Single order spectrograph

Single order (formerly coudé spectrograph or D700 spectrograph) is fiber fed medium resolution spectrograph working in the first and second spectral orders. Wavelength coverage of the second order is 4000-5100 Å (near UV to blue) with order length of 233 Å; wavelength coverage of the first order is 5100-8900 Å (green to near IR) with order length of 470 Å. Resolving power is ∼13 000 in Hα spectral region. The camera features the PyLoN 2048x512BX (E2V 42-10 BX) CCD detector (cooled by liquid nitrogen to -115°C) with a pixel size of 13.5 μm. The main advantage is that it depicts wide interval of wavelengths (475 Å in the Hα spectral region) with uniform S/N. More information about technical specification and calibration can be found here.

Camera main characteristics
Camera Chip Resolution Pixel size Reading
PyLoN E2V 42-10 BX 2048×512 13.5 x 13.5 μm 19 s
Spectrograph main characteristics
1as order
Order length
2nd order
order length
5100-8900 Å 470 Å 4000-5100 Å 233 Å ~13000 (Hα)
Optical layout
  • Slit width: 0.2 mm (0.6 arcsec)
  • Flat field lamp: Hollow cathode
  • Comparison light: ThAr lamp, 15 mA
  • Exposuremeter: just behind the slit, semi-translucent mirror, about 5% of the light to the exposuremeter
  • Collimator: focal ratio f/32, out of axis
  • Grating – works in the first and second spectral orders
  • Schmidt camera (correction plate + spherical mirror), F=700mm
A schematic of the single order spectrograph showing the optical layout and light path from slit to detector. Credit: Miroslav Šlechta.
Sample images
ThAr Calibration lamp
beta Ori
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.