Short review - Sivo nu-View spectrometer full review *.doc *.txt return to wpo review page
by Maurice Gavin - June '98
Traditionally
a pro spectroscope was attached direct to the focal plane of a Cassegrain
telescope. More common today is to feed the spectroscope via
a fibre-optic cable (FOC or cable) and isolate the spectroscope
from the telescope so no weight is imposed. The Sivo
Nu-View spectrometer takes this approach. The cable
comprises
a bundle of 50 micron fibres each 1m long. The telescope focuses the stellar
target onto the input end of the cable where the fibres are bunched
into a 0.4mm disk. The spectroscopic slit is formed from the fibres themselves
at the output end where they are teased into a single line 50 micron wide
by 1.6mm long. The 1200 line/mm grating is ~25mm square and
epoxied to an adjustable turntable. Two f/2 50mm fl achromats serve as
collimator and camera lens and a plane mirror folds the design.
Recording the spectrum: Camera focus is very critical and somewhat crudely implemented by physically sliding the camera back and forth in the exit port via the 1-1/4 inch adapter. Whilst focusing the camera it must also be rotated so the spectrum run parallel to the CCD’s longer side. A locking nut secures the camera when both are achieved. A supplied dial gauge helps to reregister the camera focus when subsequently removed and replaced. However the dial gauge will only interface with the large front plate (cooling fin of cameras like the ST-6) and with my Starlight Xpress cameras it was ineffective . The supplied exit lens can be removed altogether and the image recorded with a lens attached to the camera applied to the exit port. This permits precise and easy focusing via the lens’s helical mount but is prone to causes some vignetting and the camera needs support.
At the telescope: Setting up the spectrometer and coupling it to the telescope is best done in daylight. First the telescope finder is accurately aligned with the cable’s target area - the lunar crescent makes a good reference. Now aiming at a bright star, the telescope focus is adjusted whilst viewing the downloaded images. Typically a sharply focused star will occupy only part of the 0.4mm diameter target area. For a brighter target it is prudent to slightly defocus the telescope image so the spectrum is fully illuminated but of lower intensity so the image is not saturated. Using my 30cm Meade LX200 the longest exposure attempted was 20 minutes on b Lyrae in the Ha region and the bright emission lines were clearly recorded. To record other portions of the spectrum the micrometer is adjusted and the procedure repeated. An evening’s output will be fair low as target stars get fainter.
The 1.6mm high slit projected onto a CCD may occupy more than 100 rows of pixels providing a large sampling area (after dark and flatfield calibration). Pixels are binned in columns (under software control) to extract the complete signal. Binning is not limited by the CCD’s A/D converter but may have a fuller dynamic range. MS-DOS Software for conversion to FITS and spreadsheet readable format is supplied.
Conclusions: The price tag is relatively modest for
a small box of optics with no electronics. However in a modern disposable
age the spectrometer’s simplicity is its major asset and with care
it will give a lifetime's service.
Footnote: Since preparing this review a Mark II version
of the spectrometer has become available which effectively doubles the
resolution.