Intermediate Dispersion Spectrograph (IDS)

The Intermediate Dispersion Spectrograph is the principal common-user CASS instrument based upon the principle of a folded flat-input, flat-bed instrument. There are two camera ports. These being the 235 or 500mm foci. The straight through position (below the slit unit and filter trays) directs light to the Faint Object Spectrograph (FOS). n.b. It should be noted that although this instrument is still physically attached to the IDS, it was de-commissioned sometime ago so no further mention will be made of it here.

Slit, Slit Shutter and Dekker unit

The SLIT and DEKKER mask unit are the first components the light reaches after passing through the A&G box into the IDS. The maximum slit length is 4 arcmin, (Long slit). The slit unit comprises of two precision ground steel  jaws driven by a stepper motor which can move in or away from each other so producing the required slit aperture. Hettich inductive switches flag when the jaws are in a fully open or closed state. The slit width is variable from 0.216 arcsec to 10.82 arcsec. The slit gap is measured using a A.S.L. SLVC transducer. This is connected to an A.S.L. DC-DC signal conditioner with the output from this going to a signal conditioning board in the MMS crate.

The slit jaws are polished so that reflected light can be passed back up to the TV camera in the A&G box for SLIT VIEWING. See A&G box.

The SLIT SHUTTER is driven from a solenoid valve operating a pneumatic piston. This is the main shutter for the IDS and NOT the camera shutters which serve only as dark slides. Originally, the slit shutter was driven from the IDS MMS controller. This is no longer the case. A connection is now made to the MMS controller from the SDSU shutter control module. The slit shutter board in the controller has been modified and now only acts as a buffer for shutter control. Hettich switches provide the OPEN and CLOSE status signals.

The DEKKER unit (named after an American astronomer who first came up with the idea) consists of a slide fitted with a Dekker mask above the slit jaws. The Dekker mask consists of a series of polished plates with accurately machined slots designed to produce an aperture across the slit of a known value in arcseconds. Several Dekker masks are available for the IDS (some with different sized slots and others with combs) that can be interchanged through the Observers Port (SLIT) door.

The Dekker slide is moved by a stepper motor and has upper and lower limit (Hettich) switches fitted. The position is encoded using a Penny Giles pot; the output going to a signal conditioning card in the MMS controller. A friction pad driven by a pneumatic valve acts as a clamp.

It should be noted that the ICS software has ability to change the value that determines the definitive Dekker positions, 1, 2, 3 etc. thus small corrections can be made to where the mask will position itself over the slit.

The Slit and Dekker assembly can be removed and replaced with a Multi-slit assembly consisting of ten parallel slit-lets each 16 arcsec long with a 7 arcsec seperation. This was mainly used with the FOS.

Filter Slides

The IDS has two filter slides. These being the BELOW SLIT neutral density and colour filter slides: BSND and BSCF. The below slit filters also form part of the slit assembly and can be accessed through the Observers (Slit) Port. In each slide there are three filters fitted; one position being clear.

The slides are moved using pneumatic pistons controlled by solenoid valves. Hettich inductive slotted switches operated by a flag on the slide determine the filter position. Another Hettich sensor serves as a detent switch and operates when a filter is in a valid position.


The Collimator is of 1275mm focal length, giving a collimated (parallel) beam diameter of 85mm which is presented to the grating. There is a choice of three collimator elements with different coatings for maximum reflectivity in the wavelength range of interest. The collimator is fitted in a kinematic mount to maintain alignment and can be accessed through the Collimator Port on the IDS.

The collimator is mounted on a high resolution linear drive and driven by a stepper motor. A high accuracy A.S.L. linear transducer measures the position. The output also going to the signal conditioning board in the MMS controller. When movement has stopped, solenoid valves operating pneumatic clamps lock the collimator drive to prevent any flexure.

IMPORTANT:  It should be noted that the actual movement of the collimator +/-  (where the beam is still parallel) is small, thus the CCD cryostat needs to be mounted as such that the IDS focus falls approximately mid way within the collimator range of movement.

Hartmann Shutters

These are housed in front of the collimator and consist of two vanes; left and right, which are used for fine focusing. The Hartmann vanes are controlled by solenoid pneumatic valves and pistons. Hettich switches detect whether they are in an IN or OUT position. These are put into the beam alternately to measure the top to bottom shift in the spectral lines from an arc lamp as seen in the image from the detector. By moving the collimator +/-  a small amount, the beam varies sufficiently enough to reduce the Hartmann shift to acceptable amounts. n.b. Fine focusing of the IDS using the collimator and Hartmann shutters is an automatic procedure, but the labourious task of adjusting the CCD cryostat rotation and tilt (using the capstan screws) is still done manually.


The grating is the heart of the spectrograph and is what disperses the light into its component wavelengths so producing a spectra. The lines seen in the spectrum are a natural phenomena related to emisson lines in hot gas or absorption lines produced due to the chemical composition of the stars. The spectrograph images the slit onto the detector, but the grating causes each wavelength to be marginally shifted.

The grating consists of a optically flat plate with rulings; the spacing depending on the dispertion required. Some gratings can have as much as 600 lines per millimetre or more. Several gratings are available of both low and high dispertions for the IDS. The spectrograph is 'tuned' to the required wavelength by changing the angle of the grating with respect to the collimated beam.

The grating is mounted into a kinematic seat to ensure optical alignment and clamped to prevent movement. n.b. It should be noted that the grating can be inserted in TWO ways. This enables the BLAZE angle created by the rulings to point to the camera in use (235 or 500). An arrow on the grating should point either to the left (500) or right (235) depending on what camera is is use.

The grating angle to wavelength relationship is critical thus the position angle must be known to a high precision. This is achieved used an Ferranti 20 bit absolute intelligent encoder with a resolution of 0.001o complete with error checking built into the serial data string. This is decoded by the IDS controller and used in the final positioning of the grating. However, if the error checking bits are set then the resolution decreases by a factor of 1000  i.e. to only 1o resolution.

Due to failure of the Ferranti encoder and not having a reliable spare one, it was decided to replace this device with with and INDUCODER 16 - bit absolute encoder with SSI output. The description of this project can be found here. The new encoder emulates the old Ferranti encoder and has some additional features included.

When changing the grating, the grating shutter moves across the beam and the grating carrier moves parallel to the door of the Grating Port to allow access. As with the slit and collimator observer access ports, the grating port door is interlocked (solenoid clamped) to prevent light leaks. An ids_change command must be issued to unlock any doors on the IDS.


The two cameras are of the folded short-Schmidt type with focal lengths of 235mm and 500mm. These giving scales at the detectors of 29.4 and 13.8 arcsecs respectively. At each camera there is an additional optical component known as a field flattener. These basically flatten the field to the correct size for the detector in use. These were interchangable with different optics (when the old IPCS detector was used), but as both ports now use a CCD detector, this is no longer the case.

Solenoid operated pneumatic valves and pistons operate the camera shutters (dark slides) and were designed in such a way to automatically close on the press of a panic button in cases such as high illumination on the detectors. Again, a throw-back from the IPCS era. However, it is important that the IDS camera port NOT in use has its shutter closed. If a cryostat is NOT fitted to an used port, light could enter and ruin the observations.

IDS Controller

The IDS controller is an MMS (Micro-Modular microprocessor System) based around the 8 bit Motorola 6800 processor and was designed at RGO (circa 1982). The software was written in FORTH and the compiled code is held in EPROMS on the ROM boards within the crate.

All mechanisms that use stepper motors are controlled by double height eurocard boards (one for each motor) which contains the motor coil phase switching and power (4 transistors) circuits. A 6821 PIA chip handles the input signals from the IDS mechanism's associated position sensors and limit switches (Hettich slotted inductive switches) and also provides the direction and speed TTL toggles to drive the motor circuits. The velocity ramp up/down of the stepper motors is done using hardware. More information on these Stepper Motor Drive boards and the differences between them can be found here. There are also preliminary investigations and conclusions done by John Mills, his report is here.

As this instrument uses pneumatic values to control some mechansims, special PIA cards with interface circuitry are installed in the crate. A Signal Conditioning board is also present which handles the data from the SLVC transducers.

Serial communications to the 6850 ACIA board in the MMS is via an RS232 link (9600 7E1). There are two ports, one for engineering the other for computer control.  Generally only the computer port is used. The RS232 connection runs to a panel mounted on the CASS electronics rack where control can be switched between two ports on a PORTSERVER, port 14 which is the ICS computer accesses over the network, or port 12 which is a Virtual PC accesses over the network for engineering use. This Virtual PC is a Linux machine running on taurus with a dosbox running the CDSAI Engineering software. The  Virtual PC can be accesed by starting a VNC-Viewer session on your computer, then connect to: idsconsole:1 with the password: ********  A small box mounted on the panel fitted with a switches selects to where the comms line is routed. See the picture below or  Cassegrain Interconnections drawing for more details.

IDS A&G eng switch

n.b. Although only full I/O control with the PC is enabled with the switch in the PC position.  When control is via the Portserver, the PC can still read incoming data and will show if a mechanism has moved. A useful diagnostic.

Here is an image of the Engineering software user interface:

IDS Eng interface

In case you see the DOS C: prompt instead of the above screen, you can start the program by typing: cdsai <return>. If the screen is present but not updating and no data scrolling then call someone from the CFG group to restart the virtual machine idsconsole:1 ( If the screen is present but the Error Status Window says that there  is no respons and there is no data scrolling in the Data Flow Window, the instrument is switched off. This can be switched on remotely by entering the INT Cass APC webserver.


  1. Instructions on the CDSAI program, this document is a scanned version in pdf format, as the original document was lost.
  2. The IDS technical drawings have been digitised, organised and put onto the BSCW server.
  3. Document describing differences of the various SMD cards
  4. Document decribing the initial study by John Mills on the SMD cards
  5. Documentation on the Grating encoder substitution
  6. Refer to the INT Technical Manuals: 45  46  47  48   for full details and circuit drawings for the IDS.
  7. Refer to the INT Technical Manuals: 54 and 55   for the circuit drawings for MMS boards.
Last updated: 20 July 2015  rjp