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At the start of the night

Before you acquire the first of your targets, you need to do the following.

• General system setup

  Before dinner, ensure the DMS, CCD system, autoguider and system computer are running properly (see Section 3.2).

• Setup the CCD readout mode

  The CCD should be set up with the correct windowing so that the required part of the CCD is read out. Some CCDs also have a fast readout mode which reduces the readout time at the expense of increased noise. Normally you will need the full CCD area and the slow readout mode to minimize noise.

• Setup up the Telescope Control System

  Check with your Support Astronomer and Telescope Operator that the correct version of the TCS software has been loaded. The TCS needs to know that the instrument being used is LDSS-2 so that it sets up the correct coordinate transformation matrices internally - in particular it needs to know the offset between the zero points of the LDSS-2 and telescope rotation systems, i.e. which telescope rotator position puts the LDSS-2 slit direction at PA=0. Normally this would be done by a command such as INSTRUMENT LDSS.

• Determine the telescope rotator centre

  As part of the normal startup duties, the Telescope Operator will determine the rotator centre of the telescope. This is the projection of the centre of rotation of the Cassegrain rotator on the sky. To determine this, a bright star is selected and the rotator moved through 180. The rotator centre is at the middle of the line joining the before and after positions of the star on the direct TV. The Telescope Operator will write in the log the position of this centre in terms of the TV coordinate system. It is vital to realize that this coordinate system is changed if the reverse scan switches on the TV system are altered to make the TV image invert left-right or up-down so make sure that the same switch settings on the TV system are used throughout.

  This is an important step since the field of view of the telescope, by default, will rotate around the rotator centre. If the rotator centre is incorrect, stars will appear to gyrate in position with the sidereal period when the rotator is tracking the sky.

• Refine the telescope pointing model

  Another standard startup procedure for the Telescope Operator is to determine an update for the global telescope pointing model by a sequence of observations of positional reference stars. This is initiated with the TCS command CALIBRATE. An alternative option is to use a previous pointing model (CALIBRATE/LAST) but this is not recommended, especially for the first night that LDSS-2 has been put on the telescope. In general, it is always best to go through the positional calibration sequence in case something has changed from the previous night. The more time you spend now to get the telescope pointing right, the less problems you will have later on acquiring your fields. The Telescope Operator will examine the residual pointing errors. They should be within accepted bounds.

  Now check that this all works correctly by acquiring a positional reference star to make sure it comes in close to the rotator centre.

• Focus the telescope onto LDSS-2

  The telescope must be focussed onto the plane of the mask which is coincident with the input focal surface of LDSS-2. You will already have setup, or checked the best internal focus for LDSS-2 using Hartmann tests (Section 4.6). Make sure that the reference filter which you used to setup the reference focus value is selected and check that the correct focus value is in operation ( LFILTER filter should be all you need to do).

  Then set the focus to the approximate value using records of previous satisfactory values. The Telescope Operator should be able to find these in the log. The telescope should be pointing at a field with stars of about 14th magnitude (with the field of view of LDSS a random patch of sky will do).

  The telescope can then be focussed onto the detector using the ICL procedure FOCUSIMAGE. At the ICL interface type:-

  FOCUSIMAGE

  The procedure will ask a number of questions, then do a number of short exposures, stepping the telescope and the telescope focus between each, and finally read out the CCD frame. The star images can then be analysed for the smallest image, and thus the best focus position, using any of the following options: the X-FIND and Y-FIND procedures on the DMS; imexamine in IRAF on the Sparcstation; or LEXT on the Vaxstation. As a guide to the parameter values: the initial focus should be set to about 0.3 mm below the estimated focus for this instrument; the focus step should be about 0.1mm; and the telescope step around 20 arcseconds. The procedure inserts a double length telescope step between the first and second focus positions.

  At this stage it would be a good idea to adjust the direct TV focus to give the sharpest images for this telescope focus value so that you can assess the seeing and/or focus easily by examining the images on the direct TV system. The TV focus is changed from the ICL VAXstation using the command TVFOCUS.

• Determine the LDSS-2 centre

  It is necessary to relate the LDSS-2 centre as measured from the matrix mask to the position of the rotator centre. So far the Telescope Operator has determined the rotator centre with respect to the TV coordinate system and it is necessary to relate this to the LDSS-2 coordinate system. This is accomplished by putting a brightish star on the rotator centre as accurately as possible. Then take a short direct image with no mask and measure the centroid of the star, preferably using CURSOR/GFIT in LEXT. The position of the star gives you the position of the telescope rotator centre.

• Setup the acquisition reference position

  The acquisition reference position is the point on the TV system display that the Telescope Operator uses for acquiring a target. After acquiring the target blindly (using GOCAT etc..), the Telescope Operator will use the handset to position the target onto the acquisition reference point.

  The acquisition reference position should be the centre of field rotation. Ideally it should correspond to the LDSS centre (as determined above) rather than the physical rotator centre determined initially. The amount of mismatch that can be tolerated is determined by the size of the reference star holes in the mask. If the offset is much less than the hole size (default 10 arcsec diameter), say 2 arcsec, you can use the rotator centre as the acquisition reference position.

  If the offset is larger than this, it is probably worthwhile asking the Telescope Operator to set an APERTURE OFFSET on the TCS. This will set the telescope to point at the correct LDSS centre, and more importantly, if the position angle is changed, the field will rotate about the aperture centre rather than the real (physical) rotator centre. The TCS command used is

  ENTER APERTURE 0

  where (,) is the offset in TV system coordinates. To determine how the TV system and LDSS-2 coordinates are related, ask the Telescope Operator to use the Handset in RA-Dec mode to move the telescope by the RA and Dec offset calculated from the position of the rotator centre in the LDSS-2 coordinate system. The handset display will then give the equivalent offset in APOFF coordinates. It this which the TCS needs as the last two arguments of the ENTER APERTURE command. It's best to do all this at PA=0 to avoid confusion.

  This position defines both the LDSS-2 centre and the acquisition reference position, and the Telescope Operator should mark it on the TV system display with a cursor so that there is a visual reference mark on which to acquire your targets.