6 The Multiple Instrument System (MIS)
6.1 Introduction
The Multiple Instrument System (MIS) comprises three discrete and independent modules which can be used with a suite of analytical instruments: finder, guider and filterwheel (Figure 6-1).
The guider module is equipped with a video camera for object acquisition and guiding. (There is no longer a finder camera.) In addition, the finder module contains spectral calibration and flat field lamps.
The filter wheel module, which also contains a shutter, is used with imaging detectors and some spectrographs when employed as image reduction cameras.
The MIS modules are attached to the instrument rotator (Chapter 8) and changing the rotator angle will alter the orientation of the finder and guider fields.
The operation of the MIS is linked with several other units which must be turned on for acquisition and guiding (Figure 6-2). Only those items located in the observing room are normally powered down at the end of the night. Note the MIS hatch, which must be open to admit starlight!
The MIS modules communicate through the MIS control crate which is located in the right center instrument rack of the computer room. There are only two controls on the front panel of this rack: a power switch and a reset button. Fuses are located on the back panel.
Figure 6-2: MIS Control Schematic
Figure 6-3: Finder/Guider System Checkout
6.2 The MIS Cameras
The guider are equipped with a low-light- level video cameras which give a real time display on the GUIDE MONITOR. (There is no longer a FINDER camera).
The camera head is a Fairchild 3000F CCD with 378 x 488 pixels, a 14 mm image diagonal and a 4:3 aspect ratio. The CCD has a low light level dynamic range of 1000:1 and a minimum useful illumination of 5 lux. The spectral response peaks in the red at 8000A. The camera is equipped with a coherent fiber optic faceplate which allows it to be coupled (using a drop of immersion oil) to a four-stage Varo electrostatic image intensifier.
Each camera head has an independent power supply, mounted on the instrument rotator flange. These units are normally left ON, with the SYNC switch set to INTERNAL; they must be powered down when the camera cables are disconnected to avoid damage to the CCD.
For an image to appear on the FIND/GUIDE MONITOR two units must be powered up:
Power to the image intensifier is controlled by the guide CAMERA POWER SUPPLY. If the Digital Video Integrator is being used (typically for spectroscopic work only), it should be set to DIRECT to see starlight unless you have already set the digital levels. The Digital Video Integrator is only capable of processing signals from one camera and so the CAMERA SELECTOR is used to switch to the desired camera and only apply voltage to that image intensifier. The applied voltage must be in the range 0 to 6.75 volts and the normal operating range is 4 to 6 volts. The response is very non-linear and very rapid, so turn the gain up slowly (taking 3 to 5 seconds) while watching the FIND/GUIDE MONITOR. A crowbar circuit trips the power if more than 6.4 volts is applied. Turn the gain all the way down to reset the crowbar. Also turn the gain down when switching between cameras or when slewing the telescope to a new field.
After power has been applied to the image tube it may take 30 seconds (or many minutes, depending on the humidity) for the charge to bleed away; be careful when moving to brighter standard stars.
The FINDER camera (no longer present) is equipped with neutral density filters (Figure 6-1) to allow the brightest stars to be used to reset encoder positions. Note that the GUIDER camera does not have this protection.
The cameras can be rotated in the MIS to align them along the compass points. Contact the staff if you believe there is an alignment problem. Note that the display orientation changes with position angle of the instrument rotator ( Chapter 8), and for direct versus slit plate guiding (Section 7.4).
If you experience difficulty in acquiring objects in the guider, consult Figure 6-3 for a system checkout.
6.3 The Multiple Instrument System Control Software, xmis
The MIS control crate is connected to the observer's console (mcgraw) and commands to operate the various slides, lamps and filter wheels are entered through the xmis window (Figure 6-4) using a mouse and keyboard. When the filter wheel is not used the information on the filters is not displayed in the window.
This chapter assumes you are familiar with all of Chapter 4. Start-up procedures for the software are described in Section 4.3.4.
When the software is loaded a large red box with the word INITIALIZE appears. Click on this with the mouse to initialize communications with the MIS control crate. Use the xmis SETUP menu to Initialize or Quit the software at any time. If one or more of the modules fails to initialize using xmis, but continues after pressing the reset button on the control crate, check the fuses at the back of the control crate.
6.4 Finder (Finder camera is no longer present)
The finder serves two basic functions:
The position of the main sliding double-sided mirror determines which function is accessible. When the FINDER mirror is IN the starlight to the guider and your instrument is blocked; it is reflected to the wide field finder FINDER CAMERA (camera doesn't exist).
6.4.1 Wide field acquisition of astronomical objects (obsolete)
To acquire an astronomical object with the finder:
As of 2003, the motion of the FINDER mirror is no longer controlled by the FINDER button on the main xmis, which is broken and continuously flashes "Moving". Instead there is an aluminum utility box to the left of mcgraw's two (2) monitors with a switch that is used to move the FINDER in and out.
The star should now be displayed on the guide monitor, although it will probably be out of focus. Start with a bright star if the system needs to be focussed (Section 6.6).
If no star light is visible then consult Figure 6-3 for a system check-out.
6.4.2 Spectroscopic calibration
Spectroscopic calibration is also done with the FINDER mirror IN. Light from emission lamps or flat field lamp is reflected off the underneath of the double-sided sliding mirror.
To obtain calibration arcs or flat fields:
As of 2003, the motion of the FINDER mirror is no longer controlled by the FINDER button on the main xmis, which is broken and continuously flashes "Moving". Instead there is an aluminum utility box to the right of mcgraw's monitors with a switch that is used to move the FINDER in and out.
Neon, argon, mercury-neon and xenon spectral lamps and an incandescent (quartz) continuum source are all available. A spectral atlas for each lamp, together with typical grism settings, is provided in the manual for each spectrograph.
6.5 Guider
The FINDER mirror must be OUT to allow starlight to reach the guider and your instrument.
The guider can be used in one of two modes:
An autoguider is available and is described in Chapter 7, which assumes that you are familiar with the contents of this section.
The guider has a diagonal probe whose minor axis is 25.4 mm. The probe can move in x and y directions as specified in Figure 6.5.
The probe has three basic positions accessible through the Preset pop-up menu:
6.5.1 xmis guider parameters; preset pop-up menu
The probe can be moved to the first three positions selecting the destination in the xmis Preset pop-up window. The coordinates of the guider probe, together with the message [O], [C] or [S] are displayed in the xmis window:
| Origin: | Guider: O | (0, | 0) |
| Center: | Guider: C | (14700, | 17000) |
| Slit: | Guider: S | (14700, | 0) |
6.5.2 Off-axis guiding for direct imaging
The probe can move off-axis in x and y directions as specified in Figure 6-5. The SLIT VIEWING OPTICS vignette just a fraction of the available field. Pull the plunger out if this interferes with your upward looking optics guide stars.
There are two ways to move to an off-axis position, using either absolute or incremental instructions.
6.5.3 Slit plate guiding for spectroscopy
The FINDER mirror must OUT to allow starlight to reach the guider and your spectrograph.
Use the xmis Preset menu to move the guider probe to Slit. Check that the SLIT VIEWING OPTICS are IN. Put the CAMERA SELECTOR to GUIDER. You should now see the star reflected off the slit. If you do not then consult Figure 6-3 for a system check-out.
If the slit and the finder are not collimated to each other then try a minor "tweak" on the slit viewer plunger. Use the TV monitor in the dome to check interactively.
If there is no suitable guide star on the slit plate you can swap to off-axis guiding once you have centered the object on the slit. See Section 6.5.2 for this. You will have to re-focus the guider using the guider focus hand paddle (located to the left of the find/guide monitor) and you must also change the autoguider orientation parameters (Section 7.4). To get back to viewing the slit use the xmis preset Slit command and refocus the guider again.
6.6 How to Focus the Finder (Obsolete) , Guider, and Your Instrument
This section assumes you are familiar with the preceding sections. The procedure is different for direct imaging and spectroscopy. Once everything is aligned and focussed you can note the field centers using the DVI hand paddle (Section 7.3)
6.6.1 Focus procedure: direct imaging
6.6.2 Focus procedure: spectroscopy
6.6.3 Coarse Focus of the Guider
The guider box has a coarse focus which allows instruments to sit at different back focal distances. If you cannot achieve focus with the small guider hand paddle (located to the right of the TCS monitor) then you will need to adjust the coarse focus. The electrical motor which provides guider fine focus adjustments is mounted on a movable stage. Coarse focus is obtained by translating this stage by hand using the following procedure:
6.7 Filter Wheels and Filters
6.7.1 Two-inch filter wheel
The oldest filter-box houses a wheel which holds up to eight filters. It can accommodate 2-inch square, 2-inch round, 1-inch round and other sizes by special adaptors. Any filters available at MDM will be will be loaded by the staff; please specify exactly which filters you require in your Observing Preparation Form. As of September 1995 we are no longer supposed to borrow filters from NOAO.
You can gain access to the change door while the box is mounted to the telescope, allowing more than 8 filters to be used if necessary. The filter positions are marked 0 through 7. When filter 0 is in the beam filter 4 is accessible through the filter door. A special, rather fragile, tool is used to install and remove the filters. Do not overtighten the screws. Thermal changes can cause the pressure to increase and the filters to crack!
The filter wheel is moved through the xmis window, the current filter being highlighted in yellow. To have xmis display the names of new filters installed in the wheel:
misfilter
xmis &
in an xterm window.
6.7.2 Four-inch filter wheel
In March 2004 a new filter wheel that holds up to 12 four-inch square filters will be put into service at the 1.3-m telescope. It was constructed at OSU, and uses the same xmis software as the 2-inch filter wheel. It can be used with any of the MDM CCDs, but was designed specifically to accommodate a new 4Kx4K CCD camera that is being designed exclusively for the 1.3-m. This new filter wheel uses semi-permanent filter holders. Therefore, a set of new filters will be installed exclusively in this filter wheel (see Section 6.7.4). The new 4-inch filter wheel is expected to reside exclusively at the 1.3-m.
There is also an old 4-inch filter wheel that was sometimes used at the 1.3-m. It is now expected to remain at the 2.4-m.
6.7.3 The MDM 1999 2" UBVRI filter sets
Two new UBVRI filter sets, two inches square, were acquired in 1999. The specifications were supplied by Pat Seizer, and the filters were constructed by Custom Scientific, Inc. (David Marcus) of Phoenix, AZ [(602) 200-9200]. The exact prescriptions were designed by Alistair Walker. Transmission curves and other data for these and many other MDM filters for CCD imaging at both telescopes are available. If you request any of these filters they will be installed for you by the staff.
| FILTER | Glass |
| Bessel U | |
| Harris B | GG385 2mm, BG12 1mm, S8612 2mm, WG280 1mm |
| Harris V | GG495 2mm, BG38 2mm, S8612 1mm |
| Harris R | OG570 3mm, KG3 2mm |
| Arizona I | 805/150 nm |
These filters are of the same thickness, 5mm; they were designed to be parfocal. The "Arizona" I is an interference filter with sharp cut-offs at 7230Å and 8800Å.
6.7.4 The MDM 2004 4" UBVRI filter set
In 2004 MDM received a set of four-inch square filters for use in the new filter wheel. They were supplied by Custom Scientific, Inc. to the same specifications as the 1999 2" UBVRI filters. Until we install the actual transmission curves here, just assume that they are the same as the 2" filters.
6.8 Technical Reference Section
6.8.1 Guest Instrumentation and the MIS mounting plate
The finder, guider and filterbox modules of the MIS all have the same basic 12 mounting holes (Figure 6-6). These holes, 0.625 DIA, take reducing bushings and nuts. These are available in stainless steel and nylon. An electrical isolating spacer, made of G10, is also available.
The vertical height of each box is shown in Figure 6-1; a variety of back focal distances can be accommodated by using a combination of the modules:
| Modules used | remaining back focal distance (inches) |
| none | 15.72 |
| Finder | 7.25 |
| Finder + Guider | 3.00 |
| Finder + Guider + Filter wheel | 0.50 |
If the MIS is not used at all, the instrumentation can be mounted directly on the instrument rotator (Chapter 8). The MDM 8K Camera doesn't use the MIS.
The diameter of the central hole in the FINDER and GUIDER boxes is 177.8 mm (7.0 inches), and the unvignetted field diameter of the telescope at f/7.5 is 125 mm (5 inches). The guider probe and the slit-viewing optics both vignette this field (Figure 6-5).
The slit viewing optics were designed for use with a focal plane 63.5 mm (2.50 inches) below the guider box. The optimal focal plane of the telescope, however, lies 76.2 mm (3.00 inches) below the guider. The slit viewing optics can be adjusted to accommodate slit planes from approximately 50 to 81 mm (2.0 to 3.2 inches) below the guider box. The FILTER WHEEL has, in addition to the 12 mounting holes around the perimeter which take the reducing bushings, 8 holes tapped 1/4- 20 as shown. The central hole is 2.25 inches in diameter.
The maximum total weight of the instrument depends upon its distance and distribution from the instrument rotator, and is approximately 500 pounds (225 kg). A moving platform allows heavy instruments to be raised for attachment to the telescope. Please consult the staff at an early stage if you plan on bringing out your own instrumentation.
6.8.2 MIS Controller: FORTH commands (for staff use only)
The MIS control crate microprocessor is programmed in FORTH. It communicates with the telescope control computer, tambora, through an X-windows package, called xmis, developed by John Tonry and Mark Metzger (formerly of MIT).
It can also be controlled through a dumb terminal by issuing commands: SEE THE STAFF FOR A LIST OF COMMANDS (NO USER FRIENDLY PARTS) IF THERE IS A PROBLEM USING THE XMIS WINDOW.
6.8.3 xmis software configuration (for staff use only)
Staff notice for instrument changes: The xmis software needs to know which of the three MIS modules are installed. The telconfig script is used to setup both the TCS and MIS configuration for a given telescope and instrument combination. See the Startup Guide for details.
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