Echelle Spectrograph Description

Design guidelines Design take advantage of experience gained from the use of previous longslit spectrograph that was working since october 2004 in monitorning of novae and symbiotic stars within ANS program . The mail goal was to develop a much more stable spectrograph, with rigid mechanics, in order to achieve a good precision in radial velocity measurements while maintaining a lot of choice of dispersions and spectral ranges easily available.
When we purchased gratings from Optometrics, we saw that echelle gratings were available too at an affordable price and decided to make echelle-mode an optional optical beam in the spectrograph. The picture below shows how we obtained this, using an additional mirror and a transmission grating (300 l/mm) as cross-disperser. Echelle grating is a 79 l/mm R2 grating (25x50mm) and due to the 14 degree working angle is a little overfilled by the 20 mm diameter optical beam resulting in a 10% loss of light.

Picture 1: Both echelle and longslit mode are available in our spectrograph. Switching from echelle to normal grating can be done in less than 5 minutes. Both dispersive system use the same parallel F/20 beam from the spherical collimator f=400 mirror.

Picture 2: Gratings are interchangeable. Available dispersions are 4.3 A/pix (300 l/mm), 2.1 A/pix (600 l/mm), 0.75 A/pix (1200 l/mm) and 0.2 A/pix (approx) with echelle.

Picture 3: A fascinating image of all spectral lamps swtched on. HCL Fe(Ne) lamp is the bigger one. The long blue is Ar and the smaller one in Ne. Unfortunately Fe(Ne) lamp is rich of line only in the regions 4000-6000 and over 7300 A. A similar HCL Th(Ar) lamp is mounted instead since August 2009 and as much as 400 lines ca be used for wavelenght calibration.

Picture 4: Th(Ar) ref lamp recorded in echelle mode from 4000 (left size) to 8600 A (right size). Defocus is unfortunately present at either sizes of the spectral range. PSF for spetra is somewhat better due to smaller filling of the entrance pupil. The lenght of the slit in echelle mode is 1 mm that is equivalent to 17 arcsec on the sky.

Picture 5: Data on dispersion, central wavelenght and spectral coverage of the orders that are recorded on the CCD in our echelle spectrograph. Overlap of spectral orders is complete up to 8000 Angstrom. The spectrograph camera is a Canon 100 mm F2 lens.

Picture 6: spectrum of Radial Velocity standard HR 8308. Tests on a set of radial velocity standards show that radial velocities measured with our echelle spectrograph are accurate to 500 m/sec if calibration lamp frames are recorded before and after the science exposure of 900 sec.

Picture 14: Raw spectrum of CI Cyg at beginning of outburst is the first science target of our new spectrograph in echelle mode. 6x900 sec exposure on 16th Sept 2008.

Picture 15: Orders extracted from previous 2D image containing Halpha, Hbeta and He I 5876. Amount of information on a single echelle spectrum is huge. S/N is not bad for a V=9.6 star.

From design to first light

First mechanical parts were machined on Feb 2006 in our workshop and test at the telescope started on July 2008.

Picture 1: Two projects are starting at home. Carlo and first parts of the echelle-longslit spectrograph. In both cases I had acquired some experiece before. Carlo is following Giulio and Lucia, and the echelle is following at least 3 generation of longslit grating spectrographs.
Picture 2: Some mechanical parts ready for assembly. On the left size, an hollow Cathode Lamp Fe(Ar) used for reference.

Picture 3: Time is running and Carlo is now very interested in testing mechanical parts...

Picture 4: Chassis is ready for painting black. On the background the tools I used. nothing more than a drill and a lathe.

Picture 6: Slit of the spectrograph is cutted into two stainless steel knife edged lips that are mirrorized. A decker is used to limit the slit height for echelle mode (in echelle mode slit is usualli setted to 2x16arcsec)

Picture 7: An alignement setup was prepared that simulate the telescope entrace axis with a laser beam

Picture 8: Aligmement of optical parts is going on... Light from ref lamps come throught a 1mm core optical fiber that is focalized as F20 beam onto the slit.

Picture 9: Lamps for wavelength calibration are contained in a box attached to the tube of the telescope. Available lamps are at the moment a mixture of Ar (a lot of Ar and Ar+ lines) and Ne lamp. Intensity of Ar lines redward 6995 A is reduced with filters. A HCL Th(Ar) lamp is also available for high resolution. We realized by ourselves also the power supply for this lamp. Light from the lamps is collimated with mirrors and lenses. At the end of the optical beam an aspheric condenser focuses the light into a 1mm core plastic fiber that bring the light to the spectrograph.

Picture 10: First tests with CCD mounted onto the spectrograph showed that everything was working fine. Light source was sunlight from an optical fiber.

Picture 10: Spectrograph on July 2008 is finally mounted at Cassegrain focus of the 60 cm telescope atop Campo dei Fiori mountain (1226 m) and test can start.

Picture 11: Twilight sky spectrum with solar lines. Measured spectral resolution with 2 arcsec slit is 17'000.