AS Product Design Project idea

[Peter Reader]

Hi everyone!

I've just started 6th form and I'm currently studying Product Design. As a project I think I want to build a Dobsonian telescope...

I've got a very old 114mm aperture Newtonian reflector on a wobbly equatorial mount so I thought I could use it's primary, secondary, focuser, EP's and spider for the project and build the rest.

Does anyone have any tips/plans/material ideas or any other potentially useful information before I embark on this project?

Thanks

Pete

[callump]

Hello Pete,

that would be a really nice AS project, i think.

Here are a couple of things to look at.

1. The Orion Starblast is a short small newtonian - although it comes with a traditional cylindrical tube, you could make a square tube.

Orion StarBlast 4.5 Astro Reflector Telescope | Orion Telescopes

2. For something a little different, but still a small dob - here is "a scope like Alice".

A Scope Like "Alice"

and pics:

A Scope Like Alice: Pictures

its a bit bigger scope, but you could scale down.

most telescope makers would use 12mm ply (though you would probably get away with 8mm) - but you could look at different materials - mdf, aluminium.

Good luck, Callum

[Peter Reader]

Thanks for that. I've now got a lot to think about!

As I don't own a Dobsonian I have a question: How is the telescope held in position? I can see the tube is on a rocker (I think) but how do you prevent the telescope from moving once in position?

Thanks

Pete

[Astro_Baby]

It has friction bearings - I daresay one of the Dob owners will come along and advise.

[beamish]

I've always been quite taken with this PortaBowl !

2nd one down

[Bizibilder]

Hello Peter

There are several websites that you can google for using "dobsonian design" or "dobsonian plans / drawings". I've seen them for scopes up to 10" mirror size. One site even had a cutting diagram for 8'x4' plywood sheets.

[callump]

Thanks for that. I've now got a lot to think about!

As I don't own a Dobsonian I have a question: How is the telescope held in position? I can see the tube is on a rocker (I think) but how do you prevent the telescope from moving once in position?

Thanks

Pete

Hi Pete,

the mount stays in position just by friction, as others have said here.

but the tube does need to be fairly well balanced, at the point where it turns (in altitude). If its too out of balance, then the friction won't hold it in place.

Callum

[Peter Reader]

Is a simple cylindrical tube on similarly curved mounts either side enough to hold it do you think?

[Astrokev]

Hi Pete

Sounds like a great project - good luck!

I've built 2 Dob's over the years for my 10" f6.3 mirror. I chose a hexagonal tube which, having more flat faces, gave more choice regarding where to mount accessories - finders, focussor etc. This also allowed easier fixing of the 3 vane spider that I already had.

I used very thin ply (4/5 mm I think) and, with enough internal supports (which double as light baffles) the result was incredibly strong and light (I could stand on one of the flats without any bowing of the structure).

I also felt a hexcagonal tube was more aesthetically pleasing.

Regarding stopping movement, as others have said, there will probably be enough friction to hold things steady if you get the balance right. Alternatively, you could fix a small and simple clamp that works against one of the bearings, toghtened with a simple thumb-screw (I'm sure you'll be able to find pictures of such things, as this is a common solution). Or, consider fixing a hinged bracing rod from the base of the box to the side of the tube near the focussor. The rod should slide through a fixing on the tube, and can be locked off" when required with a thumb-screw.

Lots of options.

Hope this helps.

Kevin

[Peter Reader]

Thanks for that Kevin, how did you make a hexagonal tube?

[Peter Reader]

(Some pictures of it would be brilliant for inspiration and ideas if you have any)

[Astrokev]

(Some pictures of it would be brilliant for inspiration and ideas if you have any)

No problem, I'll post some pics over the weekend. Rather late in the day right now!

Kev

[yesyes]

There were also some nice DIY "travel dobsonians" posts in this forum a few months back, like this:

http://stargazerslounge.com/diy-astronomer/109077-12-inch-travel-dobson.html

Might be a design worth considering....

[ponytale]

Hi ,nearly all your questions about telescope building will be answered here, Stellafane: Build a Dobsonian Telescope good luck with your project and keep us posted on diy astronomer section

[Peter Reader]

That's fantastic! Thanks! I'll keep you all posted on how it goes (just started the design process -build probably won't start for another few weeks yet).

[Astrokev]

Pete

As promised, here are a few pics showing the construction of my old hexagonal tube. This was made back in the 80's before digital imaging arrived on the scene, so please forgive the poor quality, but I think you can clearly see the basic approach I used.

Material was thin ply (4 or 5mm from memory). The internal tube supports were of the same material, with strips of pine screwed to each edge to give a strong anchor point for the outer tube panels, which were also screwed to the support edges. Final result was very strong.

I cut a hinged door to allow access to the primary.

Hope these give you some more ideas!

Kevin

[Astrokev]

...and a couple more showing how I attached the primary mirror cell and spider.

The mirror cell was an aluminium plate approx 10mm thick.

The blue cover for the primary was the bottom of a bucket with a cupboard knob attached in the centre!

Kev

[Peter Reader]

How did you manage to get the secondary mirror at exactly the right distance from the primary?

The pics are really helpful!

[Astrokev]

The secondary has to reflect the entire light cone from the primary, otherwise you are losing light. Knowing the radius and focal length of the primary, you can use basic trigonometry to calculate the angle of the light cone and the distance from the primary at which the diameter of the light cone equals the diameter of the secondary.

Think of a right-angled triangle, with the baseline being the f.l. of the primary and the perpendicular being the radius of the primary. The hypoteneuse is the light ray from the edge of the mirror to the focal point. The angle subtended by this and the baseline can be calculated as :

tan(angle)= radius of primary / focal length

knowing this angle, draw another right-angle triangle, with the perp being the radius of the secondary, and the baseline being the distance of the secondary from the focal point (this is what you are trying to calculate). The formula for this is -

baseline = radius secondary / tan(angle)

Once you know this new baseline (distance of secondary from focal point), simply subtract this from the focal length to give the distance from the primary.

I then moved the secondary a little further from the primary to make sure I was reflecting the whole light cone!

You may need to read this through a few times (!) but providing you're ok with maths, it's easier than it sounds. You can probably also find programmes on the web that will do the calcs for you.

Hope this helps

Kev

[Peter Reader]

Yeah I'm pretty competent with trigonometry thank goodness. I'm taking mirrors and spider from a 114mm newtonian which I don't use. I'll be able to use trig or measure the distances on it to use.

I really wanted to know how you managed to install the spider at the exact correct position.

Thanks

[yesyes]

if it's still mounted in the newtonian, you can measure that. It will have to be at the same position.

[Astrokev]

I really wanted to know how you managed to install the spider at the exact correct position.

Thanks

Sorry Pete, in that case I don't fully understand your question. As yesyes suggests - take the measurements off the existing newt.

[Peter Reader]

Sorry guys I'm not being clear. It's about the build itself. I'm not sure how easy it's going to be to get the secondary mirror in exactly the right place, is there a degree of acceptable error in this?

[yesyes]

So are you saying that for the project you need to work out everything rather than measure on the scope you're taking the mirrors from?

Regarding tolerances, I would say there is some tolerance. When you put the secondary too close to the primary, the light cone will be too big for the secondary and you're losing light; i.e. the secondary will appear to be too small and will not be able to reflect all light from the primary.

If you go further away from the primary, the light cone from the primary narrows. You should try and get the distance so that the size of the light cone is the same size as the secondary and a little smaller.

When you move the secondary too far away from the primary, the size of the secondary will be fine but you will move the focus point further into the draw tube of the focuser. So you might no longer be able to achieve focus with your eyepieces.

I hope that makes any sense... ;-)

[Astrokev]

OK I think I understand you more clearly.

On my scope, the secondary holder is mounted on a cylindrical bar which slides through the centre of the spider. This is locked off with a thumb-screw (which is in the shadow of the secondary, so doesn't obstruct any light). You can see this in the last photo I posted. this allows the secondary to be moved along the focal axis of the primary to line up exactly with the axis of the focusser.

Hope this clarifies things!

Kev