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SPECTROGRAPHS AND RELATED TOPICS


Spectrograph - Entrance Optics - CCD ST-10 - Atmosphere - Flexure tests - Ref Lamps


Flexure Tests on our spectrograph (03-Jan-2006)
In order to verify the precision that can be attained in radial velocity measurements with our spectrograph, we performed some flexure tests. Spectrograph is mounted on vertical rails on the back of the telescope as it is shown in picture 1.

Picture 1: A view of our spectrograph. It is not designed to be very rigid, infact it should move along two rails for focusing. Neon ref lamp illuminates a diffusive circle inserted in the optical path. No particular care was devoted to reproduce the entrance beam shape also with the reference lamp.

Mechanical Flexure Tests
Flexure tests were performing recording spectra of HD9138 and HD37160. For the first star the measurements started when passing at meridian and stopped 4 hours later. HD37160 was measured starting from 3h30min before meridian and up to the passage to meridian. All measurements were recorded together with a set of lamp spectra before and after the star spectrum. The picture 2 shows the spectrum shift as a function of hour angle due to telescope/spectrograph flexures. The difference between lamp and spectrum position is about 0.2 pixels and this set the maximum precision that can be attained in radial speed measurements. With the 0.39 A/pixel used, this error is at maximum 4.8 Km/sec.
Our goal is now to work on the spectrograph in order to reduce flexures and differential shift between reference lamp and science spectrum

Picture 2: A graph showing the shift in pixel of the spectrum onto CCD as an effect of differential flexures of the spectrograph at different telescope positions. Results are not satisfactory because error can attain 4.8 Km/sec.

Picture 3: Motorized Cassegrain secondary mirror allows focusing without moving spectrograph.

Picture 4: Control electronics for secondary Cassegrain mirror and spectrograph temperature readout.

Picture 5: Design of a new spectrograph with all parts included in a solid ox in order to reduce flexures.




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09/04/2004