Well after seeing some of the high quality work that people on this site have been doing I am hesitant to even put mine up here now. With said I am also looking for suggestions on what I can do to improve what I have done so far and the best ways to finish it... It seems I am in the right place.
Please bare with me everyone. I haven't worked on this project in about 5yrs (yes its been sitting that long) and I am not in the country so I don't have access to the scope or anything for reference at the moment. I would like to get this out there so when I get back in September I can finally finish it up.
The design is a basic 12.5" F5 truss tube. My first cage was built using wood rings. After much research and realizing how heavy it was I had some aluminum rings cut by the local machine shop using their laser cutter. The truss tubes are 1" aluminum from the hardware and the tube connectors are from MoonLite. The secondary holder was purchased from AstroSystems. I am still trying to figure out the best way to mount that. Currently I am using banding metal, aluminum tubes I treaded as an attachment point. While I believe it will work alright its not pretty. I am also wondering about diffraction spikes. Many people are using an arced design. Does that eliminate or reduce them considerably? At the moment I can't remember where I bought the mirror cell from. I am also looking for a good way to mount the 2" crayford focuser I have. After seeing some people using carbon fiber cloth and resin, I was thinking that might be nice vacuum bagged onto to pink foam with wood for anchor points. Not sure what would be good there. Okay, well here are some pictures. Hopefully you guys can get some ideas spun up and point in the right direction.
Does anyone have ideas for a better spider assembly? Would wire help to reduce diffraction spikes? Would a curved spider be better? Should I use another material for the center of the spider? ie stronger lighter hold collimation better?
Nice looking scope and it looks like you have used some nice components.
There is nothing wrong with using the banding as a spider. It may not look pretty but that design is widely used and is very strong. On the other hand a wire spider will give you less diffraction. Have a look at the photos at the bottom of the page here:
Rolf (Skyviking) has a thread in this section about his telescope which is worth reading.
I have built 3 wire spiders and like them a lot. They are not hard to construct - you just need to make a jig to hold it together while you thread the wires. You do need a stiff upper cage, otherwise the tension on the wires will make it go out of round. You should be able to use the astrosystems hub OK if you built a design like this (scroll half way down the page):
The strength of a wire spider is determined by the angle between the wires - 20 degrees is OK for a 12 inch.
The concern I have about your scope is the position of the secondary you have chosen. If I were to build a tall upper cage like yours, I would locate the secondary lower down to improve baffling. As it is, you will need to add a baffle to the top end to stop stray light entering the focuser. I might be misunderstanding your layout though.
I have tried curved spiders. I must have built them poorly because I saw more diffraction with them than with a conventional spider. Others whose opinions I repect love them so it can obviously be done.
Hello Greg,
It looks like an interesting project you have going, even if it has been on hold for 5 years! You could certainly use sandwich panel contruction to mount your focuser, but you would need to reinforce it at various points (eg. screw attachments) so that it does not distort when mechanical pressure is applied when tightening the screws. You could bond wood in the panel as you suggest. I am planning on using cast fibreglass plugs that I made. I'm not sure what you mean by "pink foam" for the panel? You would need to use a core material specifically designed for fibreglass sandwich panels that enables good bonding and has the required mechanical characterstics such as compressive strength (eg. core foams like Airex or Core cell, end grain balsa, honeycombs).
Thanks for the links! I was looking over both designs with much interest. To me it looks possible to incorporate both designs and still manage to use the mirror holder I have from AstroSystems. (I have a secondary mirror heater that installs in there nicely) I like the use of guitar strings since they are thin and already have and end on them making for easy installation. I have an electric guitar and it needs to be tuned regularly which means the strings stretch a bit. Not sure if it would stretch enough to matter here or not though. So the second design using a mounting block of some sort might be ideal for me. Then use the guitar strings. (wonder which ones he is using "E" etc?) To attach the stings to the cage and as a way to apply tension I was thinking using my technique of a threaded aluminum rod inserted half way through the cage truss and held to the truss with cap screws. By tightening the screw it will apply tension on the wire. It would keep the appearance clean. I'll try to draw the design tomorrow to give you a better idea of what I am thinking.
The current placement of the secondary is deceiving. It will actually be lower in the cage since the current mounts for it will go through the trusses. I just set the spider on top of the truss for the picture since the design isn't finalized.
All in all I think maybe the wire will be good way to go to lessen the diffraction spikes.
David,
Thanks! My original thought was to keep with the current materials and use aluminum. Very simple sheet drilled to allow for the focuser. What put a halt on that was mounting it to the rings. My goal is to keep the cage clean and simple. I am trying to make it look as nice as possible by eliminating unnecessary screws and attachment blocks etc. Then I was thinking just go with the 1/2" multiply baltic birch. Easy to cut, drill just put a couple screws in the top and bottom through the cage rings and presto, done. But while that may work effectively I still keep going back to the aluminum or something else besides wood. I was thinking a carbon fiber plate would be cool looking but then I run into mounting again. Similar to the aluminum. Oh, wait! I could use the carbon plate! Just mold in some mounting points with carbon cloth etc. *slaps self on forehead* How would you lay that up? Maybe use pre-made square carbon rod and epoxy it into place then shape/sand? I had also thought of just using carbon arrow shafts and using them like smaller cage trusses and mounting the focuser to that.
My friend as a vacuum pump so I could always use that in combination with carbon and honeycomb. High tech!
Thanks for the help guys! You have my wheels turning again! I have been re-inspired! Here is to finally finishing this thing.
Hi Greg, looks like a nice scope you have going there! I'm the owner of the Serrurier truss scope which Rod was linking to.
I would say from my personal experience that a wire spider is in every way superior, but it is of course also slightly more complicated to build and install. A curved spider will have to be made of relatively thick material, and while visually you don't notice the spikes around stars with a curved design the diffracted light is in fact just spreading out across the entire FOV, so you'll have less contrast overall. Depending on what you plan to use your scope for, that may be an issue or not... For visual observing you may not notice any difference. For imaging it can make all the difference - I have certainly noticed a much improved image resolution with my new scope, and that is with the exact same optics as I had mounted before in a traditional sonotube OTA with standard 4 vane metal band spider.
Another aspect of my particular wire spider that I enjoy is how incredibly easy it is to adjust. I can make ultra precise small adjustments of the string tensions with the guitar machine heads, all while looking through the focuser.
Regarding the strings, I'm using 9 gauge steel strings (0.23mm). They are actually not under much tension at all since it is the geometry of the spider that keeps it rigid, not high tension.
I just use eyebolts as the connection points for my wire spiders. I have also used bicycle spokes but there is not much thread adjustment on them and they are fiddly.
Thanks guys... Attached is a not so quick drawing of what I am thinking about to anchor the wires to the cage trusses. Basically, take a piece aluminum rod and drill it to accept the guitar wire. Drill and tap a hole to insert a set screw that will hold the wire in place. The drill and tap the end to insert a cap screw. The cap screw will not only hold the piece in place but also act as a way to tighten it up. I was thinking maybe I should insert a piece of dowel into the truss as a way to prevent the truss tube from compressing as the cap screw is tightened. That might be overkill because I don't think there will be that much pressure. Let me know what you guys think.
Hello again Greg,
I haven't built nor used a wire spider myself, but I'll offer the following thoughts:
I would be concerned about relying on a single point of friction (ie. set screw at right angles) to reliably hold the spider, especially when it is not getting any extra leverage on the wire (eg. having a few turns of wire wound around an anchor).
I'm not sure how tough the wire is, but pinching it in this manner can also damage it, so you may find your spider going 'twang' at some point.
The threads in the aluminium rod (for the set screw) aren't really adequate (wall is too thin and aluminium is a soft metal) - you will find that they will be easily stripped when you apply pressure on the set screws.
I was kind of wondering about a single point of friction myself.
Didn't think about the pressure breaking the wire but I can see that as a possibility at some point. Also the set screw threads might be a bit weak in aluminum. Crap! Back to the drawing board.....
So another thought.... I'll redraw it and you guys can check it out. I like visual stimuli my self! Imagine if you will the same cap screw and aluminum rod. However, now the end of the rod is still drill for the wire to go through and the hole is still in the top but with no threads. That is where the end of the guitar wire will drop in. (the part with the stop) Still very clean, no pressure on a wire, and no chance of slippage. The other end will be inserted through a hole in a machine screw and then wrapped around a couple times. The screw will be held in place with a jam nut.
Hey guys! Okay here is a drawing of a mount block for the secondary mirror holder. I took the idea from the link and made some changes. I could drill a hole in each cap screw for the guitar wires to pass through and then turn the screw to hold the wire on the screw similar to a tuner on a guitar. Then use a jam nut to keep the screw from turning.
I assume the reason for the 8 contact pints on the hub is because you want the wires to cross like Rolf does with his spider?
If that is the case you don't need the cap screws - just drill a hole down each side of the hub to feed the wires through. If I am not being clear I can post a picture to explain what I mean.
You'll get the most stability with crossed strings. This way you can put a larger angle on the wires without increasing the distance between the points of attachment. By having a larger angle you can achieve the same stability with less tension on the strings which will let the spider hold collimation better, and probably also let your strings last longer.
I used 8 individual strings because I didn't want any applied pressure from screws etc to weaken the strings. This way they are just fed through the holes of the central hub and gently winded up on the guitar tensioners - there are no bends or screws to weaken them.
Hey Rod!
Yes a photo of your idea would be great. I am a visual learner.
SkyViking,
Those cage trusses are 12" long so I could make a large angle...lol How far appart do you think I should space attachment points on the trusses? The block in the picture above will be 3" x 2" x .5" aluminum. The cut outs will be .25" deep. The larger holes are just for lighteneing reasons...
nightstalker,
Thanks! I wanted to do something more..... err, retro? Using maybe carbonfiber and a making minimum mirror box/cage etc. But since I am still learning this made more sence. The Moonlite connectors are great! Easy to install and make set up a breeze. I will use them again for my next large build....
SkyViking,
Those cage trusses are 12" long so I could make a large angle...lol How far appart do you think I should space attachment points on the trusses? The block in the picture above will be 3" x 2" x .5" aluminum. The cut outs will be .25" deep. The larger holes are just for lighteneing reasons...
I'd say a 45 degree angle is optimal from a theoretical point of view. That will give you maximum stability. I guess your inside tube diameter will be some 14"-15" to acommodate a 12.5" mirror, so with 12" cage trusses you would get pretty close to 45 degrees if you attach the strings at each end of the truss.
Thanks. I rough cut a block of aluminum today and the machinist is going to cut it down to the correct size and add the notches. Tomorrow I will use the milling machine to create the holes I need and bevel the edges.
Does anyone have ideas for a better spider assembly? Would wire help to reduce diffraction spikes? Would a curved spider be better? Should I use another material for the center of the spider? ie stronger lighter hold collimation better?
