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LDSS3-C multi-slit spectroscopy cookbook

Observing with a multi-slit mask

Note: The software used at the telescope for aligning LDSS3 masks requires the existence of two special files per mask (referred to in this cookbook as ".align" and ".sub" files). There are two different ways to generate these files. This cookbook describes a method which uses only IRAF. The second method, which uses the COSMOS data reduction package is described elsewhere.


Daytime Preparation:

Make sure that the Instrument Specialist has loaded the multi-slit masks that you will use that night. Remember that observers are required to submit mask files at least 6 weeks before the date of observation in order to insure that the masks will be ready for their run.

 If the LDSS3 GUI has not already been started by the Instrument Specialist, open an xterm, type "ldss3 ", and follow the instructions in the "LDSS3 Control Software" section of the LDSS3 Manual.

In the afternoon, before starting daytime calibrations, make sure that the Instrument Specialist has prepared the telescope. This includes opening the mirror covers and having the tertiary mirror facing the West Nasmyth instrument port.

Now you should take an image of each multi-slit mask you intend to use during night. Deploy the Flat Field Screen (FF-Screen) using the DCU/Clay GUI (see manual). Once the GUI appears, deploy the screen into the telescope beam. Next, turn on the Quartz lamp by clicking on the Ql button. The Slit Mask must be set to the position with the mask that you intend to use, while Grism and Filter should both be set to "Open". Now, set ExpTime to "5", Loops to "1", and ExpType to "Object". Finally, set Binning X and Binning Y to the binning which you wish to use for the mask alignment process, and then click on the "Start" button to take an exposure.

Open an IRAF xgterm and the ds9 image display tool, and type "ldss3" at the CL prompt to load the LDSS3 packages. To edit the parameters of the "lcbox" task, type "epar lcbox". These are as follows:


lcboxv2.png

Fig.1: epar of the IRAF task lcbox  

For the "mask" parameter, enter the root name of the multi-slit mask image you took in the previous step. The "output" parameter should be set to the root name (usually the name of the mask or the field) that you want the .align and .sub files to have. Next, make sure that the "sz" parameter is set to the size (in pixels) of the alignment holes as measured on the multi-slit mask image (this should be measured on the image, here we put 25pixels as an example). The "subraster" parameter can be left at its default value of 100.

While still in "epar" mode, type ":go" to execute "lcbox". One by one, the individual CCD chips of the multi-slit mask image will be displayed, and you will be asked to mark the approximate positions of the alignment boxes. To do this, center the cursor on the box and hit "space bar". If you wish to change the minimum and maximum data values mapped into ds9, hit the "d" key. If you wish to abort, hit the "I" key. Once all the boxes on a chip have been marked (or if there are no boxes on this chip), type "q" to go to the next CCD. The task will end after cycling through all of the chips.


lboxIRAF2.png

Fig.2: a screenshot of the reference box selection in the IRAF task lcbox  

The "lcbox" task produces two files. In this example the "output" parameter of lcbox was set to "2030C", thus the files will be named "2030C.sub" and "2030C.align". The .align file will be needed during the night to carry out the alignment of the mask on the field. The .sub file is used for running the mask alignment process in subraster mode, which cuts down significantly the time required to carry out an alignment (Note: as of May 2017 the subraster mode presents read-out problems and should not be used)

Run "lcbox" on the remaining multi-slit mask images.

Flats: Take a sequence of flat field exposures with all multi-slit masks that you will be using. The telescope mirror covers should still be open and the Flat Field Screen in the "Deployed" position. The Slit Mask should be set to the position of the mask to be used, Grism to the grism that will be used and Filter to “Open”. In the LDSS3 GUI set the ExpType to “Object”, the ExpTime to a value that gives ~30000counts (a reference table for exposure times for some standard configurations can be found here) and all your settings (binning/speed/gain) to the values you plan to use for the night. Click on the “Start” button to take an exposure. Look at the image and adjust exposure time if necessary. Now take a sequence of flats by setting “Loops” to the desired number of exposures (suggested number ~10) and click on the “Start” button to begin the sequence. When the sequence is over, turn off the lamps and “Retract” the Flat Field Screen. Take flat images for all the masks to be used during the night.

Bias: In the LDSS3 GUI, set Loops to the number of exposures you wish to take, ExpType to "Bias", and your settings (binning/speed/gain) to the values you plan to use at night. Click on the "Start" button to start the loop sequence. Although LDSS3 is reasonably light-tight, bias images should be taken with the dome as dark as possible.

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



Night-time operations:

Have the Telescope Operator (TO) slew to the object that you wish to observe selecting it in your Observing Catalog that should include the desired rotator mode and offsets. As soon as the telescope has slewed to the position of the object, have the TO set up on a guide star and an off-axis S-H star.

First take an image of the mask (an exposure time of 10sec on sky should be sufficient).

Next, take an image of the field. To do this, go the the LDSS3 GUI and make sure that all your settings are correct (binning/speed/gain). The Slit Mask and Grism must both be set to “Open”, Loops to “1” and ExpType to “Object”. Set ExpTime to an appropriate value so that the reference stars will be visible (if reference stars are well selected the ExpTime should not be much longer than 60sec in good conditions). Click on the “Start” button to begin an exposure. The orientation of N and E will be indicated in the QL-Tool when the exposure is read out. Make sure that this is consistent with your expectations for the mask.

Edit the parameters of the IRAF ldss3 task "lalign". Set the "mask" parameter to the root name of the mask image taken earlier in the night; the "field" parameters should be set to the root name of the field image just taken. Set the "align_file" parameter to the name of the .align file that you produced in that afternoon. The "inter" parameter is normally set to "no", but if you wish to interact with the alignment solution (which is performed with the iraf task "geomap"), set this parameter to “yes". Set the size of the alignment boxes in pixels as measured in the mask image taken earlier.


lalign.png

Fig.3: epar of the IRAF task lalign


Now run “lalign". One by one, the expected position of the reference stars will be displayed in the "ds9" tool, and you will be asked to mark the star. Use the "space bar" to calculate the centroid, or use the "m" key to mark the position manually. If you wish to change the minimum and maximum data values mapped into ds9, hit the "d" key. If you wish to abort, hit the "I" key (do not try to abort using Ctr+c). Usually the software will suggest the brightest star close to the slit as the reference star (green circle in Fig. ?). If this is not the star you have actually used (a finding chart could be useful in this step), mark the correct star either using “space bar”, if it is bright enough, or “m”, if it is not clearly visible and you need to centre it manually. Once you've marked each of the reference stars, a solution is calculated and you will be offered the chance to eliminate discrepant stars. A final solution for both a rotation offset and an offset in RA and DEC is then calculated, and you will be asked if you wish to apply these offsets. Usually it is o.k. to reply "yes". Make sure you also reply "yes" to the question of whether you want to do a coordinated offset (which moves both the telescope and the guide probes), but let the TO know before actually executing it.



exampleLalign.png

Fig.4: a screenshot of the reference box to reference star pairing process of the IRAF task lalign  

If the offsets are large (>5”), it is advisable to take a new image of your field and repeat the “lalign” procedure on this new image.  


lalign output.png

Fig.5: A screenshot of the IRAF task lalign output 

Once you are satisfied with the alignment residuals (<2 pixels) take an image of the object with the mask inserted in the beam. Run the IRAF ldss3 task "lfalign"(Note the "f" in "lfalign" which distinguishes this task from the "lalign" task discussed in the previous section). The parameters for "lfalign" are identical to those of "lalign" except that only one image, "mask", is requested and the through-mask image just obtained should be used. Also the star FWHM in pixels should be provided, after measuring it on the field image used for "lalign".

 

lfalign.png

Fig. 6: epar of the IRAF task lfalign  

The "lfalign" task will step through each of the alignment stars showing you x and y cuts looking something like the plots in Fig. 7. The dashed line in these plots shows the level determined for the sky. The rectangle shows the position measured for the alignment box, and the solid line displays the calculated centroid of the star. You can change any of these values (type "?" to see how to change them), but usually this is totally unnecessary. Once you are happy with the xy fits, type "q" to move on to the next alignment star. After marking all the stars, you will be offered the opportunity again to eliminate discrepant stars. Finally, new offsets are calculated and you are asked whether or not you wish to execute them. These offsets should be small, but it is usually worth executing them.


 

exampleLfalign.png

Fig.7: X and Y cuts for the alignment stars inside the alignment boxes (shown as rectangles). The solid line displays the calculated centroid of the star and the dashed line is the level determined for the sky. 

You should now be ready to observe your targets. In the LDSS3 GUI set the Grism and Filter to the ones that will be used (Slit Mask should be inserted already). Set ExpTime, Loops and binning/speed/gain to the desired values and click on the “Start” button to begin the exposure.

Repeat this sequence for each mask you need to align.

 

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