A newtonian
reflector suited for solar observation only
How to
realize a reflector telescope able to rival with a classic long focus refractor
for solar study
In solar study a long focus refractor is generally preferred due to its
relative insensitiveness to turbulence (closed
tube), to obstruction absence and to the reduced quantity of scattered
light (absence of reflective surfaces)
that lead to rich in contrast and sharp images. On the other hand, reflectors,
both opened (Newton, Cassegrain) and closed (Catadioptric) ones, seldom provide
solar images comparable to the best refractors, with the only exception, in a
relative sense, of long focus Maksutov (f/15 and beyond).
This brief note will describe a particular
Newtonian reflector, specially designed for solar observation in order to
reduce, as far as possible, the previous drawbacks.
The proposed optical lay-out is shown in
the following figure.
As in a
classic Newtonian, the system is made of two optical elements only: a parabolic
primary mirror and a flat window that, as will be cleared soon, works as a full
aperture solar filter and a secondary mirror as well.
Solar
light strikes a plano-parallel window angled at 45°, whose inner face is partially
aluminium coated with a transmission around 1%. So, light intensity is firstly
and drastically reduced while crossing the telescope front closing window. In
this way, the optical beam is also remarkably shifted upward by D,
due to refraction through the not negligible thickness S of optical window with a refractive index n [1]:
D = 0.707 S (n – 1)/n
Practically,
assuming a refractive index value of 1,5 for the window glass, the vertical
beam shift will be a little less than ¼ of window thickness; a fact that needs
to be taken into account to correctly design the telescope entrance opening.
In
effect, the adoption of an optical window with a certain thickness is suggested
to avoid the superposition of ghost images in the eyepiece field, due to
internal reflections of primary mirror beam on the outer uncoated face of
opening window. Anyway, the secondary image is highly weakened by the very
small transmission factor of the window partially reflective inner face, as
shown in the following figure.
The entrance beam, weakened by window
crossing, goes on toward primary mirror that, being uncoated, reflects only 4%
of incoming light. In this way, a second substantial beam weakening is
realized, with a final transmission of 0.0004% (1:2.500) on the focal plane; a
value quite suitable for photo and CCD imaging of solar photosphere.
On the
contrary, for visual observation an eyepiece filter with a transmission between
2.5% and 25% must be added, in order to obtain a total transmission,
respectively, between 1:100.000 and 1:10.000. To this aim, a couple of rotating
polarizing filters can be adopted, with a variable transmission between 5% and
25%, specially useful during mutable sky haze conditions.
From a general point of view, such an
instrument shows the double advantage to have no obstruction at all and to be
closed. Moreover, it is absolutely safe as, even if the opening window should
unfortunately fall or break, the primary focalised beam could not reach the
eyepiece in any way. On the other side, the main drawback stands on the high
cost of flat window that, optically finished, must be sized to cover the
primary mirror full diameter: practically, 1,5 times the useful instrument
aperture. This matter makes convenient to realize small to medium sized
instruments only.
To realize our instrument we chose a
primary mirror with a diameter of 65mm and a focal length of 500mm. The
plane-parallel window, finished at l/10 (550nm), was 4” (101,6mm) in
diameter and 3/8” (9,525mm) in thickness. As the following photo clearly shows,
the entrance opening was suitably shaped in a sort of ellipse to fully cover
the primary mirror aperture.
From a practical point of view, we
considered convenient adopting a squared Aluminium tube (80x80mm sided, 2,5mm
thick) in order to simplify the mechanical connection of the entrance window
support and to easily realize the correct eyepiece holder position, angled at
45° with the window plane. In this way, we could simplify the mechanical
design, making the instrument collimation through the primary support only, the
unique adjustable element in the system. The outer tube surface was
intentionally kept Aluminium finished, for a best sun light reflection. The
inner tube surface was instead darkened with opaque black enamel.
The focuser was simply realized using the
helicoid focusing mechanism of a photo lens after dismounting optics and iris.
The instrument is completed by a small finder, provided with a full aperture “astrosolar”
(by Baader Planetarium gmbh) filter, and a squared tube collar, provided with
two threaded holes (1/4” and 3/8”) for a standard photo tripod mount.
