer than 5 sec and never exceeding 28000 adu

9. Observing

Once the telescope operator has slewed the telescope to your object run 'ltoslit' and answer 'yes' to the questions. This will offset the telescope so the target ends up near the slit position.
In LDSS GUI set Filter=g' Aperture=Open, Grism=Open
In CCD window set ExpType to "object"
Take direct image of the field
Take a short exposure of the slit
Edit parameters for 'lcobject' and enter the root names for the field and slit images, the approximate x and y slit coordinates, as measured before on the stitched image of the slit, and the slit width in arcsecs (0.75 or 1.5). Use 2000 pixels for the size of the region to be displayed ('length' parameter), which will display most of the image.
Run 'lcobject'. You will have to mark the desired position on the slit and then the position of the object; then answer 'yes' in order to offset the telescope.
This is an iterative process. In succsesive runs you don't need to redisplay and measure the slit position for the same object; the task gives you the choice of skipping this stage (answer = 'yes'). Normally, the second time you run lcobject the object will lie within the slit.
To make sure that this is the case you can take an exposure to see the object through the slit.
In LDSS GUI set Filter=Open/OG590, Aperture=the appropriate slit, Grism=VPH_All/VPH_Blue/VPH_Red
In CCD Camera GUI set ExpTime, object to "object name", and click START to begin the exposure
To take a comparison move screen into place, click to turn on the He-Ne-Ar lamps, and take the arc exposure (5s for VPH_All, 20s for VPH_Blue and 1s for VPH_Red) (don't forget to remind the NA to disable guider)
If you will take a spectrum with the VPH_Red grism you must have the OG590 filter in, and you will need 3 dome flat exposures after the comparison lamp, 8s each, with the Qf lamp on.
Move screen back, turn OFF the lamps; and ask the telescope operator to go to the next object.
Take a minimum of 2 flux standards (3 is best) with the wide slit and don't take comparison spectra for those.
Observe one telluric standard with narrow slit and take a comparison lamp after it. Make sure you get high s/n in this data without reaching the saturation.

nmorrell@lco.cl