Baffle and internal baffles (3/3)

The next phase of planning and determination of the appropriate position of various SEPTA along the optical path, consists in making real shielding elements.

As it known until now, It is particularly easy to apply to your own telescope, If the study for positioning of the seven was made in a preliminary planning phase, before you build the telescope, or if it has anything to do with a telescope intubated instead dobson.
While if you want to improve an already constructed telescope, We must take account of the copycats and constraints imposed by the existing structure and solutions.
To give an example, in most home-built dobsonian telescopes, the secondary cell, is lined with a sheet of black plastic material (generally thin corrugated), as in the picture below (Fig. 1).

Because this surface is very close to the edge of the field of view (Fig. 2), It is difficult to have a detrimental effect on the light reflected or diffused, because the parameter "a" (seen in previous columns), is very small, which would require a large number of SEPTA (in blue in fig. 2), slightly pronounced and very close between them, What's not easy to do.
Not simple, but not impossible, a cheap solution, is to be able to remove one of the two layers outer laminates of corrugated, so that men away from the internal ribs. In order to do this, just insert between the tips of scissors, one by one the internal ribs, and proceed to cut it, getting a result similar to that shown in the picture below.

In this way, It is possible to obtain a sheet with applied over a series of ribs, very thin, shallow in height and adequately approximated for the purpose. Just now, Paste this sheet above the black corrugated already in the telescope, to obtain the sequence of SEPTA needed to reject the first true primary mirror reflection.
In the files below you can see some comparisons, between the effect of flipping a light frontally mail, by the standard poilionda, whose surface smooth and shiny refers to the observer much of the reflection of incident light, and corrugated with ribs discoveries that can intercept the incident light, projecting only the shadow toward the observer.

SEPTA IN THE FOCUSER

Create and position adequately SEPTA within the barrel of Focuser is not a simple thing, especially if you want to build a solution easily applicable and replaceable if necessary.
One of the possibilities, probably the easiest for those who may have, is to print using a 3D printer object that it is designed, already in its final form, which can be inserted and removed at will from the back of the barrel, just as you do from the front with the eyepieces.
In this way you can think of a piece with features similar to the one below.

From design:
The definition of the concept:
The finished prototype:

Or you can cut a variety of pipe pieces, of the correct diameter and length, that apply in the middle of a series of discetti of plastic so you get a tower composed of: piece of pipe, diskette, piece of pipe, disk and so on, pasting them together  (as seen in photo upper left, among the images above).

SEPTA IN THE MAIN PIPE

SEPTA to be placed along the main telescope tube, may be of large dimensions, in relation to the diameter of the lens, which makes it difficult to create pieces of this size,   that are beyond, appropriately for, but don't SAG too hard at every little solicitation.
As said in the previous article, the baffle itself, could also be done by a certain thickness, provided that if you cant then the inside edge making it thin and "sharpened". Use a thick material, then lets get very stiff SEPTA, at the cost of more weight, that multiplied by the total number of SEPTA needed and whereas they will be applied in places well away from the center of rotation of the cradles, could lead to problems of instability of the telescope, that will have to be rebalanced by adding weights on the other end or by changing the property.
Taking into account these limitations, the solutions may be, or cut the circular crowns of baffles of aluminium plates, wood, plastic etc, or implementing cutting and pasting together several strips of fiberglass fabric soaked in resin (fiberglass SEPTA).
The solution with fiberglass allows SEPTA edged, equipped with a Board across the outer perimeter, that besides giving structural rigidity, serve as a basis on which to apply glue or screws to attach them inside the tube of the telescope, or to attack the fabric or the black fabric that will act as a pipe in a dobsonian.
In this case you will have to sew the black fabric to form a tube, proper diameter, within which they will charge you for the various SEPTA, and which is then attached to the primary box on one side, and the other secondary cell, staying inside of tubes, Fig. 1 (differing from the usual solution that includes a black hood towel, to apply all around, externally, Fig. 2).


UPPER HOOD

As for the hood to be applied on top of the telescope, one solution may be the following:

Having as its basis a annulus multilayer wood 7-8 mm thick and the top external septum made of fiberglass, linked by 4 aluminium tubes, and overlayed with a black elastic fabric.
The black fabric was deliberately chosen elastic, because the 4 aluminum tubes were not fixed rigidly on the ends, but are held in place by rubber bands that run internally.
In this way it was possible to create a collapsible structure, self explaining and compact to use finalized.
At the bottom of the wooden base, in over, were fixed 4 buttons, that will engage easily with their 4 complementary, places on the top of the secondary cell, making it simple and fast mounting and dismounting the lens hood.

Leave a Reply