46 research outputs found
Exact Optics: A unification of optical telescope design
A perfect focus telescope is one in which all rays parallel to the axis meet
at a point and give equal magnification there. It is shown that these two
conditions define the shapes of both primary and secondary mirrors. Apart from
scale, the solution depends upon two parameters, , which gives the mirror
separation in terms of the effective focal length, and , which gives the
relative position of the final focus in that unit. The two conditions ensure
that the optical systems have neither spherical aberration nor coma, no matter
how fast the ratio. All known coma--free systems emerge as approximate
special cases. In his classical paper, K. Schwarzschild studied all two mirror
systems whose profiles were conic sections. We make no such a priori shape
conditions but demand a perfect focus and solve for the mirrors' shapes.Comment: 11 pages, LaTex ([alleqno,epsfig]{mn}), 7 Figures (eps), accepted by
MNRA
A purely reflective large wide-field telescope
Two versions of a fast, purely reflective Paul-Baker type telescope are
discussed, each with an 8.4-m aperture, 3 deg diameter flat field and f/1.25
focal ratio.
The first version is based on a common, even asphere type of surface with
zero conic constant. The primary and tertiary mirrors are 6th order aspheres,
while the secondary mirror is an 8th order asphere (referred to here for
brevity, as the 6/8/6 configuration). The D_80 diameter of a star image varies
from 0''.18 on the optical axis up to 0''.27 at the edge of the field (9.3-13.5
mcm).
The second version of the telescope is based on a polysag surface type which
uses a polynomial expansion in the sag z, r^2 = 2R_0z - (1+b)z^2 + a_3 z^3 +
a_4 z^4 + ... + a_N z^N, instead of the common form of an aspheric surface.
This approach results in somewhat better images, with D_80 ranging from 0''.16
to 0''.23, using a lower-order 3/4/3 combination of powers for the mirror
surfaces. An additional example with 3.5-m aperture, 3.5 deg diameter flat
field, and f/1.25 focal ratio featuring near-diffraction-limited image quality
is also presented.Comment: 14 pages, 6 figures; new examples adde
Fine structure of the chromospheric activity in Solar-type stars - The Halpha Line
A calibration of H-alpha as both a chromospheric diagnostic and an age
indicator is presented, complementing the works previously done on this subject
(Herbig 1985, Pasquini & Pallavicini 1991. The chromospheric diagnostic was
built with a statistically significant sample, covering nine years of
observations, and including 175 solar neighborhood stars. Regarding the age
indicator, the presence of stars for which very accurate ages are determined,
such as those belonging to clusters and kinematic groups, lends confidence to
our analysis. We also investigate the possibility that stars of the same age
might have gone through different tracks of chromospheric decay, identifying -
within the same age range - effects of metallicity and mass. These parameters,
however, as well as age, seem to be significant only for dwarf stars, losing
their meaning when we analyze stars in the subgiant branch. This result
suggests that, in these evolved stars, the emission mechanism cannot be
magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al.
2002c, and references therein). The Sun is found to be a typical star in its
H-alpha chromospheric flux, for its age, mass and metallicity. As a byproduct
of this work, we developed an automatic method to determine temperatures from
the wings of H-alpha, which means the suppression of the error inherent to the
visual procedure used in the literature.Comment: 10 pages, 10 figures, accepted for publication in Astronomy &
Astrophysics. Nature of replacement: match astro-ph and ADS title (greek
letter
Two-mirror Schwarzschild aplanats. Basic relations
It is shown that the theory of aplanatic two-mirror telescopes developed by
Karl Schwarzschild in 1905 leads to the unified description both the prefocal
and the postfocal systems. The class of surfaces in the ZEMAX optical program
has been properly extended to ascertain the image quality in exact
Schwarzschild aplanats. A comparison of Schwarzschild aplanats with approximate
Ritchey-Chretien and Gregory-Maksutov aplanatic telescopes reveals a noticeable
advantage of the former at fast focal ratio of the system.Comment: 19 page
An application of the SMS method for imaging designs.
The Simultaneous Multiple Surface (SMS) method in planar geometry (2D) is applied to imaging designs, generating lenses that compare well with aplanatic designs. When the merit function utilizes image quality over the entire field (not just paraxial), the SMS strategy is superior. In fact, the traditional aplanatic approach is actually a particular case of the SMS strateg
The atmospheric dispersion corrector for the Large Sky Area Multi--object Fibre Spectroscopic Telescope (LAMOST)
The Large Sky Area Multi--object Fibre Spectroscopic Telescope (LAMOST) is
the largest (aperture 4 m) wide field of view (FOV) telescope and is equipped
with the largest amount (4000) of optical fibres in the world. For the LAMOST
North and the LAMOST South the FOV are 5 deg and 3.5 deg, the linear diameters
are 1.75 m and 1.22 m, respectively. A new kind of atmospheric dispersion
corrector (ADC) is put forward and designed for LAMOST. It is a segmented lens
which consists of many lens--prism strips. Although it is very big, its
thickness is only 12 mm. Thus the difficulty of obtaining big optical glass is
avoided, and the aberration caused by the ADC is small. Moving this segmented
lens along the optical axis, the different dispersions can be obtained. The
effects of ADC's slits on the diffraction energy distribution and on the
obstruction of light are discussed. The aberration caused by ADC is calculated
and discussed. All these results are acceptable. Such an ADC could also be used
for other optical fibre spectroscopic telescopes, especially those which a have
very large FOV.Comment: 14 pages, 8 figures, Accepted for publication in MNRA
Recommended from our members
Guiding eyepiece for 36-inch photometer
Total length of item is 95 mm.Eyepiece for photometer used on 36-inch telescope at Cambridge Observatory. The cover arrangement was designed by Dr. Roderick Willstrop to protect the optics from stray light