ISIS is a high-efficiency, double-armed, medium-resolution (8 - 120 Ã…/mm) spectrograph, capable of long-slit work up to ~4' slit length mounted at the cassegrain focus of the WHT.
Use of dichroic slides permit simultaneous observing to be done in both the blue and red channels, which are optimised for their respective wavelength ranges.

Spectropolarimetry and imaging polarimetry are also available, as well as drift-scan mode for high-time resolution monitoring programs. Link to ISIS Polarimetry Setup

The default detectors are a large, thinned EEV42-type (2048x4100) device on the blue arm and a TEK (1024x1024) or MARCONI (2047x4611) device on the red arm. TCS:

• USER> STATION CASS
• USER> INSTRUMENT ISIS
• USER> AGSELECT CASSE
• Zeroset AZ and ALT

to zeroset on the targets switch to computer control:

• USER> AZ 298
• USER> ALT 89
• USER> ENG
• USER> ZERO AZ TAR
• USER> ZERO ALT TAR

Drive AZ and ALT slowly in + direction over the zeroset target points, then switch to computer control again

to zeroset absolute:

• USER> ZERO AZ ABS
• USER> ZERO ALT ABS

Zeroset absolute will go wrong in case an absolute encoder bulb is broken! When slewing to the first object check azimuth and altitude incremental and absolute encoders are in close agreement and updating.

• USER> ZERO ROT ABS
• USER> WAVELENGTH 0.6
• USER> TEMP ...
• USER> PRESS ...
• USER> HUMI ...
• USER> UT1UTC ...
• USER> CAL LAST
• USER> FOCUS 97.87 (27/5/2004)

Autoguider:
Use the standard uDAS autoguider for CASS with AUTOFOCUS 2900 (27/5/2004). This was last run from lpss88 (whtdas18), with 'obssys 1', 'startobssys' and 'startag AG6'. No windowing is required.
The monitor should then display a camera mimic showing temperatures (these are presently rather inaccurate) and camera status. SAOimage display tool will also appear as well as the TV Guider Control. This GUI allows you to change exposure times, take fields and set the TV and Guider going. Images are piped directly to the display tool.

GSS is used for finding guide stars, connect to LPVS3 via LAT and login as GSS. Select WHT and instrument ISI.
To move the probes, make a remote connection to whtics from the TO monitor.

TV Acquisition:

The ISIS slit is 7.5 degrees tilted with respect to the incident light beam, so that the reflected light can be picked up by the TV system to enable slit viewing while observing. There is no filter in the beam (displayed on mimic in red). The sky can also be seen in AGCOMP mirror position ("direct view") - beware the X direction is opposite to that in Slit viewing. AGCOMP position is sometimes used to do a pointing calibrate (less flexure).

To avoid problems with the TFO mechaism (Slide containing TV focal reducing optics), when ISIS is on this mech is moved to the Barrel position and then plugged out. This gives an image scale of 18 arcsec per mm.
(The other two positions of the slide are TV Scale 5 (no optics) and TV scale 12).

TVSCALE 18 allows the full unvignetted slit width of 4 arcmin to be seen on theTV.

The orientation of the sky with Cass TV slit-viewing depends on the sky position angle (PA) and the TV-scale:

      E                     Example for TVSCALE 18                       N
      ^                                                                  ^
      |                                                                  |
      |                                                                  |
      |                                                                  |
PA=0: +------>N   PA=90: +------>E   PA=180: N<------+   PA=270: E<------+
                         |                           |
                         |                           |
                         |                           |
                         v                           v
                         N                           E

The SAOimage display must be flipped the image in the X direction to agree with the HANDSET X offset direction in Slitview.
Do this by selecting X on the Zoom menu.

Telescope focus:
At the begin of every night measure the best telescope focus taking exposures at different focus positions of a standard star, using a wide slit measuring the spacial profile.

Rotator centre:
After an instrument change you are supposed to determine the rotator centre and to run the 7 star calibrate procedure, this takes about 20 minutes.
Use TVSCALE 12 and direct viewing, this is the top surface of the comparison mirror (ICL> AGCOMP). This mirror is believed to be stiffer than the slit viewing mirror.
Measure the star position on the TV screen (x,y), then rotate 180 degrees and measure again. Calculate the midpoint, this is your rotator centre.

Calibrate procedure:
Calibrate is done on the rotator centre with the rotator tracking turned off (USER> ROT MOUNT xxx).

Determine aperture offset:
There is no need for defining an aperture.

Acquisition:
When slewing to a new object watch out for limits (rotator, azimuth) and consider use of:
USER> UNWRAP ROT|AZ

Acquisition is done with the slit-viewing mirror in, taking exposures with TV. A single object is always centred next the rotator centre. The position can be changed to avoid dust particles on the slit or defects on the chip. If necessary, perform a blind offset from a reference star to the science target.
It is easy to accurately recover a position on a following night using the same guide star. If required log the following parameters: sky PA, probe position and X- and Y-position of the guide star. On the following night make sure PA and probe are in position, then type 'USER>autoguide on x y'.

Autoguiding:
GSS tells us where to move the probe to, in order to pick up light from suitable guidestars:
The probe positions are giving by the guide Star Catalogue. To get the suitable guide star use the next script;

• $> @gs

Also the GSS can be loaded using the next commands;

• $> GSS
• GSS> CONFIG WHT CASS
• GSS> ROT {sky pa}
• GSS> SEA hh mm ss dd mm ss equinox
• GSS> EXIT
• $> TYPE OUTPUT.GS

Choose a 13-15 mag star from the output list, and send the guideprobe to the required position. There is two options to move the guide probe.

Once the guideprobe is in position you take a 'field' using the TV Guider Control (if not in TV mode already). Then start guiding. Check the status on the TCS DISPLAY changes from TRACKING to A/GUIDE.

The major difference here is that during exposures, no object can be seen on the slit. This is because the polarisation optics in the beam above the slit significantly reduce the visibility of the slit reflection. During acquisition the polarisation optics are usually removed, but it is still a little more difficult than normal ISIS acquisition because a multi-aperture DEKKER (usually 3 slots in the spatial direction) is also in the beam above the slit, which limits the parts of the slit we can see in the TV view.

Polarisation optics consist of polariser and analyser in the beam above the ISIS slit: in practice these are a wave-retarding plate and a calcite block, respectively. The latter separates beams with 2 different polarisations. Instead of one spectrum we therefore obtain two representing a polarised and non-polarised signal respectively.

For linear polarimetry the polarising element is a half-wave plate (hwp), for circular polarimetry it is a quarter-wave plate.

The waveplates above the slit also alter the f ratio of the beam. To re-focus the star on the slit when the polsarisation optics are in the beam, use the following values:

• Decrease telescope focus by 0.2mm
  This can now be automated using a GUI hwp "Dfocus" setting of -0.2mm
• Decrease TV focus by 3000 (did not help visibility, NOM 10/4/06)
• Preserve ISIS focus by Increasing RCOLL and BCOLL by 9200 (SA does this).

For flats a spinning polarimeter is used (hwprotate command, also automated by the GUI).

• Put the DAT tape in the DAT drive in lupus (in the terminal area).
• Open up an xgterm.
• cd /obsdata/whta/yyyymmdd
  
• Use the tar command to write the data tape, e.g.:
  tar -cvf /dev/rmt/0n *.fit

How to trail a star along the slit:

With the slit North-South:
- USER> ROT SKY 0   (position the slit N-S)
- move star to start position right of the rotator centre
    (the star will trail to the left in this example)
- USER> MARK name   (mark start position)
- USER> GOCAT name  (put position data in edit source block)
- USER> DIFF 0 1    (set trail speed to 1"/sec)
- USER> NEXT        (start trail)
The observer must time the exposure according to the differential rate!

With the slit at any angle:
- Use M. Azzaro's program 'diffrates' to calculate the differential rate needed
    for your actual position angle. Use the commands MARK, GOCAT and NEXT as
    shown above.

To move any ISIS or AGBox elements it is necesary to do it from the new GUIs in the observer system in Taurus.

The old ICL commands

" ICL>change - select the mechanism to be replaced from the menu
ICL>dekker 0 (Dekker safely away from slit)
ICL>bfold 0 (Dichroic in outer-most position)
ICL>r(b)filta(b) 0 (Moves the filter holders in outermost-position)"

Replacing optical/dekker units:

Gratings:

1. Release grating holder mechanism by turning the switch.
2. Remove grating _gently_ and place it in its grating box.
3. Place new grating. MAKE SURE THAT BLAZE DIRECTION IS CORRECT! (otherwise you will loose a factor of 10 in optical throughput).
4. Engage holding mechanism again. Verify that grating can not move.
5. Close door firmly.

Filters:

1. Pull out filter holder. Red arm filter horizontal placed, blue vertical. Some force might be required.
2. Insert updated filter holder again. A *click* should be heard.

Dekker:

1. Dekker slide placed just above slit. Pull out, replace, push in again.

Dichroic:

1. Release unit by turning black knob attached on dichroic plate.
2. Pull plate out _slowly_, not touching any optical surfaces.
3. Insert new dichroic plate _gently_. This is the most tricky part of it all since it has to be positioned quite precisely. Take _very_ care of not hitting the optical surfaces by any obstacles inside the spectrograph (especially a vertical placed metal plate to the right of dichroic position).
4. Tighten dichroic with knob.

CAUTION: Do not touch optical surfaces of gratings/dichroic under any circumstances. It can have dewasting consequences. If dusty, use dry air only.

-------- © 2004 Isaac Newton Group of Telescopes --------
Last Updated: 10 November 2005 Juan Carlos Guerra jcg@ing.iac.es