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LDSS direct imaging

Daytime preparation:

 

Confirm with the Instrument Specialist that LDSS3C has been setup with the requested filters and any other "Special Requirements" noted in your Instrument Setup Request Form, submitted three weeks before your observing run. 

The GUI should already be running on the observer workstation but if it is not (or if it stops at any point) simply open an xterm, type “ldss3” and follow the instructions in the "LDSS3 Control Software" section of the LDSS3 Manual.

With the GUI running, take time to familiarize yourself with the controls and exercise the components (mainly filter changes, aperture mask and grisms should be set to “OPEN” for imaging). You should confirm that the filters you need 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. The focus values vary as a function of temperature. When the option “Auto” is selected on the “Focus” section of the GUI, focus values will be automatically updated if the temperature changes by more than 4 degrees, following an empirical linear fit calculated by the Instrument Specialists (for more details see v2.4 (2017-04-11) of the GUI).

In order to take daytime dome flats you will need to have the Instrument Specialist prepare the telescope. (Note: twilight or night flats are generally preferred due to considerable stray light in the dome during the day). The mirror covers will need to be opened and you should confirm that the tertiary is properly facing the West Nasmyth instrument port. 

Turn Vsub-on, set on the GUI the combination of binning/gain/speed necessary for your science observations and start the calibration sequence (for gain we recommend low and for speed we recommend fast in all cases)

Dome Flats: Deploy the Flat Field Screen (FF-Screen) using the DCU/Clay GUI (see manual). For imaging flats, the quartz lamps (Ql, Qh) controlled by the Flat Field Screen GUI are either too bright or do not appear perfectly stable, therefore, you should ask the Instrument Specialist to set the variable flat quartz lamp in the dome to an appropriate level using the regulated power supply mounted under the NASW platform. In order to clearly avoid shutter effects in the flat fields at the 1% level, it is best to keep all of your exposures greater than 10 seconds. 

The following table indicates some appropriate quartz lamp intensities and exposure times for the standard griz filters (binning 1x1, speed: fast):

 

Filter

Counts (Fast bin1x1)

Exposure time (sec)

Voltage (V)

 g'

       25000

         20

    2.5

 r'

       24000

         10

    1.8

 i'

       23000

         10

    1.3

 z'

       23000

         12

    1.0

 

Next in the LDSS3 GUI, make sure that the Aperture position and Grism positions are set to “Open” and the desired filter is in position. Set binning/speed/gain to the values that you will use for science observations, set the exposure time to the appropriate value and set the loops to the desired number of exposures for this filter. Finally, click on the “Start” button to begin the exposure sequence. Note: be sure that all other lights in the dome are off before taking any flats!

Repeat these instructions for each filter. Once you are finished, turn off the quartz lamp, retract the Flat Field screen out of the telescope beam and you can ask the Instrument Specialist to close the mirror covers.

Bias: 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/speed/gain values to whichever values you plan to use at night, and then click on the "Start" button to start the sequence.

The afternoon is also a good time to start IRAF on the observer workstation (using the link in the menu bar) and load the LDSS3 packages by typing "ldss3" at the 'cl>' 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.

Note: If you experience a large difference between the two amplifier when displaying the stitched image, add the bias section [1025:1152,1:4096] 

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

  

Twilight: 

Take twilight sky flat fields if desired. 
First, have the telescope operator (TO) slew to an 'empty field' for sky flats. In the LDSS3 GUI, set the Filter to the desired position, set ExpTime to a modestly small value (e.g., 7 seconds). Click on the "Start" button to take a test exposure, and repeat until the brightness of the sky has decreased to a reasonable value (e.g., 30,000-40,000 cts/pixel). For the next exposure, you should ask the TO to move the telescope by a small offset like 10-15 arcsec (this can be done during readout), increase the exposure time (a factor of 1.6 - 2 should be close to the correct amount), and take your next exposure. Repeat until you have obtained at least 5 exposures. Proceed to your next filter and repeat the sequence.

 

Night-time operations: 

Have the TO slew to the object that you wish to observe from your Observing Catalog File. 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 and you should be ready to observe your target. 
Note: After slewing the telescope, your target is expected to be close to the center of rotation of the detector (x:,y: as measured on June 2017). This is quite close to the amplifier limit. If you prefer to center your source in one of the amplifiers you may use the iraf task “ltoslit”. 
First you need to stitch your field image, using “lstitch” and decide the position where you would like to have your target.

 

lstitch.png

 Fig.1: epar of the IRAF task lstitch 

 

Let the TO know  that you will need to run “ltoslit” since they need to turn off guiding and SH. 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 values of the desired position. Binning  should be set to the values used to take the stitched image on which the positions has been measured.

 

ltoslit.png

 Fig.2: epar of the IRAF task ltoslit 

 

The script will ask you:  “Send RA/DEC offset to TCS?”, if the offsets seem reasonable type “yes”, if you do not want to send the offsets, type “no”. Once at the offset position, the TO can start guiding and S-H by setting up a guide star and an off-axis S-H star. You should now be ready to observe your target.

 In the LDSS3 GUI, set the Filter to the desired position, make sure the Aperture and Grism positions are "Open", set ExpTime and Loops to the desired values, and then click on the "Start" button to begin the sequence.

 

 

 

 

 

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