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 :
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.
 For details see, for instance: Jenkins, White – Fundamentals of Optics – McGraw Hill, 4th ed., pp.24-29