Working with ISIS

Home · Search · About ING · Astronomy · Public Information · Engineering

WORKING WITH ISIS

ISIS is a high-efficiency, double-armed, medium-resolution (8 - 120 Å/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.

The default detectors are a large, thinned EEV12 4096×2048 device on the blue arm and the RED+ 4096×2048 device on the red arm. Pixel scales are 0".2/pix in the blue arm and 0".22/pix in the red arm. Spectral resolution will depend on the grating in use and the slit width. Two low noise frame transer devices (QUCAM2 and QUCAM3) are also available for fast spectroscopy mode, both having a pixel scale of 0".19/pix.

Spectropolarimetry and imaging polarimetry are also available. See below for general information and use of Spectropolarimetry and ISIS Imaging Polarimetry Setup which includes instructions for TOs (this needs to be checked).

Documentation

ISIS Astronomy page.

CAGB Softwware manual

ISIS Software manual

Acquisition Tool manual

Startup

TCS:

USER> STATION CASS
USER> INSTRUMENT ISIS
USER> AGSELECT CASS
USER> FOCUS 97.85 (2011)
ZEROSET (az, alt, rot)
CAL (last or faint)
TVSCALE: 9200 (2011)
AUTOGUIDER FOCUS: 2800 (2011)

ISIS mechanisms can be controlled with the GUI:

Preparation

Telescope focus:
At the start of every night the best telescope focus is measured by taking exposures of a standard star at different focus positions , using a wide slit (8 arcsec) and measuring the spatial profile. The observer will do this.

Rotator centre:
After an instrument change you are supposed to determine the rotator centre and to run the 7 star calibrate procedure, which takes about 20 minutes. This would only be done on a service night, unless there are pointing problems, in which case it can be done anytime.
Use TVSCALE 12 and direct viewing, this is the top surface of the comparison mirror (TO> 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 to define an aperture.

Observing

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

TV Acquisition:

For a descpiption of acquisition using the Acqtool, see the ACAM Acqtool User's Guide

The ISIS slit is tilted 7.5 degrees 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 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. The sky can also be seen in AGCOMP mirror position ("direct view") - beware the X direction is opposite to that in Slit viewing. In slit viewing mode the full unvignetted slit width of 4 arcmin can be seen on theTV.

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


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

There are focal reducer optics for the TV with two possible positions: TVSCALE 12 or TVSCALE 5. The default position is TVSCALE 12. In this position, a 18"/mm optical barrel is deployed to permit viewing of the full slit with AG4. With TVSCALE 5, about 60" of the centre of the slit is seen.

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: telescope offsets, sky PA, probe position (r and theta) and X- and Y-position of the guide star as seen on the TCS. On the following night make sure PA and probe are in position, then type 'USER>autoguide on x y'.

Autoguider:
Use the standard uDAS autoguider for CASS. Check that the position of the "Autoguider to TCS Selection" switch is in the correct position. Standard focus for the autoguider is 2800 (2011), this can be set from the GUI or from the TO prompt using the command:

TO@taurus> autofocus 2800

This was last run from udasdev2 (whtdas18), with 'obssys', '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. DS9 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. Be careful not to allow the guide box to drift outside the area of the CCD as this causes guiding to fail.

GSS2 is used for finding guide stars. To start the GUI, click on the WHTGSS icon on the desktop of the osadisplay2 machine, enter the password and the "gss2".

Alternatively, open a new terminal window and type:

[whtobs@osadisplay2 ~]$ ssh -Xl gss whtgss (password is the same as ever)
[gss@whtgss ~]$ gss2&

Documentation for gss2 can be found here.

GSS2 tells us where to move the probe to, in order to pick up light from suitable guidestars. The probe positions are given by the guide Star Catalogue. The Cass autoguider has a FOV of ~1' X 1'.

Choose a 12-14 mag star from the output list, if possible, and send the guideprobe to the required position. There are two options to move the guide probe.

1.-

Using a telnet connection to TAURUS. user: whtobs, pswd:***** in the command line >obssys, and we have a prompt TO>

To send the command to the guide probe use:

TO> prag <autoradial> <autotheta>.

When the AG probe reaches position the message PRAG <XXX> <XXX> will appear at the TCS prompt.

One can also move in just autoradial or autotheta (for example to improve the position of the guide star on the AG chip:

TO> autoradial <a number in the range 0-40000 microns>

TO> autotheta <a number in the range 0-180000 millidegrees>
(NB: autotheta of 0-35,000 will vignette the slit viewing camera!)

2.-

To move the autoguider we can use the Graphic interface from Taurus.

If the guide star needs to be recentred, a new prag command can be sent. To move the guide star up, increase autoradial and to move it left, increase autotheta (and vice versa). Size of movement: to move the guide star from the right edge to the left edge, increase autotheta by ~6000, and to move it from the bottom edge to the top, increase autoradial by ~12,000. 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.

POLARIMETRY

Note that this section may contain information relevant to both imaging and spectropolarimetry and could be a bit mixed, except for the spectropolarimetry acquisition section which is just for specpol.

The polarisation optics significantly reduce the visibility of the slit reflection, so that during exposures, no object can be seen on the slit. Acquisition is also a little more difficult because a multi-aperture DEKKER (usually 3 slots in the spatial direction) is also in the beam above the slit, which reduces 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.

Telescope Focus:

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 about 0.2mm with respect to the standard ISIS focus. This should be checked at the start of the night, with the polarisaion optics in the beam. Focus with spectropolarimetry using the Half-Wave Plate in was around 97.7 (July 2012)
This can now be automated using a GUI hwp "Dfocus" setting of -0.2mm (the observer will do this if they want to)
It may be necessary to decrease the TV focus by 3000, but it's probably not necessary since the focus isn't very different
Preserve ISIS focus by Increasing RCOLL and BCOLL by 9200 (SA does this).
A corresponding decrease in the autoguider focus of a few hundred will probably be needed too.

Working in polarimetry mode will mean frequent focus changes as different focus positions may be required for acquisition, linear polarimetry and circular polarimetry. It is reccomended that the observer adds all necessary focus changes to their observing scripts, particularly if they are doing both linear and circular polarimetry. The SA should determine the correct focus offsets on the first night of the run. It is probably NOT necessary to refocus simply for acquisition, therefore if you are doing just one type of polarimetry then you can use this focus value for the science and it will be OK for the acquisition too, so focus changes during the night are not needed.

Flats:

A spinning polarimeter is used (hwprotate command, also automated by the GUI, also done by the observer). (Is this for imaging or spectropolarimetry? Not done by TO anyway!)

Acquisition for Spectropolarimetry:

Usually a fixed Sky PA such as 90deg will be used all night. For acquisition, the polarisation optics must be removed so that the target and slit can be viewed on the acquisition TV, otherwise nothing will be seen. e.g. to remove the Half-Wave Plate, do in taurus: hwout. Acquisition can be done using AGSLIT (this worked OK in July 2012). No use of AGCOMP was needed. The appropriate dekker should be in place throughout, e.g. the dekker called POL18ARCSEC, which has a gap of 18 arcsec between the centre of the central dekker slot and the centre of the left or right dekker slot.

First mark the position of the slit on the TV and also the positions of the dekker slots which are going to be used. This could be the central one and one of the side ones for example. A 2-second exposure was sufficient to see the dekker slots and the slit. The slit may be located towards the top of the dekker slots. With no TV windowing, the slit was found at y=457 (July 2012). Acquire the target onto the intersection of the slit and the dekker slot you want to use. There might be some strange-looking reflections of the star around the edge of the dekker slot, this is normal. Start guiding when well acquired and wait for a couple more readouts to make sure the acquisition is good. Remember that the polarisation optics need to be put back in before observing (the observer might need reminding) or the TO can do it in taurus with: hwin to put the Half-Wave Plate back in for example.

Important: The dekker (and maybe the slit) positions will change very significantly due to flexure and you must not rely on the markers for any subsequent acquisitions!

Grating/Dekker/Filter/Dichroic change (Usually done by SAs)

ISIS & AGBox elements may be moved from the new GUIs in the observer system in Taurus, or via the TO> prompt. Useful commands are:

TO> agslit (put the agbox into slit viewing mode)
TO> agcomp (put in the comparrison mirror to take arcs)
TO> arcoff (turn off the calibration lamps and put the slit viewing mirror into place)
TO> isis_change_filter (unlocks the slit door and move the dekker to the out position to allow access to the filter mechanism)
TO> dekker out (moves the dekker to the 'out' position)
TO> rfilta 0 / rfiltb 0 (moves the red filter a / b mechanism to the outermost position)
TO> bfilta 0 / bfiltb 0 (moves the blue filter a / b mechanism to the outermost position)

More commands can be found in the software manuals for CAGB / ISIS.

Replacing optical/dekker units:

Gratings:

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

Filters:

Move the filter out (0-position) and pull the filter holder out gently. The red arm filter is placed horizontally, the blue vertically. Little force should be required.
Insert updated filter holder again. A *click* should be heard.

Dekker:

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

Dichroic:

Remove first RFILTA below. Release unit by turning black knob attached on dichroic plate.
Pull plate out slowly , not touching any optical surfaces.
Insert new dichroic plate gently. This is the most tricky part of it all since it has to be positioned quite precisely. Be very careful not to hit the optical surfaces againt any obstacles inside the spectrograph (especially the vertical metal plate to the right of dichroic position).
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.

Miscellaneous

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 other 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.

Last Updated: 6 July 2012 by Fiona Riddick