Baffle and internal baffles (2/3)


SEPTA in the main pipe:

Once the length of the tube, You can begin to determine where they should be placed internal SEPTA, in order to block the off-axis light entering the telescope, and reflected or scattered by internal surfaces, before this I can get on the primary mirror or focal-plane shutter.

When a beam of light hits the surface of an object, This reflects both believed that speculate, and the relationship between the two depends a lot of the same physical and geometric characteristics of the surface. There are various models to describe how light is reflected in various directions, and choose which one is the one that reflects as closely as the reality is not simple. This however, It is important if you volgiano make numerical analysis of detail through special baffle performance software, While as regards the determination of placement of SEPTA, that is the aspect that most concerns us, a first approximation can be ignored.

The following video shows the procedure to determine the location of the baffles inside the tube of the telescope. As you can see, the procedure is very simple and is certainly more understandable through the Visual example that not using a thousand words:

As seen in the video the location and extent of the baffles depends very much on the inside diameter of the tube, or by the parameter "a". By varying this parameter will change, In addition to SEPTA and their position radial extension, even their number. As you can see from the image below, a lower diameter pipe, top (so with "a ' less), requires a higher number of SEPTA to reject the first reflection, compared to a solution with higher diameter pipe.

Though a large number of SEPTA, Besides being wasteful from the point of view of implementation, does not represent an optimal solution, because the edges of the SEPTA are themselves a source of diffused light, that cannot be in any way screenshot. Therefore   is good to reduce the number to a minimum.

As is always the case so, you have to seek a compromise solution, between the number of SEPTA and containment of pipe size, that may not be overly large in relation to the diameter of the lens, for obvious reasons of size and portability.

SEPTA in the barrel of Focuser:

The same reasoning for led tube, This can extend even to the focheffiatore tube.
The unwanted light, can get to hit the inside wall of the barrel, thus reflected or diffused towards the focal plane. It is good therefore predict small setti here too, in the same way as described in the previous video.
We start by placing the outermost septum near the end of barrel, so that goes to the light cone of the desired field of view. From here you can begin the process of positioning of the other SEPTA.
The limit at which must not be placed other SEPTA, will be to the fact that, None of the eyepieces will have to go to beat the last septum, When we can insert it into the Focuser.

It is important to note how the SEPTA also inserted inside the barrel of Focuser, could help reduce the minimum height of the tube (explained earlier in the article 1/1), and then the dimensions of the telescope.


Since the edges of the SEPTA are themselves sources of stray light, We must take particular care in achieving the same, making them the finer and sharp as possible. For reasons of rigidity, It is not easy to make very fine and SEPTA can not be deformed, so Alternatively, can be implemented with a certain thickness and then wedge this bevelled inner edges, placing then the flat face to the outside of the tube and the side slanting towards the mirror (as shown in Figure):

A thick septum, with the flat edge, has a circular area where off-axis light can bounce off without being in any way caught or screenshot, being able to get on the primary mirror. Unlike these, SEPTA with the sharp edge do not possess this land, and the light, that first was reflected to the mirror, now continues its path until you reach the next septal wall, coming thus adequately screen.

Leave a Reply