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My Newtonian



(Not perfect, but easy to build)





General

This is the second Newtonian I have built. I am quite satisfied with it although it is not perfect. It's weight is about 9 kg, it has a focal length of 1177.5 mm and this results in a focal ratio of f/5.8, which allows me to observe deep sky objects and in a limited way planetary objects too. Adjustment of the two mirrors is done within a few minutes with the aid of a laser collimator. Due the fact that my brother Titus (a carpenter)helped me with the wooden tripod, I was soon able to balance my scope in an equatorial way. My Newtonian consists of the following parts:

25 cm Tube
8" Main Mirror, aluminiumized
62mm Secondary Mirror, aluminiumized
4-arm Spider Mounting for Secondary Mirror
2" Friction focuser. with Motofocus (NGF4)
TeleVue Finder scope
Modified 2" Finder Scope TASCO
Heavy Mirror Cell "Kosmos"

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Improvement of Optical Quality

To improve the optical quality of my Newton and to reduce reflection to a minimum, I used malted walnut shells (coffee mill!!) spread over a thin and still wet layer of black painting inside the tube. After drying I covered this powder with another layer of a black non reflecting spray painting.

Furthermore I added two blends made of heavy paper and black velvet, which cover the outermost part of my main mirror (about 1 mm). These rings reduce reflection effects but produce a small amount of turbulence in the tube. By these measurements I was able to improve contrast considerably. Additionally I covered all screws and metal parts of the focuser. with black painting. The effect was astonishing.


Alignment

One critical thing was to align the focuser in a way it pointed exactly right angled to the secondary mirror. Especially with the NGF4 this is not a very easy task. As a kind of help I drilled a very small hole (about 1/2 mm in diameter) into the tube just opposite the focuser. With the aid of my a laser collimator. I can adjust properly the focuser in a couple of minutes.

For precise focusing during observation I cut a circle of black heavy paper with the inner diameter of the tube. In this circle I cut out two somewhat smaller circles with some 2" diameters, placed to the edge of the outer circle. Placing this mask at the front of the tube I get a double-image of a single star, when my ocular is even slightly out of focus. Only if the ocular is placed right at focal point I get a clear image of a star. This is very important for CCD observation, as focusing through a CCD camera can be much more difficult than it is for unaided viewing.

Further Scope Accessories

For solar observation I constructed a mask of mylar film in a tube, that has about the same or larger diameter as the telescope tube itself. The important thing was to tilt the film slightly in his plane, as there are no more reflecting "ghosts" possible anymore. Never try to look at the sun with your eyes!! This mylar film is able to reduce solar light so far, that it can be collected on a projector film (chalk paper frame on a rod).




Further notes on my homebuilt worm gear



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Email me at daniel.sch@spammenot@bluewin.ch
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