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

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

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List of Figures

• Fig. 2-1: The Cassegrain Twin Spectrograph with CCD detectors on the 3.5 m telescope.
• Fig. 2-2: Graphical User Interface for TWIN and CCD control ( Under construction !)
• Fig. 2-3: Optical layout of the TWIN
• Fig. 3-1: Electronic control boxes on the TWIN housing
• Fig. 3-2: Comparison/Calibration light source unit

List of Tables

• Table 2-1: Basic instrument parameters of the TWIN
• Table 3-1: Comparison/Calibration light sources
• Table 3-2: Slit apertures
• Table 3-3: List of gratings
• Table 3-4: Grating angles as function of wavelength for various gratings

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 Å per mm) in the wavelength range from 3200 to 11000 Å. 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 Å, 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 Å and from 5000 to 10000 Å, 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 Å , 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 Å at 36 Å 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

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 Å

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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 [Å] 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]	deg	min	deg	min	deg	min	deg	min	deg	min	deg	min	deg	min
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]	deg	min	deg	min	deg	min	deg	min	deg	min	deg	min
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

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.

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