One of the things that can be done to improve the performance of a telescope, is associated with undesirable phenomenon of stray light, due to reflection of radiation from external sources from surfaces before the telescope's detector (or of the eyepiece), which are characterized by a certain roughness (Hence the diffuse reflection).
The baffles are devices to help reduce interference from to sources of direct lighting, or spread by internal surfaces of the telescope, they tend to decrease the contrast in images or even to provoke the so-called ghost (Ghost images).
Generally in all telescopes, to try to reduce the side effects of these disorders, scorch the inside of the pipe with low reflective black paint, so that the reflected radiation from the tube is minimal. In addition to this though, SEPTA can be inserted (walls in annulus) which are designed to intercept any light reflected or diffused by tube, primary mirror schermandola, in a sense, as you do with your hand on your forehead when you want repair by sunlight.
These devices are absolutely paramount in optical telescopes and space probes in the rooms, as well as protecting the sensor from interference sources, due to the Sun or to the black body radiation of the planets, allow the instrument's first passive thermal control auks. Are almost always present even in refracting telescopes or in good quality reflectors, but practically never used in dobson. In fact for those accustomed to watching from very dark skies, the baffle is not essential, since they fail the main sources of disturbance light (Sun, Moon, artificial lights etc), but for all those who often find themselves observing in sub-urban skies where artificial lights can tarnish your image quality, or for those who observe double stars or objects even with the presence of the Moon in the sky, then this device can improve the performance of your telescope.
As mentioned above, SEPTA present inside the telescope must be positioned in such a way as to prevent at least the first reflection directed to the mirror by any portion of the surface of the inner tube. Well built and designed baffle, may force the light to make several reflections before you can reach the primary mirror, reducing the intensity of the scattered radiation at each reflection. Anyway for amateur applications the Elimination of first reflection directed to the mirror, is more than enough to raise the quality of the images observed.
EXAMPLE for a NEWTONIAN TELESCOPE
- Determination of the minimum height of the tube.
NELL's image alongside shows the Schematic of a Newtonian telescope, where one can distinguish, the primary mirror diameter "D", the secondary mirror, the Focuser, the inner wall of the telescope tube diameter Dt and the cone of light that opens to the sky at an angle equal to the field of view of its instrument (on one side) and with a certain size of the illuminated field (on the other). "A" means the space between the edge of the mirror and the pipe wall.
Once you have this data, You can start by determining what the minimum required length of tube.
The most trivial criterion is to make it a little higher spider attack points of the secondary, but this is a suboptimal solution dismissive and. A better policy, is to cut it to a length that, that no light can get directly inside the illuminated field in the focal plane.
To determine this fee, you have to draw a line from the lower end of the illuminated field to the top, and more inside the telescope, Focuser barrel. Extending this line until it meets the inner wall of the telescope across, We get the minimum height to which terminate the pipe. In fact every other line departing from an altitude higher than the minimum just found, fail to enter the field light up of the telescope, as incoccerebbe before the barrel of Focuser. in the case of a closed pipe, the mirror line to this, who descended in the direction of the primary, It doesn't matter, as the tube or any black cloth (in a dobson) Shield rays coming from this area.
If your telescope is not present in the pipe, talking a bath towel around the lattice structure, It is good to place a hood that covers the entire area between the two red lines above are plotted (as you can see in this picture).
If the tube is too short, then with a height lower than the minimum, as is often the case when the choice is to make him finish just above the attachment points of the spider, It happens that any light source, artificial and not, that is within the dashed red Cone, in the image below, would be able to arrive directly on the photo sensor or the eyepiece greatly degrading images.
In the case of a dobsonian, generally the rule of thumb is to minimize masses and dimensions, thing that causes it to produce very thin secondary cells, and not able to screen unwanted lights. To remedy this, many amateur astronomers are constructed of cylindrical removable hood, to be placed on top of the telescope, they have dual screen functionality against the outside lights and protection against fogging and condensation on the secondary.
One example is that described in This article by Julius.