I am new to this site. My name is Daniel and I am building a 18” truss telescope. The question I have is what diameter should the upper cage be compared to the 18” F/5 primary. The mirror is 457mm diameter and I made the inside upper cage at 520mm, is this too big? I’ve been reading a few diy jobs and I’ve seen that the cage should be only half an inch bigger than the mirror. Mine being 2.7” bigger I’m starting to think it’s too big and I will have trouble finding focus. My secondary is 3.1”. If I can have any feedback on this it would be much appreciated.
Finding focus won't be an issue. I don't think 2.7 inch wider is much of a problem. As it allows space for focuser baffles, filter slides etc. You can use a smaller cage which makes the size of all the components a little smaller which helps if space is an issue.
Try playing around with newt to see what works best:
BTW are you member of the ASV? if so why not come to the Instrument Making section meeting next Saturday afternoon. There are people there who could help you.
520mm ID UTA should be fine, just keep your focuser board a little closer to the inboard edge. I made my UTA 502mm ID and 602mm OD and the height of the cage is 300mm.
With an 18" F5 primary and 3.1" secondary you can have up 330mm from the centre of the secondary to the focal point. The Feathertouch FTF2015BCR uses a 6mm base plate, 37mm focuser body and if you rack the focuser out 11mm to accommodate various eyepieces, you end up needing 54mm for that.
330mm - 260mm - 54mm = 16mm
So locating your focuser board so that its outside face is 16mm from the UTA inside face will give you 330mm as your "D2" which means you end up with a fully illuminated 1/2" field which is good for general use.
That should leave you a distance from your primary to the centre of the secondary of 1955mm or thereabouts.
One thing to check is how much does your secondary holder obscure the mirror? If the holder has a lip that cover part of the mirror you should check the actual clear minor axis measurement.
My calcs were based on the outer 1.5mm of the secondary being covered by the holder thus leaving a 3" minor axis....
I set my focuser board a fair way in so that I had easy access to the SIPS element to be able to remove it for collimation
Oh ... and to IIS Daniel, where you will get lots of free advice whether you want it or not.... We can also help to empty your wallet in record time....
I am only a beginner at scope building, just finished my second one, but happy to help if needed. My first build was an 18" F5.6.
Thanks for the reply. If I keep the bigger cage would this mean I will need to shorten the truss poles to make sure the focal length reaches the eyepiece. This is my very first build and so far I feel I am doing ok. I made my own spider and secondary holder. I also made my own mirror cell using plop. The only thing I’m having issues with is how long to cut the poles. And having a bigger UTA I wasn’t sure if I would be loosing any light hitting the secondary.
The papers that came with the mirror says 2000mm from primary to secondary, then 308mm from secondary to eyepiece, but my secondary to eyepiece is just over 400mm, so I wasn’t sure if I could get into focus being further away. Or just re make the rings a bit smaller as my spider is easily adjusted to fit anything between 18” to 25”.
I had a look at Newt but some things on there confuse me a little bit. As for being a member If the ASV, I am not. I would love to come to one of your meetings though. I was trying to upload some photos of what I’ve built so far but I can’t upload for some reason. But thanks heaps for your help
I only posted this question about an hour ago and already got a few replies, what your saying makes perfect sense. I still think I need to re make the rings though, they are only 40mm wide not allowing enough room to move the focuser board in and out if needed. Plus there isn’t much room to tighten the thum screws for the pole connector I made. If I re make the rings what do you think the best ID size would be?
I am using the focuser that comes with the Sky-Watcher 12” Dobsonian. Am I measuring the focuser wrong. I was told to measure from base of focuser to when it’s half way out
Hi Daniel. Awesome work you're doing! I built a 10" truss dob and was having an issue regarding the focuser to secondary. I came up with making the secondary section independent of the truss to allow more or less focal length.
By loosening the clamps I can also remove and rotate the secondary section.
I also converted my 12" dob to collapsing type which makes it easier to allow for focusing issues and is great to transport around. I hope this helps
Thanks Astro_nutt for your input. Your projects look amazing, same as Matt, your UTA is beautiful,
Another quick question, I tried using 3mm ply for the UTA wall. As I bend it around it starts to snap and crack, 3mm is the thinnest ply I can find. Is there anything else I can use or do I need to soak the ply before trying to bend it into shape? Apart from that I think everyone has answered all my questions. I thank you all so much for your help. I was recommended to this site by a friend of mine and I will definitely recommend my Astro friends too. This forum is amazing how strangers make you feel welcome and will help out someone they don’t even know.
I would also love to attend a star party if anyone knows of any that I can attend, I am in Melbourne,
Thanks again for all your answers and help. Much appreciated
Many people use a thin black plastic to line the inside of the secondary cage. However thin ply is structurally stronger and doesn't warp in the heat (like in a hot car).
If you use a thin ply, then look for 1.5mm thick ply. You can get hoop pine ply that thickness from plyco but the sheet size is probably far too big for you. I believe some hobby stores stock thin ply in small sizes but I have never bought it.
I have used black plastic art folders which are quite cheap.
I used 0.8mm thick 'post forming laminate' for my first UTA baffle.
Makes for a great baffle. I actually thinned it down to 0.6mm with a belt sander to make it more flexible and 25% lighter.....
With this stuff you need to make sure it is warmed before bending it too much.....
Thanks Rod and Matt for the help. I think I should be all sorted now with all the information everyone gave me. I will post photos as I progress, still going to take a few months until I’m finished as I can only work on it one night a week.
And Matt it looked like the first UTA photo you uploaded was made of carbon fibre. I would love to see what the rest of your scope looks like. That UTA looked beautiful
Thanks Rod and Matt for the help. I think I should be all sorted now with all the information everyone gave me. I will post photos as I progress, still going to take a few months until I’m finished as I can only work on it one night a week.
And Matt it looked like the first UTA photo you uploaded was made of carbon fibre. I would love to see what the rest of your scope looks like. That UTA looked beautiful
Thanks again for everything from everyone
Daniel
Looking forward to your progress photos Daniel.
The baffle is a 0.5mm sheet of twill weave blue kevlar/carbon fibre. The focuser board is 3mm thick carbon fibre...
I am just finishing the scope this week by adding encoders and the Nexus DSC.
Hi everyone, thanks heaps for all your help and input. I thought I would attach a photo of my UTA, I wouldn’t have been able to do it without your help
Thanks again. I will upload more photos of the finished scope in a few weeks, hopefully I will be done by then. I did have an accident and dropped my secondary mirror on my concrete driveway and it’s now all badly scratched up. But I will get anew one soon.
to IIS Daniel, where you will get lots of free advice whether you want it or not.... We can also help to empty your wallet in record time.... 
