Instrumentation

Ondřejov Echelle Spectrograph

Ondřejov Echelle Spectrograph (OES) is a fibre-fed high-resolution spectrograph. The spectrograph is installed in a temperature-stabilised room. The light is directed via optical fiber (core diameter 0.1 mm) from the primary focus to the coudé room where it is reflected to an echelle grating (made by Richardson Grating Laboratories) with the size of 408×165×74 mm with 55 gr/mm and a blaze angle θ=69°. The optical mirror reflects the light further to a equilateral prism (made by TOPTEC Turnov, CZ, glass Schott LF5) with apical angle of 55°, which separates the orders that are then brought to a focus on the detector. The detector is a nitrogen-cooled EEV 2048×2048 pixel CCD (Versarray 2048B, EEV 42-10-1-36B) with a pixel size of 13.5 μm and a dynamical range of 65535 ADUs. The read-out noise is of about 3.5 e− rms, and a dark current of 1 e−/p/hr. In addition, the spectrograph is fed by a calibration lamp. Also, the iodine cell is mounted on the telescope-spectrograph interface, providing a secondary spectral calibration source.

A more detailed technical description with mechanical setup and all optical elements can be found in the report from the installation phase of the OES (Koubský at al., Ondřejov Echelle Spectrograph – OES, Publ. Astron. Inst. ASCR, 92:37–43, 2004).

Technical Specification and atlas of ThAr lines

Guiding System

Fig. 1: A sketch of the OES showing the optical layout and light path from the slit to the detector. A – slit, B – collimator, C – echelle grating, D – first parabolic mirror, E – small flat mirror, F – second parabolic mirror (optical elements D, E and F are parts of so called white pupil, making the light beam more narrow), G- prism, H – objective lens and dewar wessel. Credit: Miroslav Šlechta.

Parameters for Optical Elements

  • Beam diameter at collimator: 142 mm.
  • Collimator focal ratio: f/32.
  • Angle of crossdisperser: 55°.
  • Objective focal ratio: f/1.8.
  • Camera objective lense focal length: 200 mm.
  • Slit width: 0.6 mm or 1.8 arcsec.
Fig. 2: Upper image – the mechanical parts holding the optical elements of the Ondrejov Echelle Spectrograph. Lower image – the detector with the dewar wessel. Credit: Miroslav Šlechta.

Grating

  • Effective dimensions: 157×412 mm.
  • Echelle (Milton Roy): 54.5 g/mm.
  • Blaze angle: 69°.
  • Material: Zerodur.
  • Made by Richardson Grating Laboratories.

Prism

  • Dimensions of the base: 144×118 mm (118 mm is the length of the diffraction edge).
  • Height of the prism from the base: 140 mm.
  • Diffraction angle: 54.5°.
  • Material: LF5.
  • Density: 3.25 g/mm3.
  • Made by Walter, Faculty of Mathematics and Physics, Charles Univ., Prague, CZ.

Objective

  • Canon EF lens 200, F1.8L Ultrasonic.
  • f=200 mm.
  • Focal ratio: f/1.8.
  • Diameter of input beam: 111.1 mm.
  • field of view: 12°.

Dewar and CCD chip

  • Detector: Versarray 2048B (for more details see here)
  • Chip: EEV 42-40-1-368.
  • Dimensions: 2048×2048 pixels.
  • Size of pixel: 13.5 x 13.5 μm.
  • Gain: 2.
  • Reading noise: 10.
  • Cooling: liquid nitrogen.
  • Operational temperature: −110°C.
  • Made by Roper Scientific.

Wavelength coverage and performance

The wavelength coverage of OES is from near UV (3753 Å) up to near IR (9195 Å). The resolving power is R=51600 at 5000 Å (R∼40000 in Hα) and spectral sampling is 2.4 Å/mm. The spectral range is covered by 56 usable orders. The number of spectral orders range from 92 to 36. The individual spectral order covers ∼70 Å in the near UV region and ∼145 Å in near IR regions. In blue, spectral orders overlap, thus, it is possible to merge them. The overlapping interval is ∼20 Å in near UV. However, the overlapping intervals gets shorter towards red and the last (short) overlapping intervals are between spectral orders covering 5782-5879 and 5879-5977 (the overlap is less than 1 Å). Between the orders covering 5879-5977 and 5978-6079, there is already a gap (∼1 Å). The gap is gradually increasing and around H the gap between consecutive orders is ∼13 Å and even ∼63 Å around 8800 Å. For more details about OES see also Kabáth et al., 2020, PASP, 132, 5002.

SNR vs. Exposure time

The appropriate exposure time can be roughly estimated using Fig. 3. It is seen that for the signal-to-noise ratio (SNR) of ~65, an 1-hour exposure time is needed for a 10-mag star. However, the particular SNR depends not only on the zenith distance but mainly on the observing conditions. The limiting precision regarding the radial velocities is in the order of hundreds m/s for faint stars. By using the iodine cell, precision is about a few tens of m/s.

Fig. 3: The exposure time needed to get desired SNR with OES. Credit: Miroslav Šlechta

Single Order Spectrograph

Single order coudé spectrograph works in the first and second spectral orders. Wavelength coverage of the second order is 4000-5100 Å (near UV and blue) with order length of 233 Å. Wavelength coverage of the first order is 5100-8900 Å (red and near IR) with order length of 470 Å. Resolving power is ∼13000 in Hα spectral region. The detector is an PyLoN 2048x512BX pixel with nitrogen-cooled CCD (E2V 42-10 BX) with a pixel size of 13.5 μm, cooled by liquid nitrogen to −115°C and a dynamical range of 600.2 ADUs. The main advantage is that it depicts wide interval of wavelengths (475 Å in the Hα spectral region) with uniform S/N.

Technical Specification and atlas of ThAr lines

Guiding System

Optical layout

Fig. 1: A schematic of the single order spectrograph showing the optical layout and light path from slit to detector. Credit: Miroslav Šlechta.
  • Separating glass plate – the glass plate is installed in the tube connecting the coude room to the telescope. This tube is corresponding to the hour axis of the telescope. The light beam is going through this tube to the slit head. The glass plate divides space in coudé room and the dome and thus it prevent from the draught.
  • Turning prism + eyepiece – the astronomer should be able to look to the field of view of the telescope (although it is not used very often). Then one can put a prism to the optical path and redirect the light beam to the ocular with thread cross.
  • Dichroic mirrors – there are 4 dichroic mirror for the optimal separation of spectral orders: red, infrared, blue, and aluminium. However, the profit is not very important and thus, mainly due to auto-guiding (see individual chapter in link below) we use aluminium mirror.
  • Slit width: 0.2 mm (0.6 arcsec).
  • Exposuremeter: just behind the slit, semitranslucent mirror, about 5% of the light to the exposuremeter.
  • Spectral filters – for the separation of spectral orders.
  • Collimator: focal ratio f/32, out of axis.
  • Grating – works in the first and second spectral orders.
  • Schmidt camera (correction plate + spherical mirror)
  • Detector: PyLoN:2K (for more details see here)
  • Camera: Schmidt camera, focal length 700mm.

Slit head

  • Flat field lamp: Hollow cathode
  • Comparison light: ThAr lamp, 15 mA
  • Slitwidth: 0.2 mm (0.6 arcsec)
  • Spectral filters: to separate spectral orders

SNR vs. Exposure time

The appropriate exposure time can be roughly estimated. It is seen that for the signal-to-noise ratio (SNR) of ~30, an 1-hour exposure time is needed for a 10-mag star. However, the particular SNR depends not only on the zenith distance but mainly on the observing conditions.


The exposure time needed to get desired SNR with the single order spectrograph. Credit: Miroslav Šlechta

Photometric camera

Fig 1: The primary focus with the CCD camera for optical imaging. Credit: Miroslav Šlechta.

The photometric camera is installed in the primary focus. The focal length in primary focus is 9 m (focal ratio f/4.5) but the photometric camera changes the effective focal length in primary focus to 7 m and focal ratio to f/3.5. The change between fibers (i.e., spectrographs) and photometric camera can be done within 1 minute.

  • Camera: G2 MarkII 3200 (Moravian instruments)
  • CCD chip: KAF-3200ME, 2252×1544, pixel size 6.8×6.8 microns
  • Readout noise: 9 ADUs
  • Gain: 0.93
  • Cooling: Peltier
  • Effective focal length: 700cm
  • Field of view: 7×5 arcminutes (rotated 45 degrees with respect to North up, East left)
  • Resolution: 0.2 arcsec/px
  • Filters: u’, g’, r’, i’, z’, and 3nm Hα

Photometric filters