My telescope is made of plastic tube
using the objective lens of 40 mm F20 from Orbys (2000 yen)
The focal length is 800 mm and the eye piece is Or 6 mm so the power is 133 times .
The inside of the tube is seal with black piled sheet to get clear image without reflection lights .
Although I omitted the inside rings but the diameter of the tube is wide enough and the focal length is son long that I can get very clear image of the shapes of the planets .
Adding to this ,as the diameter of the objective is small , this telescope is not so sensitive to the influence of the air current . I can use it in a bud seeing condition .
So I use this scope more often than any other my telescope .