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The choice of the spectrograph design (Fig.2) is determined by the quantity R, the overall dimensions of the PF cage and the restrictions on the rigidity of the structure of the device.
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We have chosen
mm, which at the PF relative aperture of 1:4 yields
mm. The collimator is assembled according to Newton
scheme, the dimensions of the flat refracting mirror are such that it falls
within the central shaded part of the collimated beam. The diameter of the
shaded part is determined by the relation between the main mirror diameter
and the PF cage diameter. As the camera's objective a fast 4-lens aplanat is
used having a focal distance Fcam=140 mm and a relative aperture of
1:1.8, the resolution at the centre is no less than 80 gr/mm, that over the
field at a distance of 3 mm from the edge along the frame (
) diagonal is 35 gr/mm. These parameters allow a two-pixel
resolution over the whole area of the CCD used (
) to
be realized. The light transmission is 0.9, the drop in illumination over the
frame (
) is no more than 20%. An echelle grating
of 75 gr/mm,
, is used, the size of the shaded area is
, the concentration in the working order is 70% in
fractions of the reflected light. As a cross-dispersion element a grating of
300gr/mm, working in the first order, is used, the size of the shaded area
is
, the concentration in the working order is 90%
in fractions of the reflected light. The instrument is equipped with
additional cross-dispersion gratings 600 and 1200 gr/mm. The orientation of
the echelle grating conforms to case ``C'' according Schroeder and Hillard
(1980), i.e.
, and
. Such a configuration
permits the centres of all optical elements to be arranged in one plane.
The slit part is a turrel mounting with several deckers of fixed sizes. The
sizes of the working rectangular entrance diaphragms are
arcsec
and
arcsec. The inclination of monochromatic slit images due to
the non-zero
angle value is made up for by changing the orientation
of the decker. In the preslit part are mounted the optics of the TV guide
and the optics of the calibration channel, the latter being thick
fiber-optics passing radiation to the spectrograph entrance. All the optical
and mechanical units are mounted on a single frame, on the opposite side of
which (i.e. outside the optical part volume) the electronic units of the
calibration and control systems are fixed (design and execution of V.I.
Fateev). The proposed design of the spectrograph made it possible to minimize
deviations of the centre of gravity of the structure, reduce its weight and
use such an orientation of the CCD cryostat that enables observations at the
prime focus of over 15 hours without replenishing liquid nitrogen.
The CCD
system incorporates the cryostat camera and the controller unit, connected by
a cable 3 m long. The controller is housed in a commercial 19-inch casing
and is connected to the computer (two coaxial cables 200 m in
length).
The system ensures control by the CCD in the frame mode. The read-out of a
frame fragment and the binning over the two coordinates are possible. The
read-out time of a frame of pixels is 101 s with a
reduction rate of 12 kpixels/s. The signal reduction channel is characterized
by the following parameters:
16-digit analog-to-digital conversion;
charge-code conversion coefficient -- 2.3e-/ADU;
read-out noise -- 6.5e-;
bias level -- 1600 ADU;
stability of the bias level -- 0.5 ADU;
integral nonlinearity of the channel -- no higher than 0.05%;
differential nonlinearity -- no more than 0.5 ADU.
The image is aligned on the CCD frame as follows: 1160 work
pixels are oriented along the spectral order, 1040 pixels are
oriented in cross-dispersion direction. The pixel size is microns. The CCD angular scale is 11.45''/mm or 0.37''/pix. With
2.5'' seeings the spectral order is 14 pixels in height, i.e. one
can work with a 4-pixel binning across the main dispersion, reducing twice
the caused impact by read-out noise. The spectral resolution
estimated from spectra is R=20000.
Observations are performed according to usual way (calibration by continuous and line spectra, registration of the spectrum of a fast-rotating star, dark-frames) that can be modified if it is needed for a particular programme. With a properly organized sequence of observations, about 90-95% of night time is used for signal registration. The registration of observational data is performed in the environment Nice (Knyazev and Shergin, 1994). The reduction of images is made in the MIDAS system.