I'm building another newt and I thought I'd post some pics of the progress. I have only done about two days work and am surprised how fast it is coming together.
I am up to building the secondary holder and would like to know what size secondary I need, anyone have an idea? It's an 8 inch f5 system, and will be dedicated to imaging only.
Thanks Ken, the disks for the secondary holder are 3x50mm. It is a new design I'm trying, it has only one adjustment screw! And is more sturdy and rigid than any secodaries I have made before, I'll post some more pics of it when it's done.
Thanks for the link, I put in my specs and it reckons with a 63mm mirror I'll get a 32% obstruction with a 20.68mm 100% illuminated focal plane. And with a 70mm mirror I'll get a 35% obstruction with a 29.84mm 100% illuminated focal plane.
Which one do you reckon would be best? I will be using APS-C size sensors (24mm) most of the time but might upgrade to full frame later on.
probably going slightly bigger than I need is not a bad idea.
How did you fasten the Al tube in the upper section to the wooden ring and in the mid-section as well?
why not ply rather than mdf? wouldn't it be harder to seal the mdf from moisture?
How did you fasten the Al tube in the upper section to the wooden ring and in the mid-section as well?
why not ply rather than mdf? wouldn't it be harder to seal the mdf from moisture?
Cheers
Alistair
Thanks Alistair,
I drilled hole in the wooden rings and hammered the tubes in (they were a nice tight fit) I'll pull them out and glue them in when I'm ready.
I used MDF because I had some laying around and also there's less chance of chipping when cutting, I find I can get a better finish with it as well.
As to sealing, I have never had any problems, With a coat or two of exterior varnish or epoxy resin you'll be right for many years.
I'd suggest the larger secondary as well as otherwise you'd have vignetting.
If I understand correctly, the increase to the central obstruction would only be an issue if its for planetary work.
What is the weight so far and how are you mounting the primary?
Keeping the focuser as close as possible to the rings without the drawtube protruding too much is ideal.
so focus with a dslr could be say 5mm from fully racked in. Focal point for CCD's would be further outward as distance to body is a lot lower than dslr's.
Alistair, just checked the weight on mums kitchen scales and it comes up to 2.3kg's (thats just the bare frame as seen in the pics) I'm not really worried about keeping the weight down as this is going to go on my horse shoe mount on which weight isn't an issue.
You would have a heart attack if you knew how I'm mounting the primary, so I won't mention it just yet
Thanks for your help Ken, Simon, and Alistair regarding the secondary size, 70mm she is
I got a little bit more done on the secondary holder this arvo after school, here is a few pics.
A bit more progress, installed a light shield around the upper section made from 0.5mm al sheet, attached the spider vanes, and glued together the secondary assembly.
I'm now wondering about secondary offset, is it needed? and if so how should I do it? I did some quick calcs and I think the offset is around 4.5mm, does that sound right?
The next stage is building a heavy duty focuser, any ideas on a good design?
OOoooer ! Another Serrurier truss build ! Nice work. Interesting secondary holder construction, I gather it 'hinges' on the two trapped ball bearings while restrained by the spring and adjusted by the single cap bolt. Nice and simple. Can't wait to see how you are going to mount the primary mirror. I've redesigned and modified mine several times now for various tasks. So easy having good open access to all the connections and components.
Brent, et al,
I'm sure that Mark Serrurier is turning in his grave.
His revolutionary design concept was a "truss" design where the deflections of the top and bottom sections were balanced - this maintained the alignment of the optical axis.
The "hinged" attachments also allow for the parallel deflection of the top and bottom rings to maintain their parallel alignment.
Not many (if any) of the amateur truss designs should be called "Serrurier Truss" designs.....
(Notwithstanding all the great design work he did, he had to "compromise" his final design for the 200" due to the Coude access. I have copies of his original design paper etc. etc. Ask me about Sir Barnes Wallis' "rigid dick" telescope tube design - now that IS an original!!)
The "hinged" attachments also allow for the parallel deflection of the top and bottom rings to maintain their parallel alignment.
hi Ken,
Interesting point, would you have links to examples of these hinged attachments?
Most of the serrurier truss telescopes in observatories seem to have the truss nodes welded in the centre and extremities.
this was one I could find http://en.wikipedia.org/wiki/Serrurier_truss
VLT is another example where the nodes are bolted to the centre frame http://www.cosmicdiary.org/fmarchis/.../01/pano9.jpeg . not sure if its built on the same concept.
for the parallel deflection to work, would the truss nodes need to be a material that can compress or stretch? else how would they deflect even if the hinges allow small movement of the nodes?
that "Rigid dick" telescope design sounds interesting!!. couldn't find any references though. how does it work structurally?
Alistair,
The "hinged" ends on the larger telescopes approximate a pin joint.
The effective section of the truss tube is significantly reduced to allow it to deflect and act as a hinge.
The "Rigid Dick" truss was a series of parallel tubes each of which could be pumped up hydraulically to change their effective strength....think about it ;-)
I believe the trials Sir Barnes Wallis did verified the capability of the system, but it was eventually rejected due to concerns about oil leakage - such is life!
The "Rigid Dick" truss was a series of parallel tubes each of which could be pumped up hydraulically to change their effective strength....think about it ;-)
Ha ha, I get it, that's pretty radical..
would love to see modelling of the hinge joints and the structure as whole. I've seen modelling of the large fork structures but not the truss.
I'll dig through my notes and find the Surrurier paper for you.
(I did my Hons degree with a design thesis on the 98" Isaac Newton Telescope. I had the pleasure of working at Grubb Parsons for a while and getting involved with their design guys back in 1970)
wow, that's so cool. I like the wooden ladders between the fork of the Isaac Newton scope. that's pretty massive.
on the serrurier topic, are there any real world measurements of deflection at the ends when its horizontal? I presume there would be measurements to validate the design.
Any finite analysis structural program given the correct inputs will quickly give you the deflections.
It's structurally a simple arrangement...much easier than building a bridge!!
(Having said that....I saw the calc sheets - logarithms/ slide rule in those days. And in the end they actually measured the end deflections under various loads and machined a bit more of the diameter of the stubs!!!! until they got to spec.)
OOoooer ! Another Serrurier truss build ! Nice work. Interesting secondary holder construction, I gather it 'hinges' on the two trapped ball bearings while restrained by the spring and adjusted by the single cap bolt. Nice and simple. Can't wait to see how you are going to mount the primary mirror. I've redesigned and modified mine several times now for various tasks. So easy having good open access to all the connections and components.
Thanks Brent, your correct about how the secondary works, it is a design I saw some time ago and have wanted give it a go.
The last few scopes I have built have had solid tubes so I thought it would be fun to try a truss.
With a coat or two of exterior varnish or epoxy resin you'll be right for many years.
