Cassegrain TWIN Spectrograph
3.5 m Telescope Calar Alto
User´s Manual


  1. General Information
    1. Basic Configuration of the TWIN
    2. Optical Diagram of the TWIN
    3. Summary of Basic Parameters of the TWIN

  2. Operation of TWIN and CCD cameras
    1. Starting TWIN
    2. TWIN control GUI
    3. CCDs control GUI

General Information

Basic Configuration of the TWIN

The Cassegrain Twin Spectrograph (TWIN) of the 3.5 m telescope has been designed for spectroscopic observations of point sources or extended objects at intermediate spectral resolution (typically 20 to 150 /mm, 0.30 to 2.25 /pix) in the wavelength range from 3200 to 11000 . With an assumption of one pixel resolution (i.e. 15 micron) this leads to spectral resolutions in the range of R=3000 to R=14000. The TWIN includes two separate spectroscopic channels ("Blue" and "Red") behind the common entrance slit aperture. The light from the slit is divided into two beams by means of a dichroic mirror ( 4 beam dividers available with cross-over wavelengths near 4500, 5500, 6800, and 7500 , respectively). Each channel consists of its own components including filter, shutter, collimator, grating, camera and detector. The optics of the "blue channel" and the "red channel" have been optimized for the regions from 3200 to 5500 and from 5000 to 10000 , respectively. The TWIN can be converted into a single-channel instrument by replacing the beam divider by a mirror acting as a "red beam selector".

Various reflectance gratings of 154 x 206 [mm] ruled area are available which can be used in either channel. The folded Schmidt-type cameras provide an external focal plane of 7 x 40 [mm] area, such that various CCD detectors can be easily attached to the TWIN. An eight-position aperture wheel in the focal plane of the telescope allows selection of various predefined slit widths. In each case the slit length is 240 arc sec. The aperture wheel is replaceable. Various spectral lamps and a continuum light source have been built-in (matching the exit pupil of the telescope) which can be selected for wavelength calibration or "flat fielding".

The electronics of the TWIN and of the two CCD detectors are controlled by a workstation via an EPICS data base system. The TWIN and the CCD cameras are operated by a graphical user interface.

The TV guiding and acquisition system of the 3.5m telescope will be used for viewing or guiding the object on the reflecting slit mask or for offset guiding. The autoguider can be used either on the slit view, acquisition field or the offset field.

The whole instrument (including the TV guider) can be rotated on the Cassegrain instrument flange in order to select the position angle of the entrance slit. Rotation is performed normally using the command CASFL at the tecs35 console. The PA of the the slit is PA(Slit) = PA (Cass) + 90.

Optical Diagram

The optical diagram is presented in Fig. 1-2:. The f/10 beam from the 3.5 m telescope enters the TWIN at its entrance slit aperture. A total of 8 different slit apertures, mounted on a replaceable aperture wheel, can be selected. The maximum length of the entrance slit is 210" (unvignetted) or 240" (vignetted up to 30% on one side).

The light reflected by the aluminized aperture mask (slit) is fed into the TV guiding and acquisition system of the telescope. A retractable mirror can be brought in front of the slit, in order to feed the light from the comparison/calibration unit onto the slit. In that case, all light from the telescope will be blocked from entering the spectrograph. The light beam from the comparison/calibration sources matches the f/10 beam from the telescope (same position of exit pupil). Each of the comparison/calibration light sources is selected by a rotatable diagonal mirror within the comparison/calibration unit.

The light entering the TWIN through the slit is divided into two beams by means of a dichroic mirror, tilted 18 degrees. The cross-over regions for the 4 available beam dividers are near 4500, 5500, 6800, and 7500 , respectively. The beam divider unit contains additional flat mirrors in order to bring each beam into its particular spectrograph channel. The light path and the optical components of either the "blue channel" or the "red channel" are nearly identical and will be described below. The beam divider unit can be removed from the TWIN and replaced by a "red beam selector" containing a single mirror. In that case all the light is fed into the red channel. Each spectrograph channel includes a filter carrier (for blocking unwanted spectral orders of the grating) and a shutter in the light path between the beam divider and the collimator. Each collimator is an off-axis parabolic mirror of 185 mm diameter and 1460 mm focal length. The collimator mirror can be moved (manually) in the direction of its axis in order to make focus corrections in the exit plane of the cameras of the TWIN.

FIG 1-2: Optical diagram of the TWIN. Indicated dimensions are quoted in mm.

The collimated light is refracted towards the spectrograph camera by (interchangeable) reflectance gratings of 154x206 mm2 ruled area. The angle between the incident and (central) diffracted light beam is 37 degree. The diffraction angle of the grating can be set with an accuracy of 3.6" (about 0.25 at 36 per mm). Each camera is of the "folded Schmidt-type" consisting of a corrector plate of 210 mm diameter, a spherical mirror of 210 mm diameter and 230 mm focal length, and a large diagonal flat mirror between corrector and spherical mirror. The diagonal mirror has a central hole, permitting the convergent beam from the spherical mirror to reach the external focus (about 15 mm behind the exit flange of the camera housing). The curved field of the camera is made flat by means of an aspheric field flattener lens. (For the currently used CCD-detectors, i.e. SITe, 2000 x 800 pixel a 15 micron, these lenses are specifically designed serving as windows of the CCD-dewars). The useful size of the camera field is 7mm x 40 mm.

Summary of basic instrument parameters

Scale at entrance slit 1" = 170 µ
Length of entrance slit 240"
Focal ratio of collimators f/10
Focal length of collimators 1460 mm
Ruled area of gratings 154 x 206 mm
Focal length of cameras 230 mm
Maximum field of cameras 7 x 40 mm
Scale at camera exit (slit direction) 1" = 26.8 µ     1 pix = 0.56"
Slit apertures Width ["] 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 3.6, 10.0       Length 240"
Position angle Adjustable by rotating the spectrograph at the cassegrain flange
Order sorting By beam splitter and/or colour filters
Beam splitters Available at 450, 550, 680, and 750 nm
Filters for order separation WG295, WG360, OG515, RG610, BG39, Calflex X1
Detectors SITe-CCD's with 2000 x 800 Pixel 15 µ (CCD)
Comparison lamps ThAr, HeAr, FeNe, Ne
Flat fields Quartz-halogen bulbs (+ various filters) (built-in). Dome light and screen
Grating list

Operation of TWIN and CCD cameras

Starting TWIN

To start an usual observing run follow the next steps:

TWIN control GUI

Press any button in the GUI to get a description of its function

Calibration lamps

The spectrograph includes a comparison/calibration light unit ( Fig. 2-1) for wavelength calibration or flat fielding of the images. A retractable comparison mirror in front of the slit reflects the light from that unit onto the slit. If the mirror is in that position it will block all light from the telescope.

FIG 3-2: Comparison/Calibration light source unit.

The comparison/calibration unit provides for 5 different light sources, to be selected by a rotatable diagonal mirror within that unit. The presently available sources are:

Position Light source
1 He-Ar glow lamp
2 Fe-Ne hollow cathod
3 Th-Ar hollow cathod
4 not used
5 not used
6 Incandescent lamp (for flat field)
7 Ne glow lamp

Note: Individual light sources can be modified by color or neutral density filters to be inserted in front of the sources within the comparison/ calibration light unit (operation restricted to staff members).

To manage this unit, click on TWIN-GUI "calibration" button and the next small GUI will appear:

Select light source (He-Ar, Th-Ar, Fe-Ne, Ne or continuum), switch light on and change mirror position in. When the exposure it is finished, switch light off, change mirror position out and click on exit.


The slit aperture wheel provides for 8 different positions, each representing a predefined slit aperture. The presently available slits are:

Position Slit aperture
mm arc secs
1 1.00 x 40 5.9 x 240
2 0.15 x 40 0.9 x 240
3 0.20 x 40 1.2 x 240
4 0.25 x 40 1.5 x 240
5 0.30 x 40 1.8 x 240
6 0.35 x 40 2.1 x 240
7 0.40 x 40 2.4 x 240
8 0.60 x 40 3.5 x 240

Mask position

A holder for slit masks that can be moved along the slit has been implemented. Moving it along the slit allows for masking bright stars on the reflecting slit jaws, that otherwise would cause the TV-camera to go in overload.


Various gratings are available to be used either in one or both channels of the TWIN. Changing gratings is a delicate procedure and therefore restricted exclusively to the authorized personnel. Please note that changing gratings is time consuming and not at all recommended to be done during an observing night. A grating angle of zero indicates that the zero spectral order will be located at the center of the camera field.

The relation between the Grating angle THETA [degree] and the wavelength at the center of the spectrum WL [] is given roughly by

WL = sin THETA * 18967000 / (m * a)

where "m" denotes the spectral order and "a" the number of grooves per mm of the grating.

The grating angle can be changed clicking on the "Red-Grating" or the "Blue-Grating" button and entering the new angle values in the small GUI that it is shown.

Anyway, it is easier and recommended to use the "Calc Grating" button. This way, after choosing the channel and order in the new GUI, just enter the central wavelength click on "Calculate" and "Set Grating Angle", and the grating will be accurately positioned.

Order separation filters

Although most of the light of unwanted spectral orders it is blocked by the dichroics, the transmission (or reflection) of these it is not a 100% for the complete wavelength range. For this reason, in some configurations, order separation filters are needed.
Filters can be inserted into the light beam of either channel using the filter carriers of the TWIN. A carrier can take only one filter. The filter carriers are operated manually using the handles at the bottom plate of the TWIN

Installing filters into the filter carriers is a delicate operation restricted to the authorized personnel.

Available filters for order separation: WG 295, WG 360, OG 515, RG 610, BG 39, Calflex X1.

At the TWIN-GUI can be read which filters are installed in both arms, and if these are in or out of the light beam


Actually four beam dividers are available for separation at 4500, 5500, 6800 and 7500 .
As can be seen in next graphics, the red part of the light beam it is not 100% transmitted. For this reason, and for some configurations (grating, central wavelength, etc ...) some undesired reflections can appear. The effect it is avoided when using the red beam selector mirror.
At the TWIN-GUI it is shown which one it is installed.

CCDs control GUI

With this GUI we manage the two CCD-Cameras of TWIN spectrograph. Buttons and fields on the left part (blue background) are referred to the blue-arm camera and the ones on the right side (red background) to the red-arm camera. Description of any button or field can be get it clicking on it and some common characteristics are listed here:


Choosing one of the entries will open the following popup-Windows:

Camera Electronic

If you get many error-messages like "no serial connection" or "timeout in select" or if you only get noise instead of an CCD-Image, push this button and wait until you get the message in the System Messages lines "Press start for a new exposure". Attention! The following 3 images after the reset may have a slightly higher noise level!

Chip Info

... with some detector information like chiptype, overscan, prescan and postscan and upper limits for x and y.

Dewar Temp.

... with the actual Dewar Temperature. Pushing the button "Get Temp." will update the information if possible.


Choosing one of the entries will open the following popup-Windows:

Focus Parameters

... with two entry fields: Number of rows to shift during a focus exposure and the number of focus-loops.


... with the toggle-button to switch between saoimage/saotng or another quicklook-system like midas. If you push the button, the image will be automatically loaded into saoimage or saotng if one of these programs is open.


...will show the status of disks.


Here, you can load a saved parameter-file by double-click or click & Load, save the actual parameters in a parameter-file (give a Filename and press SaveAs), view the entries of a parameter-file or delete a file. Special: the 4 files user_def1-4.par are related with the 4 buttons "USER DEF1-4" in the Main-Window. This means, if you save a configuration in one of these files, you can load them by pressing the related button.

Change Output Filename

The text-field shows the last Image-Filename (except this is the first exposure or something's gone wrong and the Filename has already been updated). It will be checked and updated when starting an exposure. You can change the prefix, image-path and observer-name in the window that will pop up after pushing the "Change"-button.

Exposure parameters

Enter the desired exposure time into the entry-field "time". Accuracy is up to 0.1sec. 0 sec is allowed.

Choose (by pressing the right mouse button) the desired exposure-type. The type is also saved as Fits-Keyword

A dark exposure means, the shutter is kept closed during the whole exposure-time

A science exposure means, the shutter is open for the desired exposure time.

A flat exposure is the same as a science exposure, only the keyword in the Fits-header will be different.

A calibration exposure is similar to flat o science exposure, but ensure that the calibration lamp will switch on and mirror in when the exposure start (if it is not done in advance).

The toggle-button "Mode" switches the Image-Filename between test0001.fits and the specified Image Filename. The file test0001.fits will always be overwritten and not incremented.

CCDs Configuration

The text-fields show the actual area of interest (in x1/y1 x2/y2) plus the binning information (H-bin means horizontal binning, V-bin vertical binning factor).

The push-button "Full Frame" will set the the coordinates of the area of interest to a full frame image at the selected binning.

If you want to change the x/y-parameters or the binning, push the button "Change".

In the following popup-window you see the text-entry fields for the area of interest and the binning. x1-y1 is the lower left corner and the lower limit for x is 2.

The fields for readout-speed, duration, mean, noise, sensitivity and gain are not yet implemented.

The lower part of the configuration window shows some detector information like chiptype, overscan, prescan and postscan in physical pixels (15 µ).

Readout Speed

Actually only one readout speed it is available, labeled "normal". It takes aprox. 55 secs. to read the full unbinned chip.

Object Name

Enter into this text-entry field the Object Name of your observation. If exposure mode "normal" is selected then any object name is required. For "test" mode this field can be left blank.
This entry is deleted every time after an exposure. You can get the last name back by pushing the button LAST NAME. The object name is saved as Fits-Keyword in the header of the image file.


The text-entry line for comments is saved twice as Fits-Keyword in the header of the image file, one at the beginning of the exposure and the other at the end. This means, you can enter comments about clouds, seeing, etc ... during a longtime exposure.

Start Exposure

When clicking on this button an exposure is started with the actual parameters. Obtaining all the telescope and instrument information, as well as reserving disk space for the image-file, it is done before the real exposure start and takes only 2 or 3 seconds.

During exposures, the remaining exposure time is displayed as a green bar-graph below the System Messages. If the remaining exposure time is greater than 3sec, you can stop the exposure by pressing the STOP button.
This will close the shutter and activate the 4 buttons on the right: Change ExpTime, Resume, Readout and Abort .

Clicking on Change ExpTime a new window appears where the exposure time can be changed.

With resume, you re-open the shutter (not during dark-exposures of course) and continue with the exposure. The dark-time in the FITS-Header will differ from the exposure-time.

With read out, you terminate the exposure and start immediately the readout process. The exposure-time in the FITS-Header will be the actual elapsed time with shutter open.

With abort, you dismiss the exposure completely.

Synchron Mode

When the asynchron button is pressed up, you can manage the blue and red CCDs cameras in a completely independ way.
Working in synchron mode anything you type or select in the input-fields (black background) of one camera will be writted in both. And more important, when clicking on a START button the exposure will start in both cameras.
Attention !! synchron mode is not recommended for short exposures (few seconds) because some times the shutter doesn't work properly.

Santos Pedraz Marcos
Last modified: Mon Jun 18 02:30:45 GMT 2001