17.5" reincarnation

[AstroJunk (Jonathan)]

Originally Posted by mental4astro

Ethos, Argo, Eq platform, what more?

ServoCat

[tnott]

Re the balance issue : with a standard Obsession-style dob, you can fine tune the balance near the end of construction by shifting the mirror cell up and down. This is not possible in some other designs.

If you make a scope like Babcock's 12 incher, then you can build the top ring and mirror box etc.first. But, you need to work out the weight/distance maths before you can decide on the size of the side bearings, which lead in turn to the dimensions of the rocker and ground board.

As I got all my parts pre-cut, I had to work out the weights right from the beginning.

If you are thinking about DSC then having central pivot points is handy. Horseshoe or crescent shaped side bearings may need a double encoder arm to be attached.

It's fun working out different scenerios before settling on a final design.

[mental4astro (Alexander)]

Hi all, I spent the weekend nutting out the design, and finally have settled on a hybrid between an Albert Highe and light weight dob.

As I only use pen and paper I don't have a CAD design to post. So I shall try to describe the idea.

*Three pole truss design.
*Single ring secondary cage.
*Enclosed low profile mirror box.
*Large diameter removeable bearings (still tossing up with single position or variable).
*Circular ground board with ball bearing pivot like the following:
http://www.geocities.com/ruedigerhei...amt3_small.jpg
http://www.geocities.com/ruedigerhei...ker1_small.jpg


The rocker box would be beefier than the above to accomodate future electronics.


I've made an estimation to the weight of the 50mmx1.6mm size poles I'll be using, and there is little difference when compared to using 8 one inch poles, about 200g. Very stiff poles, so clamp design and build critical for the small quantity.

One thing I'm asking, would anyone have a 114mmX500mm reflector they'd like to contribute as a finder scope. I've put a posting up on this under the Trade/Wanted forum. This modest little scope I think would work well under the poor Sydney sky. Further details can be tracked down under the classified ad:

http://www.iceinspace.com.au/forum/s...ad.php?t=45677

Tnott, I will be following the construction sequence you mentioned as it allows for most versatility to accomodate any design changes.

Mental.

[lacad01 (Adam)]

Should be pretty streamlined and compact by the look and sounds of things. Looking forward to tracking your progress

[tnott]

Looks good.

2 things to think about:

1. The flex-rocker design attached has no central pivot point to mount an encoder. If you are not going to add DSC in the future then no problem, it is a good design that transfers the weight of the altitude points directly to the ground ring. So the rocker does not need to be as beefy and it saves weight.

If you do want to use DSC then replacing the ball bearings with a central pivot bolt would make more sense. You can retro fit a central pivot point later, as I did in the 16" Tridob, but it was bit of a pain and I only did it that way because I did not plan for DSC from the beginning.

2. The ground ring in this design looks like it is not rigid enough. If you look at other flex-rocker iterations then they often have a more rigid ground ring made up of two layers of ply separated by a framework.

Either the design has to:
A) have a rigid rocker transferring the weight to teflon pads situated directly above the feet of a (flexible) groundboard, or

B) a flex-rocker with teflon pads attached to it directly underneath the altitude bearings, transferring this weight to a rigid ground board.

If you have both a relatively flexible rocker and groundboard then you'll be in trouble, esp. with a 17.5" scope.

Examples of a flex-rocker/rigid groundboard approach are Mel Bartels' Tridob and Dan Gray's 28".

A good example of a more minimalist rigid rocker with a central pivot point and (flexible) groundboard is the Obsession UC (& my 22"). This design would not take up any more room or be much heavier than the one in the link from your post, especially if you used a triangular groundboard or cut pie-shaped pieces out of a round one.


Just food for thought................

[mental4astro (Alexander)]

Hello Tnott,

Firstly, thanks very much for your interest and guidance. Much of the design evolution I'm undertaking comes from your lead.

I agree that the rocker of the link I put up is light. I would still look at a double layer of ply for the base of the rocker. This puppy will still be heavy for sure.

I am not rushing into DSC's. Later maybe, however thanks for the forsight of the need of a central pivot for the use. I'll probably just remake the rocker and base should I feel so inclined. Or if I totally botch-up this style of pivot, I'll default back to a central pivot.

Placing the teflon pads below the alt-bearings I had not thought of, ! I'd be looking at this regardless of design.

Question, why are these rocker-ground-board designs called 'flex-rocker'?

[Rod]

It's called a flex rocker because the rocker is made deliberately flimsy so that the weight of the scope pushes the bearings into contact with the groundboard. The azimuth bearings are located directly under the altitude bearings. Because of this thin ply or aluminium is all that is needed to connect the three or fours altitude support points. I have made two of these for my 12 inch dob. It is surprising how light this style of mount can be. It is also very stable. If you make a flex rocker, do not make it stiff with double thickness ply, there is simply no need to and your base may well be uneven if it is overbuilt.

The mount in the links you gave is not a flex rocker and would not be as stable as one. I agree with everything Tim said and yes look at Mel's tri-dob and Dan Gray's designs. They are really well thought out. I wound up retrofitting a pivot on mine too for simplicity so it looks like a very simple version of Dan's.

Rod.

[tnott]

Yep. Thanks Rod.

Mental, the teflon pads should only be fixed under the Alt. bearing if going with a flex-rocker/rigid ground board design, with the laminate glued to the top face of the ground ring.

If going with a rigid rocker/thin ground board design they need to be fixed above the feet of the groundboard, as in The Dobsonian Telescope, with the laminate on the underside of the rocker.

If you re-read A) and B) from my previous post I think you'll get the idea. The two approaches are like mirror images of each other.

If you do go with the flex-rocker approach the only slight down side is that the laminate on the ground ring faces up and can collect dust. I found that I had to wipe the laminate and teflon pads of the 16"Tridob each time I went out for a viewing session to keep the Az. motions nice and smooth.

[mental4astro (Alexander)]

Rod & Tnott, does this mean three or four Az teflon pads? From what I see, it's four.

And how about the feet to the ground, three or four?

Still following the 15lb per sqr inch loading rule?

[Rod]

The tridob has three az pads. One under the central fin at the back and two under the folding az bearings at the front. Dan Gray's design has four az pads, one in each corner.

I would always start with 15lb per square inch rule. You need to use virgin teflon. I used some generic teflon I bought a long time ago and was never happy with the azimuth movement using it. I replaced three of the pads with bearings, installed smooth formica and kept one teflon pad. This gave very nice motion for my scope.

Most flex rocker designs wind up with very large altitude bearings because you want to make the altitude supports short and stiff. Make sure you can accommodate this with your design.

Rod.

[tnott]

Yep. The only reason the Tridob uses 3 Alt. pads on the bottom of its flex-rocker is that it had 2 front bearing fins and 1 at the rear. Use four for two standard Alt bearings, underneath each of the Alt. teflon pads.

I and others just sat the ground ring directly on the ground with this design - no feet. No reason you couldn't fit three feet to the bottom to make it even easier to rest on uneven ground - but the ring has to be really rigid!

The other approach always has three feet underneath the groundboard, with the teflon pads directly above each.

BTW - another advantage with using a rigid rocker/flexible groundboard for a larger scope is that you can shim around the central pivot to make the motions smoother in Az. As dobs get bigger the feet and therefore the teflon pads above them get further apart, which means more resistance.

Using the right materials and psi on the pads worked OK on the 16" Tridob because the design was so light, but shimming around the central pivot on the 22'' really helped improve the motions in Az.

If you are at all worried then I would go with the more standard but economical design as in scopes like the Obsession UC. It is simple to build, will work and you can add encoders later if you want.

[hickny (Peter)]

Alex,
I remembered reading somewhere about a portable telescope that wuld fit into a "carry on case". This website http://www.litescope.eu/litescope%20products.html has an interesting design.

[mental4astro (Alexander)]

Hello everyone.

Firstly, Hickny, the 'litescope' you mentioned is a very nice design. I've also seen it used by two other ATM'ers, Moonsilver scopes, the other being made by a fellow IIS member, 'scopemansit', who has posted pics of his beautiful work in these forums. However, I'm not sure if these last two are commercial ventures.

I have been experimenting with the Odyssey II, changing one finder and adding a second.

I've had a whinge on the mental gymnastics needed with traditional straight-through finders. Having tried right-angle-correct-image (RACI), they don't do it for me either. I sought out a correct image straight through, and grimaced at the price. So, I made my own from a 7X50 pair of binoculars.

I asked IIS members if they have had experience with making these. Thanks to all who replied to my query, served to inspire and create, .

I cut the binocs to keep the focusing mechanism on the eyepiece without the diopter, keeping the other eyepiece to act as a focuser for a second finder to make, .

The mounting mechanism is based on another common system: the secondary holder of a Newtonian, ! Simple and it works !

The reticle was a matter of finding the focus of the eyepiece, which thankfully fell inside the tapered internal flange ring. The wire was donated by a diused short phone lead. It is about 0.1mm in diameter. The centre of the eyepiece was found using a geometric trick from year 8 high school (I'm glad I paid attention that day!). Two tiny holes (0.5mm) were drilled at each wire terminus position to wind the loose wire ends and tension the wires in place.

This modified finder is a little heavier than the Unitron finder it is replacing, but it is only a prototype. The refined end design will be even lighter. I kept the prismatic form of the binocular because this system creates the correct image orientation.

The second finder I attached towards the rear end of the scope. The Odyssey focuser being so high, and the truss design also requiring a step ladder (I aint the tallest bugger), finding new objects often had me repositioning the ladder two or three times because the original gross positioning was significantly out. So this second finder was my answer. Works a treat too, . Not once did I have to do a double take. The finder came borrowed from another scope.

I hope all this made sense. If not, look at the pictures!