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Low Dispersion Survey Spectrograph (LDSS-2)

Mask design
LEXT startup files
New software for telescope TWEAKs
Running LEXT at ING

New high-resolution red grism available

A new high-resolution red grism has been developed in Durham, and is available on a collaborative basis. The first on-sky results have been successful. Contact Richard Bower (

New LEXT-users please note:

The installation and subsequent mastering of the LEXT software is not trivial. A safe estimate of the time needed to install LEXT and to design the masks for your run is about 2-4 weeks.

We have had several cases of very late entries of mask designs as the result of underestimating the effort required to design masks. Please realize that we can only guarantee to manufacture your masks on time and with reasonable quality if you send in your masks not later than one month before your observing run!! This means that you will be on schedule when you start installing LEXT about two months before your LDSS run.

The LDSS+LEXT manual states that we can make masks on the spot, BUT THIS IS NOT TRUE! Depending on the number of slits, the manufacturing of one mask will take about 1.5 to 3 hours. Given the number of masks that are usually used for one total LDSS run, it is clear that the manufacturing of the masks needs to be planned in the schedule of our mechanical section well before the actual observing run. Therefore, we need to have your mask designs one month before your run.

Handy hints for LEXT mask design

Hint 1) Read the LDSS + LEXT Users' Manual (for now, skip the parts about telescope tweaks and data reduction). This manual contains almost all info that you need for designing masks. All other info can be obtained from the page you are reading now.

Hint 2) Be sure to have accurate and internally consistent coordinates, for all fiducial stars and science objects.

Hint 3) Select fiducial stars with a visual brightness of about 17 or 18 mag (if you are observing in BRIGHT time, you will need brighter fiducial stars). Brighter stars might saturate the chip when doing a science exposure. If possible, select a set of stars with uniform brightness.

Hint 4) For mask design it is vital to use the right lext.startup and lext.set files. See below for links to these files; be sure to get the ones that go with the detector you will be using.

Hint 5) While designing masks, make sure that slits and fiducial holes fall within the field region that is transparent to the collimator. An absolute minimum of 3 fiducial holes within the collimator FOV are necessary to be able to tweak the telescope during field acquisition. Use the following command to plot the collimator boundaries, directly after executing the LEXT 'predict_spectra' routine:

  • plot/circle Xcenter Ycenter 580 (if using the TEK and SITe setup files)
  • plot/circle Xcenter Ycenter 933 (if using the LOR setup files)
where Xcenter and Ycenter should be replaced by the coordinates of the chip centre as specified in the lext.set file.

Hint 6) The maximum number of slits on a mask is 100. This does not include fiducial holes. High quality slits can only be guaranteed for masks with less than 40 slits.

Hint 7) Although the LDSS/LEXT manual states differently, the width of your slits and the diameter of the fiducial holes should not be specified in the .CAT/.MFR files different from 1.5 arcsec and 10 arcsec respectively. If different slit widths or hole sizes are required, please contact your support astronomer.

Hint 8) Send in your mask designs, i.e. the .CAT files and .MFR files, not later than one month before your observing run!! This will ensure that your masks will be manufactured in time for your run.

LDSS field acquisition now implemented in IRAF

New TWEAK routines run under IRAF. These routines are used to accurately position the telescope on the requested field. This means that the observer does not have to run LEXT while observing. Please note that LEXT still has to be used for mask design.

HELP on these IRAF routines can be obtained with IRAF running on lpss3 (altair):

These routines are part of the local IRAF package ldsstweak. To obtain general help on, and an introduction to, this new package type in IRAF.

The IRAF routines use the LEXT catalog files, or .CAT files, and the lext.startup and lext.set files. Therefore, please send (email) your Support Astronomer the LEXT catalog files you used for mask design. The LEXT mask designing proces will eventually lead to files that hold mask manufacture information, the .MFR files. Please send these files to your Support Astronomer as well.

Running LEXT at ING

The LEXT application at ING is run on the SunOs UNIX cluster - that is, lpss1 and lpss2. It can be run from any account, but when you are actually observing with LDSS we ask you to use the account observer.

There are a few things that you should do before starting up LEXT:

  • Check that the DISPLAY environment variable has been set appropriately. If it has not, type the command xdisplay at the shell prompt.
  • If you have logged into a UNIX cluster node from, say, the lpss3 console, make sure that you have enabled the Xwindow protection for that cluster node to write to the console. For the lpss3 console, this would be done using the xhost command on lpss3.
  • Check that the environment variable LDSS_STACK is defined, and is (ideally) pointing to your current working directory.

After all that, type the command lext at the shell prompt and LEXT will start up.

To start up the graphics display type device xwindows from within LEXT. Alternatively, you can try xmake xwindows or xmake x2windows or xmake x3windows (etc.) from the shell prompt. If neither works try starting up the graphics display from a different computer. LEXT will be able to access the display even when the process is running on a different computer.

The next step is to ensure that you have the correct versions of several configuration files for the detector you wish to use. This is the subject of the following sections.

The correct LEXT files

LEXT is used to design LDSS masks, to derive telescope tweak values when observing, and to reduce LDSS data. Besides being absolutely certain of the accuracy of your astrometry, the most important consideration when designing LDSS masks is to ensure you have up-to-date versions of the LEXT start-up files. Similarly, when using LEXT during observing, these start-up files are fundamental to the accuracy of any derived telescope tweak values. The start-up files are:

  • lext.startup - This file defines the image size of the CCD when read out and then invokes lext.set.
  • lext.set - This file defines the chip centre coordinates, focal plane distortion and a number of default parameter values.

These files should be in your current working directory when running LEXT. Be warned that you can run LEXT without these files in your working directory, in which case LEXT will load template files from its release directories. Any result from using these files is likely to be wrong.

There are start-up files for both detectors available for use with LDSS: TEK2, LOR1 and SIT1. When designing masks for LDSS it is very important to use current operational versions of these start-up files. If you do not, then you may find some of your designed spectra fall slightly off the chip, either in the wavelength or the spatial direction, when you get to the telescope. The reason for this is that the physical chip centre is never at the image centre, as a result of the underscan/overscan regions used for bias strips and also because of small offsets of each chip in its cryostat.

You can download the start-up files for each chip from this Web page. If you click on your intended detector, below, you will be presented with the associated LEXT start-up files:

Reading image files into LEXT

IRAF format image files, .IMH files, can be read by LEXT. The LEXT application expects image data to be in a specific orientation when read in. However, CCD cryostats in use at ING have their CCD chips mounted in various orientations. The LEXT READ command has a parameter which controls this orientation. To ensure that the correct orientation is used in all LEXT procedures, the LEXT procedure READIM should always be be used to read images. To read the data in the correct orientation into LEXT, there should be a file


in your current working directory. If there is not, then instructions for copying the correct file to your current working directory can be found under your chosen detector:

Up-to-date documentation

The most up-to-date documentation for the LDSS instrument and the LEXT software is the on-line version. However, some small updates may not have found their way into the on-line version yet. Here, the most reliable source of information will be the LDSS folder in the WHT Control Room. Clearly, if a major change is likely to affect observers remotely designing LDSS masks, they will be informed by the ING Support Astronomer responsible.

Please read the LDSS manual in the WHT Control Room when setting up, it may save you valuable time when on the sky.

Problems, problems, problems

If you get stuck, don't waste valuable observing time trying to sort it out yourself - get one of the ING staff onto it. Your Duty Engineer or Support Astronomer will know who to contact if they cannot solve the problem themselves.

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