I've started working out the dimensions of the various components of the three pole truss scope.
Because the secondary cage will be just a single ring of hardwood ply, I'll be using 15mm, one or two sheets, glued, should I use? I'm looking at a rim dimension of 50mm.
Mental
Last edited by mental4astro; 21-05-2009 at 08:50 AM.
Here are some pics of the original incarnation, with some cosmetic alterations by my kids and me.
As the Odyssey II is right now, I can only manage to fit the scope into the car and allow seating for me and one other passenger, and very poor visablity around the car.
I also need help in loading and unloading her as she is so heavy and bulky.
I'm hoping to reconfigure her to a more manageable three pole truss design, like that of Albert Highe's.
Below is a site, not of Albert's, but better shows what I'm wanting to achieve:
One thing I found, after I collimated her, the secondary mirror looks to be undersized. Is this much of an issue for a visual scope?
When you put your eye at the focal plane, you should be able to see all of the primary mirror reflected in the secondary. If not then the secondary isnt catching all of the light collected by the primary mirror and you have in effect a reduced aperture scope. Makes it harder to collimate too.
If your focal plane is high enough out of the focuser for all your eyepieces with some to spare, you might be able to pull the primary back a bit to help.
When I look in, the image of the primary 'overhangs' the secondary. I have to move my head a bit to catch the edge of the primary all around.
The secondary also seems to be set a little low so I can view the edge of the primary more easily at the 'top' of the secondary than the rest, with the lower edge of the secondary needing much more movement on my part to view the respective edge of the primary.
You can use several programs to check the optimisation of the optical components of a scope(eg. secondary size, top ring inner diameter, etc). "Newt" is a common one but I used a speadsheet by Andy Martyn instead. Also does focuser and behind secondary baffle calculations. Just enter in your own values and it will do the job. Email me if you want a copy as it won't attach here.
For my 10 inch I used a single thickness of 12mm Gaboon Ply. For the 22" I used 2 layers to get 24 mm. Stiffness depends on design too. On both these scopes the 3-vaned spiders are bolted to where the poles attach to the top ring. The main potential for flexure is then the focuser board. I would go with 15mm then reinforce around the focuser if necessary.
50mm between the inner and outer rim sounds about right - also need to leave room for the thick poles/clamps if going with a Highe design.
Poking around Highe's and Dobstuff's websites should give you plenty of food for thought.
The kids are available for commisioning. Prices start from a bag of lollies and chips.
I did some calculations using the info from the good book 'The dobsonian' for the secondary, and also measured the thing. It's all good. The actual secondary's minor axis is if anything a couple of mm oversize.
I think what's happened is that the actual put-together of the Odyssey may not be optimal, The tube may be a little short for the new mirror cell and spider. The original spider was just a single strip of metal sheet running across the diameter of the tube. Savings had to be made somewhere for the prices the Odyssey's originally were offered at.
Now knowing the optics are matched I'm not as concerned.
I'll still take you up on the offer Tnott, serves as a double-check, .
I compiled a list of ply components to be made, and the final assembled volume using a Highe design is the same as this current Odyssey (of the OTA). The outside diameter of the spider ring is the same as the Odyssey's tube.
However, that is where the similarities end.
Mental
Last edited by mental4astro; 25-05-2009 at 11:14 AM.
Regarding fasteners, I never bothered with stainless steel screws etc. and have never had corrosion problems. Part of the reason is that I could not get all the fittings I wanted in stainless - the scopes are a mixture of what I could find.
But, I don't live next to the coast like you do - so maybe get what you can in stainless.
Also, I generally painted/varnished the wooden components before I bolted/screwed everything together as that was easier for me - don't know if a bit of varnish sticks to and protects zinc-plated bolts like wood.
I usually spend a lot of time getting the weight/distance maths right in these ultralight scopes because the balance depends a lot on the size of the altitude bearings. The greater the radius of the bearings the more the scope is bottom heavy and visa versa. I get the weights from the internet re. the wood, poles, optics, focuser, spider, telrad etc. I then calculate the amount of wood, aluminium etc. and do a best guess, fudging the scope to be a little bottom heavy as in "The Dobsonian Telescope".
The advantage of the type of Highe design I used is that I could slide the side bearings up or down a bit to fine tune balance, though sometimes the bigger variations have the side bearings fixed to the mirror box. You can also stuff counterweight in the poles or on a sliding counterweight on the outside of a pole in this design.
But it is better to get it as close as possible first
The use of stainless screws and bolts would be really only on components not painted over, such those holding the spider and mirror cell, which may occassionally require adjusting or to remove the mirror for cleaning, and varnish would only impeed. The Odyssey now has stainless components on these parts.
I am looking at using the largest alt bearings practicable. I can see the advantage, and small bearings only retard performance, the bigger the better. I also like the 'horn' like effect of protruding bearings, .
I'll be using steel brackets to hold the spider in place. I've got an old steel bed frame, from which I'll cut out the brackets, predrill the necessary holes and chrome plate to preserve. Their position on the top of the ring will not be seen by the mirrors, so I'll make a feature of them by being polished and shiny.
While on the topic of alt-bearings, any suggestions on REMOVEABLE alt-bearings?
I'm glad you like the artwork, AstroJunk. Thought I'd have a bit of fun. The 'blank canvas' was too tempting not to have a go at. Plus these things are to be enjoyed. I have to put up another photo, this time showing the other side as my boy asked "where's my picture, papa?".
The side bearings on the Highe design I copied are actually attached to the two side poles, not the mirror box. The rear of the side bearings just sit up against the mirror box without being bolted to it.
So when you take out the poles from the mirror box, the side bearings come with them. You can slide the side bearings off the poles too if you want but I don't bother:
The attached photo is of a Plettstone scope. The side bearing is clamped to the pole in two places like mine and Albert's, but there is also a bolt running through into a wooden side piece on the mirror box for added strength. This means that when you take the scope apart, the poles are pulled out of the side bearings which stay attached to the mirror box.
In this design the whole relationship between the side bearings (2 layers, one slightly oversize to keep it in place), mirror box and rocker must be very precisely done as it all has to fit together just right to work.
If you use another design or attach the side bearings to the sides of the mirror box you can just use knobs and threaded inserts or T-nuts so you can take them on and off - or similar. Here's a couple with removable bearings:
I'm looking at a scope somewhere between an orthodox Highe and Babcock's 10" traveller. I just want to introduce a little more rigidity into the mirror box with a semi-closed mirror box. I can avoid clamping onto the poles this way as I'm not to sure I can achieve the degree of accuracy needed.
Alan Scott employs a variable position bearing system on his 16" scope. This way he can adjust the balance of the scope according to what he sticks onto the cage or focuser. Cool, . Fidely but nothing big and heavy to potentially fall on the primary to abjust balance:
I like this idea. The big radius bearings also help to lower the profile of the rocker box.
Man, I'm torn between heading to the toolshed right now, but I really would like to use this puppy in a dark sky site before she's out of action for a while, ,,,, ,... .
Alex, you might be interested in these couple of designs for your scope.
the first two twenty inch scopes where built by Peter Bobroff of NSW or Camberra? and Allen Thomson and Graham Long of Queensland
The Twenty five inch is owned and built by Norm Papas from Toowoomba
I hope they can give you some more ideas
Pictures taken at Queensland AstroFest about 1994/5?
You should be able to do most of the construction without putting the other scope out of action. I guess it depends on how many parts you are using from the old one.
I normally only install the optics just days before first light.
The only disadvantage of not clamping the poles to the side bearings is that you increase the flexing length of the poles a bit and you have to make a structurally stronger mirror box to fasten the bearings to, as you have said.
Reducing the size of the rocker box by using large altitude bearings would help in transport (unless the other components stacked in it are higher anyway).
You know Alex, I hear ya. I've been bitten by the aperture bug since looking through the big dob at Crago (Bowen Mtn). Now I've got all sorts of evil-schemes running through my head and I think I'll contain it to getting one of those el-cheapo 16" GSO truss dobs after tax time. I've already the plans in my head to convert it to a more traditional, elegant truss tube design. I even picked up the Dobsonian bible by Kriege today from Bintel, that's how obsessed I've become
Anyone know where to find the rarest of the Odyssey's, the 30"er. I've started sniffing the air and seeking the wind's direction every time I step out of the house (shake, shake,...twitch).
Ron, love the metal work on those two 20". Be nice to have those skills and tools at my disposal.
The az. system used in those two makes me go ooohhh! Had toyed with the idea, but too complicated for me. Be nice though. Strong, light, beautiful.
Thanks everyone for your encouragement, help and guidance.
Started to put pen to paper sorting out the components. Cut out little templets to figure out the most efficient use of the ply and cut sequence. The ply has been sourced, and I'll be having the supplier do the gross cutting for best square results. Should be able to finalize the design over the weekend. Box idea being favoured, just need to figure out configureation for the poles. I don't want to go to four poles, sure is easier, but I'm up for the challenge of the geometry of three, .
After reading "The dobsonian", I'm much more at ease in not tieing my nickers in a knot over the balance of the instrument until the OTA is completed. I can concentrate on the structure first. Just have to keep reminding myself of this, . This book has been a good guide on how to plan the sequence of a scope's build.
I've started writing to another IIS member who's in Uruguay (it's a really head spin to talk astro in Spanish, facinating take though as Fernando is a professor of astronomy there). He tells me that the largest scope there is 20". This Odyssey was the largest in Tassie for a long time while it was based there. I cannot imagine just how big the 40" monsters are, .
.
, focuser, and finder.
. Fidely but nothing big and heavy to potentially fall on the primary to abjust balance:
,
,
,
, 
,... 
.
.
Alex, you might be interested in these couple of designs for your scope.
.
. Ethos, Argo, Eq platform, what more?
