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[1/4] My Homemade 488mm (19") F4.85 Dobsonian Telescope (2006-2011)

I have to admit that Iím hooked on telescope making. After I finished my high school in 2006 I needed to start a new project. I was thinking different options and sizes of mirrors that I could make. I donít know why but I ended up doing a half a metre mirror! Of course I was terrified first and I wasnít that sure that Iím able to do it. But then I remembered that I had the same kind of feeling with my 300mm mirror and it turned out very well. I didnít know if this was about aperture-fever or just pure madness but soon I had more than 10kg of glass front of me waiting for grinding. On these pages I have described how I built my half a metre Dobsonian telescope.

Designing the Telescope

Optical Design

My goal was to make a telescope with primary mirror diameter of 500mm and focal ratio F5. After finishing the mirror the final diameter came 488mm and focal ratio F4.85. I used the free OSLO software for ray tracing. From the graph below one can see the ray traced optics of my telescope. Parallel rays entering the telescope are reflected from the 488mm concave primary mirror and the focal plane is then folded to the side of the telescope using a flat 88mm secondary mirror. The secondary mirror is positioned 4,34mm away from the optical axis (offset) in order to centre the fully illuminated field.


Because the shape of a primary mirror in a Newtonian telescope is parabolic, there is no spherical aberration. However, a parabolic mirror results in coma aberration. This is why images of point like objects such as stars are no longer point like but look like little comets when observed away from an optical axis. The aberration increases when the focal ratio decreases. So itís not a big problem in telescopes with focal ratios more than F6. However, because my telescope is F4.85 the coma is clearly visible on the edge of the image.

So why didnít I make my telescope with F6 in order to have better image quality? The simple reason is the physical size of the telescope. Even if the focal ratio is F4.85 the focal length is already equivalent to 2367mm. With F6 the height of the telescope would become too large for me. Therefore I had to compromise between the physical size and the image quality of the telescope.

Optics

Primary Mirror Diameter D 488 mm
Focal Length f 2367 mm
Focal Ratio F F4.85
Secondary Mirror Diameter d 88 mm
Fully Illuminated Field 20 mm
Secondary Mirror Coverage (d/D) 18 %
Areal Secondary Mirror Coverage 3.3 %
Offset of Secondary Mirror 4.34 mm
Theoretical Resolution 0.24"
Limit Magnitude ~16

The spot diagram below shows the image quality over 0,5į field. It is clear that the coma aberration is the dominating factor that reduces the image quality. Note that the effect is greatly exaggerated and you can find the scale on left from the last row of spots. According to this spot diagram an image of a star is spread by 0,09mm when observed on edge of 0,5į field.


Mechanical Design

I decided to use heaps of time on designing required mechanics for the telescope in order to prevent stupid mistakes. At that time I was actually studying mechanical engineering and had therefore access to Catia V5 software. I used Catia to model even the smallest screws in order to calculate the centre of gravity precisely.

The first mechanical part I designed was the primary mirror support. Because the mirror is relatively thin (thickness to diameter ratio slightly less than 1/10), the primary mirror needs to be supported very well. Otherwise the mirror will bend under its own weight and loses its desired shape. I used the Plop software to calculate optimized positions for the 18 mirror supports. The graph below represents the deformation of 488mm mirror on optimized mirror cell (deflections in mm).

I used a quite common flotation type primary mirror cell with 18 points. These points are attached to the tips of six triangles. The whole system is self adjusting by allowing the triangles and the bars supporting the triangles to move slightly. Hence the forces are distributed evenly across the mirror. When the telescope is pointed towards horizon the mirror will be supported by a wide and thin aluminium band. Again forces are distributed evenly across the side of the mirror. There is a protective tape between the aluminium band and the mirror of course. I also included three columns with safety nails that prevent the mirror flying off the cell when the telescope is transported on bumpy roads to observing sites. Behind the mirror cell is the back plate where the cooling fan and switches are mounted.


The main design of the telescope is based on Dave Kriegeís Obsession telescope. This design includes a plywood mirror box and round upper secondary cage that is supported by aluminium truss structure. This is then mounted on a low Dobsonian mount that makes the pointing of the telescope possible.

Yleiskuva kaukoputkesta

More detailed look to the upper secondary cage. Two plywood rings are connected with four 30mm carbon fibre tubes. It is necessary to minimize the weight of the upper secondary cage in order to keep the sizes of the mirror box and the mount minimal. Between the two plywood rings a thin aluminium light shroud is mounted. The upper secondary cage houses the secondary mirror, focuser and two finder scopes. There is a conventional 50mm finder scope and also additional Celestron laser finder.



The image below shows the bottom of the telescope. The main mirror cell is mounted inside the mirror box.



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