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

Please read chapter News to get information about the actual status of the TWIN spectrograph.



Contents

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  1. Preface
    1. List of Figures
    2. List of Tables
  2. General Information
    1. Basic Configuration of the TWIN
    2. Optical Diagram of the TWIN
    3. Summary of Basic Parameters of the TWIN
  3. Operation of individual components of the TWIN
    1. Comparison/Calibration light sources
    2. Slit Apertures
    3. Position Angle of Entrance Slit
    4. Gratings and Grating Angle
    5. Filters for Order Separation
  4. Operation of TWIN and CCD cameras
    1. General Remarks
    2. Description of the TWIN Control Program
    3. Description of the CCD Control Program
  5. Appendix
    1. Efficiency Curves of Gratings
    2. News


Preface

This user's guide is intended to serve as a reference document for users of the 3.5 m Cassegrain Twin Spectrograph (TWIN) in connection with a CCD detector system.

A description of the CCD-dectors in use can be found in the CCD detector overview .

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List of Figures

List of Tables


General Information

Basic Configuration of the TWIN

The Cassegrain Twin Spectrograph (TWIN) of the 3.5 m telescope ( Fig. 2-1:) has been designed for spectroscopic observations of point sources or extended objects at intermediate spectral resolution (typically 20 to 150 per mm) 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".


FIG 2-1: The TWIN Spectrograph with CCD-detectors on the 3.5m telescope.

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 3.5 m 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 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.


Graphical User Interface for TWIN and CCD control ( Under construction !)

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Optical Diagram

The optical diagramm is presented in Fig. 2-3:. 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 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, at an incidence angle of 18 degree. 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 2-3: 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.

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Summary of basic instrument parameters

Table 2-1: Basic instrument parameters of the TWIN:

Scale at entrance slit: 1" = 170 mm
Length of entrance slit: 240"
Focal ratio of collimators: f/10
Focal length of collimators: 1460 mm
Ruled area of gratings: 154 x 206 mm2
Focal length of cameras: 230 mm
Maximum field of cameras: 7 x 40 mm
Scale at camera exit (slit direction): 1" = 26.8 mm
Cross-over wavelength of dichroic mirrors: 4500, 5500, 6800, 7500

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Operation of individual components of the TWIN

Comparison/Calibration light sources

The spectrograph includes a comparison/calibration light unit ( Fig. 3-2:) for wavelength calibration or flat fielding of the spectrograms. 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. 2-2).



FIG 3-2: Comparison/Calibration light source unit. Filters can be inserted in front of each lamp.

The comparison/calibration unit provides for 7 different light sources, to be selected by a rotatable diagonal mirror within that unit. The presently available sources are listed in Table 3-1.

Table 3-1:. Comparison/Calibration light sources

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).

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Slit apertures

The slit aperture wheel provides for 8 different positions, each representing a predefined slit aperture. The presently available slits are listed in Table 3-2.

Table 3-2:. Slit apertures

Position [mm] [arc sec]
1 0.10 x 40 0.6 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

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Slit postion angle

The actual position angle of the slit PA (Slit) depends on the position angle of the telescopes Cassegrain instrument flange PA (Cass), namely,

PA (Slit) = PA (Cass) + 90 .

The slit angle can be changed by rotating the entire TWIN (including the TV Guider of the telescope) on the telescopes Cassegrain instrument flange. Rotation is performed normally using the hand paddle at the main mirror cell of the telescope. The PA (Cass) (= "PWCAS") will be indicated on the diplay box ( Fig. 3-1:) at the main mirror cell of the telescope (after pressing the corresponding button on that box). It is strictly recommended not to rotate the TWIN unless all cables can be carefully watched which run from the instrument to the telescope main mirror cell. The instrument flange must be unclamped before and clamped after performing a rotation using the hand paddle.

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Gratings and grating angle

Various gratings are available to be used either in one or both channels of the TWIN. The gratings and their basic characteristics are listed in Table 3-3. Efficiency curves for most of the gratings are collected in the Appendix. 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.

Table 3-3: List of gratings of the TWIN

No. grooves blaze angle blaze recommended application
1. order range order dispersion channel
[lines/mm] [degrees] [Å] [Å] [Å/mm]
T01 830 19 7700 6000-10000 1 52 red
3200-5000 2 26 blue
T03 600 17 9500 7500-11000 1 72 red
3800-5500 2 36 blue
T04 600 13 7100 5500-9000 1 72 red
3200-4500 2 36 blue
T05 1200 17 4700 3700-6500 1 36 blue
T06 1200 26 7100 5500-9000 1 36 red
3200-4300 2 18 blue
T07 400 9 8000 5500-11000 1 108 red
3300-5000 2 54 blue
T08 600 9 4700 3500-6500 1 72 blue
T09 600 22 12000 8400-11000 1 72 red
5500-7500 2 36 red
3500-4500 3 24 blue
T10 830 30 11500 8400-11000 1 52 red
5500-7000 2 26 red
4500-5500 2 26 blue
T11 270 5 7200 5500-11000 1 160 red
3200-4500 2 80 blue
T12 600 7 3800 3100-4800 1 72 blue
T13 300 4 4700 3500-5500 1 144 blue
5500-7000 1 144 red


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 * 1896700 / (m * a)

where m denotes the spectral order and a the number of grooves per mm of the grating.
A more exact computation of the grating angle is done by using the program twinangle on any of the Calar Alto workstations. (The program is made available by entering getastro caha).

Table 3-4 presents the grating angles required for given wavelengths and gratings. (Please note that a second order grating of 600 g/mm will act in the same way as a first order grating of 1200 g/mm, etc.). Back to Contents


Table 3-4a: Grating angle as a function of the central wavelength WL [Å] and the virtual number of grooves per mm. (The virtual number is the actual number multiplied by the spectral order number).
Table for blue part of spectrum.
WL 300g/mm 2600g/mm 800g/mm 1200g/mm 1660g/mm 1800g/mm 2400g/mm
[Å] degmin degmin degmin degmin degmin degmin degmin
3200 2 54 5 48 7 45 11 40 16 15 17 40 23 53
3300 2 59 5 59 8 00 12 03 16 47 18 15 24 40
3400 3 04 6 10 8 14 12 25 17 18 18 49 25 28
3500 3 10 6 21 8 29 12 47 17 50 19 24 26 17
3600 3 15 6 32 8 44 13 09 18 21 19 58 27 05
3700 3 21 6 43 8 58 13 32 18 53 20 33 27 55
3800 3 26 6 54 9 13 13 54 19 25 21 08 28 44
3900 3 32 7 05 9 28 14 17 19 57 21 43 29 34
4000 3 37 7 16 9 42 14 39 20 29 22 18 30 24
4100 3 43 7 27 9 57 15 02 21 01 22 53 31 15
4200 3 48 7 38 10 12 15 24 21 34 23 29 32 06
4300 3 53 7 49 10 26 15 47 22 06 24 05 32 57
4400 3 59 8 00 10 41 16 09 22 38 24 40 33 49
4500 3 04 8 11 10 56 16 32 23 11 25 16 34 42
4600 4 10 8 22 11 11 16 55 23 44 25 53 35 35
4700 4 15 8 33 11 26 17 17 24 17 26 29 36 29
4800 4 21 8 44 11 40 17 40 24 50 27 05 0 0
4900 4 26 8 55 11 55 18 03 25 23 27 42 0 0
5000 4 32 9 06 12 10 18 26 25 57 28 19 0 0
5100 4 37 9 17 12 25 18 49 26 30 28 56 0 0
5200 4 43 9 28 12 40 19 12 27 04 29 34 0 0
5300 4 48 9 39 12 55 19 35 27 38 30 11 0 0
5400 4 53 9 50 13 09 19 58 28 12 30 49 0 0
5500 4 59 10 01 13 24 20 21 28 46 31 27 0 0
5600 4 04 10 12 13 39 20 45 29 20 32 06 0 0
5700 5 10 10 23 13 54 21 08 29 55 32 44 0 0
5800 5 15 10 34 14 09 21 31 30 30 33 23 0 0
5900 5 21 10 45 14 24 21 55 31 05 34 03 0 0
6000 5 26 10 56 14 39 22 18 31 40 34 42 0 0
6100 5 32 11 07 14 54 22 42 32 16 35 22 0 0
6200 5 37 11 18 15 09 23 05 32 51 36 02 0 0
6300 5 43 11 29 15 24 23 29 33 27 36 43 0 0
6400 5 48 11 40 15 40 23 53 34 03 0 0 0 0
6500 5 54 11 51 15 55 24 17 34 40 0 0 0 0
6600 5 59 12 03 16 10 24 40 35 17 0 0 0 0
6700 6 05 12 14 16 25 25 04 35 54 0 0 0 0
6800 6 10 12 25 16 40 25 28 36 31 0 0 0 0
6900 6 15 12 36 16 55 25 53 0 0 0 0 0 0
7000 6 21 12 47 17 10 26 17 0 0 0 0 0 0
7100 6 26 12 58 17 25 26 41 0 0 0 0 0 0
7200 6 32 13 09 17 40 27 05 0 0 0 0 0 0
7300 6 37 13 21 17 56 27 30 0 0 0 0 0 0
7400 6 43 13 32 18 11 27 55 0 0 0 0 0 0
7500 6 48 13 43 18 26 28 19 0 0 0 0 0 0

Table 3-4b: Grating angle as a function of the central wavelength WL [Å] and the virtual number of grooves per mm. Table for red part of spectrum.
WL 270/mm 300g/mm 400g/mm 600g/mm 830g/mm 1200g/mm
[Å] degmin degmin degmin degmin degmin degmin
5500 4 29 4 59 6 39 10 01 13 55 20 21
5600 4 34 5 04 6 46 10 12 14 11 20 45
5700 4 39 5 10 6 54 10 23 14 26 21 08
5800 4 44 5 15 7 01 10 34 14 42 21 31
5900 4 49 5 21 7 08 10 45 14 57 21 55
6000 4 53 5 26 7 16 10 56 15 13 22 18
6100 4 58 5 32 7 23 11 07 15 28 22 42
6200 5 03 5 37 7 30 11 18 15 44 23 05
6300 5 08 5 43 7 38 11 29 16 00 23 29
6400 5 13 5 48 7 45 11 40 16 15 23 53
6500 5 18 5 54 7 52 11 51 16 31 24 17
6600 5 23 5 59 8 00 12 03 16 47 24 40
6700 5 28 6 05 8 07 12 14 17 02 25 04
6800 5 33 6 10 8 14 12 25 17 18 25 28
6900 5 38 6 15 8 22 12 36 17 34 25 53
7000 5 43 6 21 8 29 12 47 17 50 26 17
7100 5 48 6 26 8 36 12 58 18 06 26 41
7200 5 52 6 32 8 44 13 09 18 21 27 05
7300 5 57 6 37 8 51 13 21 18 37 27 30
7400 6 02 6 43 8 58 13 32 18 53 27 55
7500 6 07 6 48 9 06 13 43 19 09 28 19
7600 6 12 6 54 9 13 13 54 19 25 28 44
7700 6 17 6 59 9 20 14 05 19 41 29 09
7800 6 22 7 05 9 28 14 17 19 57 29 34
7900 6 27 7 10 9 35 14 28 20 13 29 59
8000 6 32 7 16 9 42 14 39 20 29 30 24
8100 6 37 7 21 9 50 14 50 20 45 30 49
8200 6 42 7 27 9 57 15 02 21 01 31 15
8300 6 47 7 32 10 04 15 13 21 17 31 40
8400 6 52 7 38 10 12 15 24 21 34 32 06
8500 6 56 7 43 10 19 15 35 21 50 32 32
8600 7 01 7 49 10 26 15 47 22 06 32 57
8700 7 06 7 54 10 34 15 58 22 22 33 23
8800 7 11 8 00 10 41 16 09 22 38 33 49
8900 7 16 8 05 10 49 16 21 22 55 34 16
9000 7 21 8 11 10 56 16 32 23 11 34 42
9100 7 26 8 16 11 03 16 43 23 28 35 09
9200 7 31 8 22 11 11 16 55 23 44 35 35
9300 7 36 8 27 11 18 17 06 24 00 36 02
9400 7 41 8 33 11 26 17 17 24 17 36 29
9500 7 46 8 38 11 33 17 29 24 33 0 0
9600 7 51 8 44 11 40 17 40 24 50 0 0
9700 7 56 8 49 11 48 17 52 25 07 0 0
9800 8 01 8 55 11 55 18 03 25 23 0 0
9900 8 06 9 00 12 03 18 15 25 40 0 0
10000 8 11 9 06 12 10 18 26 25 57 0 0
10100 8 15 9 11 12 17 18 38 26 13 0 0
10200 8 20 9 17 12 25 18 49 26 30 0 0
10300 8 25 9 22 12 32 19 00 26 47 0 0
10400 8 30 9 28 12 40 19 12 27 04 0 0
10500 8 35 9 33 12 47 19 24 27 21 0 0

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Filters for order separation

In most applications filters are not required to block unwanted spectral orders, since the dichroic mirror serves for the same purpose. Nevertheless, 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 ( Fig. 2-1). Installing filters into the filter carriers is a delicate operation restricted to the authorized personnel.

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

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Operation of TWIN and CCD cameras

The CCD cameras

Both the red and the blue spectrograph channel are equipped with an independent CCD camera. In the current version specially coated CCD detectors from SITe are used which have 2000 x 800 pixels with a pixel size of 15 micron. Information on the detectors is given in the CCD detector overview .
See also the CCD manual for operation of the CCD's.

Overview

Both the TWIN spectrograph and the CCD cameras are controlled and operated by the same workstation using the Calar Alto EPICS data base system. There is one Graphical User Interface (GUI) window for the TWIN spectrograph and one common (GUI) window for the CCD-camera of the blue and of the red channel. Standard Calar Alto GUI's are used as known from several other instruments.

Log in, start, ...

  • Login at the X-terminal as obs35. The Password is given to you by the Calar Alto staff.
  • An instrument list is showing up where you should select the item TWIN + CCD.
  • Invoke the TWIN-GUI by entering start_twin in a xterm-window.
  • Invoke the CCD-GUI for the TWIN-CCD cameras by entering start_twincam.
  • Set the path(s) for storing the datafiles (*.fits) of red and blue channel in the CCD-GUI.
  • Start one or two MIDAS-sessions from that data directories.
    Note: In the moment images have to be converted (intape/fits ...) and loaded by the observer.

    Operation of the TWIN spectrograph

    Basics

    The shutters and the exposure times are controlled by the CCD window. So, only the grating angles, the slit width and the settings for the calibration frames are entered at the TWIN control window.

    A detailed manual for using the CCD cameras and the TWIN spectrograph during observation is under construction.

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    Appendix

    Efficiency Curves of Gratings


    Grating No. 1


    Grating No. 4


    Grating No. 5


    Grating No. 6


    Grating No. 7


    Grating No. 8


    Grating No. 9


    Grating No. 10


    Grating No. 12

    News

    The TWIN spectrograph is currently being upgraded.
    In a first upgrade in october 1996:
    1. Two new SITe CCD-detectors with 2000 x 800 Pixel and a Pixelsize of 15 micron have been installed.
    2. 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.
    3. Aditionally to the beamdivider at 5500 three more beamdividers are now available for separation at 4500, 6800, and 7500 .

    A second upgrade was performed in May 1998. The following status is achieved now:

    1. The TWIN-program and the CCD-program are now running under a graphical user interface (GUI).
      In the moment they are implemented on the CASTOR workstation.
    2. Log-in Procedure:
      Login at the X-terminal with obs35.
      The Password is given to you by the Calar Alto staff.
    3. An instrument list is showing up where you should select the item Twin + CCD.
    4. Invoke the TWIN-GUI by entering start_twin in a xterm-window.
    5. Invoke the CCD-GUI for the TWIN-CCD cameras by entering start_twincam.
    6. Set the path for storing the datafiles (*.fits) in the CCD-GUI.
      All data files are saved to disk into this subdirectory under /disk-c/obs35.
    7. Start one or two MIDAS-sessions from that data directory/ies.
      Note: In the moment images have to be converted (intape/fits ...) and loaded by the observer.
    8. Working with the new GUI's should be more or less self-explainable. A complete new version of the manual is just in progress.
    9. The optics of the Schmidt cameras of the TWIN have been updated. This should lead to an increase of about 30% in light.

    U. Graser, 23-JUNE-1998

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    Uwe Graser
    graser@mpia-hd.mpg.de