Building my ultimate telescope: 300mm f/4 travel Dobsonian

[Roel]

I recently started building my ultimate telescope: a 300mm f/4 air travel Dobson.

Why 300mm f/4?

- There's no need to use a special observing chair: a standard chair is comfortable enough to view from zenith to horizon. When traveling on an airplane I don't want to take an observing chair with me!

-A 300mm truss Dob is about as large and heavy as I can take on an airplane. (mirrors as hand luggage, the rest (fully stuffed with socks, underwear etc.) in the big suitcase. Apart from that: an aperture of 300mm is not only large for a travel scope, but it's also large enough to be my main telescope back home.

- The OTA will fit on the back seat of my car fully assembled (no need to assemble and disassemble in freezing cold nights.

For the primary mirror I chose a 27mm thick Orion UK mirror with Hilux coating. The thin mirror will cool a lot faster than the standard thickness mirrors, and is also much lighter for transport. I have ordered the mirror, but it has not arrived yet. Nevertheless I began building a few parts of the telescope.

First the secondary cage. I made it from two rings of lightweight poplar multiplex (6mm). The inner "wall" is made from flexible 3mm multiplex. It is incredibly lightweight and pretty strong. The tension of the spidervanes will strenghten it even more. Inside diameter: 320mm (12,5"), outside diameter 350mm (13,8"), total height: 110mm (4.3")

I hope the complete telescope will be finished in september when I'm going to a very small village in Germany for an observing weekend underneath its extremely dark sky.

[Steem]

That's great to hear! I'm the third friend by the way. Call us the three musketeers.

[Rusty Strings]

Just curious, is multiplex another word for plywood?

[Roel]

Just curious, is multiplex another word for plywood?

That's right, at least in the Netherlands it's called that way...

The past few days I have made some progress on the telescope.

- The secondary cage is almost finished. I installed the focuser and the red-dot-finder and I glued the truss connecters onto the bottom ring.

- I cut the altitude bearings. I copied the design by Dieter Martini, because I have always found that design the nicest way to keep the bearing wide, solid, lightweight, and still attractive. The outside diameter is 45cm (17,7"), but because they're not complete half circles, they will fit inside the box that will be approx. 38cm (15"). I have not rounded the ends yet. I'll do that when the box is ready. I can then cut them so they'll fit snugly inside.

-Also the 6-point mirror cell is not completely finished yet. I have to know the exact outside diameter of the primary down to the millimeter to apply the edge support bolts. The backside support however is functional. Three aluminum "see-saw" beams will support the mirror at the right points with evenly distributed pressure. The extra height it will add underneath the mirror is only 15mm (plus the height of the collimation springs)

A drawing of the cell design can be found here: Ontwerp en bouw 300mm f/4 Kofferdobson (update 3 aug. 2010) Roelblog

[beamish]

I have to say that I'm very impressed with the simple elegance of the design of these Dutch dobs. You're clearly very skilled with a router too!

Way beyond my DIY skills, however I am keen to know how you made the securing connectors for the truss poles and how do you connect the poles together ???

[Roel]

Thanks Carlo!

But I never use a router, I have a small budget for tools and I make everything with a cheap electric drill, a cheap table saw and this extremely cheap jigsaw (don't laugh):

I bought it in a supermarket.

I will post pictures of the trusses and the connections later, when I completed those.

[beamish]

I'm not laughing-I'm crying !!

I would never have thought you could do that with supermarket cheapo ! Those cutouts look great- of course that just reflects yet further the quality/skills you have grrrrrrrrr:D

[Roel]

Update!

I've build the primary cage and I've cut the altitude bearings to fit inside. 2 holes in the "front" for fans, 4 holes in the "back" for the outgoing airflow. A 3rd fan will be mounted on the rear side of the primary.

And that was all I could finished before the primary mirror arrives. I need to know not only the exact diameter for applying the edge support, but also the exact weight of the mirror for balancing the scope and positioning of the bearings.

[Starblazer]

That is one hell of a nice job so far mate

[weareborg]

That's a lovely looking mirror box and side bearings you've made there. Considering they were all done with basic woodworking tools, you must be very proud of your result. Keep up the good work. Are you leaving the wood its natural colour, or staining it???

Andy

[brantuk]

Nice one Roel

[Polar Bear]

Excellent work, looking forward to following this build.

Paul

[Roel]

Thanks everyone!

Today I put in the spider (made by Sumerian, it's the only part I don't have the skills for to make it look nice), I placed black flocking paper inside the cage, and I finished the outside with a transparant semi-gloss stain.

[Rusty Strings]

Excellent design and build Roel. I flatter myself that I'm good with wood but your skills with a jigsaw are just brilliant.

[Roel]

Update!

I've painted the mirror box and installed the mirrorcell using three springs. The trusses are cut 1" longer than I calculated (the mirror still hasn't arrived yet) and will be cut to the exact lenght when I've got the mirror. The connection will also be slightly different than what you see in the picture, this is just a test.

I also installed two switches to independently control the surface-side and backside fans.

I just hope that the primary will arrive before the 5th of september, I've been planning an observing trip to the countryside of Germany and the idea was to use the newly build scope to observe there....

[Demonperformer]

Really nice job ... i'm very envious

[Keiran]

Wow... Nice job... this project is on the cards for me soon, I hope

[weega]

[cut]The trusses are cut 1" longer than I calculated...[cut]

can you tell me the size (and thickness) of the "L-section" of the trusses? I'm planning to rebuild the trusses of my 200mm f/4.5...

thanks and clear skies!

ciao

dan

[Starblazer]

Very nice indeed!!!

Seeing that vinyl just made me want a dobsonian turntable lol. Imagine those at the DMC championships!