DIY 18” truss dob, UTA size?

[Dbroz]

Hi everyone

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.

Thank you all in advance

Daniel

[Rod]

Hi Daniel

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:

https://stellafane.org/tm/newt-web/newt-web.html

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.

Rod

[Kunama]

Daniel,

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

[Kunama]

Oh ... and :welcome: 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.

[Dbroz]

Hi Rod

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

Daniel

[Kunama]

How is your D2 distance 400mm ? Seems unlikely that with a 260mm UTA inside radius you could have 140mm from inside of UTA to focal point>>>>

What focuser are you using? got any pictures?

[Dbroz]

Thanks Matt for the warm welcome,

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?

And again thank you all for your feedback

Daniel

[Dbroz]

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

[Dbroz]

I’m having trouble trying to upload photos. It keeps saying upload error

[Dbroz]

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[Dbroz]

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[Dbroz]

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[astro_nutt]

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.
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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
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[Dbroz]

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

Daniel

[Rod]

Hi Daniel

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.

Hope that helps.

Rod

[Kunama]

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.....

[Dbroz]

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

[Kunama]

Looking forward to your progress photos Daniel. :thumbsup:

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.

There's a build thread here: http://www.iceinspace.com.au/forum/showthread.php?t=164454 and if you're really bored, there's another to be found here: http://www.iceinspace.com.au/forum/showthread.php?t=135679&highlight=guinevere

[Dbroz]

Wow Matt that is beautiful,

[Dbroz]

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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.

Once again thank you all

Daniel

[Kunama]

Looking good Daniel, keep the pictures coming.:thumbsup:

[Dbroz]

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Almost finished, just a few more adjustments and a bit more painting, I was able to focus with my 25mm eyepiece last night but I could not focus with any eyepieces below 25mm, my poles are still about 30-50mm too long. I will be cutting them down tonight and hopefully be able to get the rest of my eyepieces into focus. If anyone has any suggestions on how I can improve this telescope to look or function better I am happy to hear your thoughts. Collimation holds well as I turn left and right, but up and down I do notice that the secondary does move slightly when I go from horizon to zenith. It doesn’t move much but it moves enough to allow the laser to go off the centre mark by about 15mm. I’m guessing it’s the weight of the secondary. I did use a thick foam double sided mirror tape to hold the secondary mirror to the holder. I might change it to silicone once my new secondary comes in. But if you have any suggestions about anything that would be great.

Thanks a lot guys.

Daniel

[Kunama]

How much tension do you have on the spider? I have mine tight enough to play G sharp......or an octave above :thumbsup:

[Dbroz]

My spider is pretty tight. I’m pretty sure it’s the thick tape I used, the mirror is a bit loose , I was playing around with it before and noticed that the tape is too thick and it makes the secondary wobble. I got the poles to the correct length, so all eyepieces can reach focus, but I’m still having issues with the 10-6mm eyepieces, the higher I go in magnification the more the objects come out like a blob or blur. With the 25mm I can’t seem to get a sharp image, the bright stars are looking like arrow heads or seagulls. Could this be due to my secondary mirror being so scratched up as it was dropped on my driveway and has deep scratches everywhere? Or is it collimation? Or did I not let the mirror cool enough, my primary is 50mm thick, and I only had it outside for about 30 minutes before I looked through the eyepiece. And as I move up in magnifation it gets worse with each increase.

Thanks
Daniel

[Kunama]

Daniel,
Sounds like you have a few things going on in there....
Start by securing the secondary so it doesn't wobble about, then if you have a laser, check that the reflected laser spot hits the centre of the primary and doesn't move about when you move the scope around.

Depending on what you use for collimation, get the primary collimation done. I use either a barlowed laser or Cheshire eyepiece at the moment. Then move the scope around to check that the collimation holds while you do this. (Check for any movement in azimuth and altitude)
I have my spider/secondary set so that my laser spot will only move off the centre of the primary if I shake the scope and even then the movement is less than 1mm.

At F5 you will still see coma at outer half of the field (seagulls) without a Paracorr, this will be more pronounced with some eyepieces...

Your 10mm and 6mm eyepieces are pushing the magnification up to 225x and 375x and that will require the mirror to be cooled to ambient and steady seeing.

A 2" thick primary, if exposed to warm temps during the day, may never get to thermal equilibrium at night, forced cooling by fans will help but much depends on how fast the outside temperatures are dropping.

If you have a large fan (typically used inside house) and 240v available you can use that to cool the mirror much faster.

I doubt that the scratches would be the culprit to the extent you're describing, my money is on the collimation shifting, coma inherent in the optic at F5 and lack of thermal equilib.... The 'woolly' stars become smaller and smaller points as the mirror gets closer to ambient :)

[Dbroz]

Thanks Matt

I worked out for sure that the thick tape is definitely not good as the weight of the secondary moves as I bring the scope up and down. I’m using both a laser and Cheshire. I will try to cool the primary as best I can. As for coma should I get a coma corrector? I was told a F/5 shouldn’t need one. When I look a say Jupiter for example, with the 10-6mm eyepieces it seems as there is a double image and they are slightly off resulting as a big blur. Is this coma too? It’s hard to explain exactly what I’m seeing. I also don’t know how well figured my mirror is. It is 20 years old, the original owner still had it in its original box as he never got around to building his scope. It was made by Nova optics. Do you think it would be wise to test the mirror? And if so can I do this myself at home? I would even know how to test the mirror. I might join the asto association club if they are allowing new members. Anyway I appreciate all the help you have given me so far, mich appreciated

Thank you
Daniel

[Rod]

Hi Daniel

Your scope is looking great!

I have a 12 inch F5 and don't use a coma corrector. I have one but just don't feel the need for it so I don't think that's your issue.

I agree with Matt it's probably a collimation issue brought on in part by the tape. Check also for how stable your truss system is. It looks well made but I found with mine that if some of the knobs weren't tight enough the truss could slip a little.

I'm sure you know this but alight with the Cheshire first to check all the optical elements look concentric. Just use the laser for the final touch up.

I would recommend you join the ASV. You can do that anytime and members of the Instrument section can help you. They can also test your mirror. If you are in the south eastern suburbs, MPAS is a good group too with many helpful members.

Rod

[Kunama]

I doubt that what describing is from the primary mirror. I am no expert on Newtonians or on any scopes of that matter but until you rule out the secondary movement you won't be able to get the collimation right.

Is the issue the same with other eyepieces?

The lack of detail at higher powers could simply be too much magnification for the seeing conditions...

Joining the ASV is the way to go there is a wealth of knowledge and experience at the ready to help diagnose issues like this.....

[Dbroz]

Yeah the issue is the same with all eyepieces from 15mm-6mm, the 25mm is ok but still not pin point, the seagulls are not just at the edge but in centre as well. But like you said it’s most likely due to collimation. The scope is so high that I collimate while horizontal, once I begin to lift it will come out of collimation. I have plenty of tome to work it out and fix. Thanks again for all your help

[Kunama]

Try to collimate with it at around 35º-45º rather than horizontal then you know the mirror is resting agains the cell's glides.

[Dbroz]

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Thanks Natt for all your help. I got the poles at perfect height and all my eyepieces come into focus now. Wow what a view, I can see so much more than my 12”. It’s amazing the difference between the 2. Now all I need is my new secondary and to properly make my ground board and finish painting the inside and outside of the box. Then I need to make a shroud. But apart from that I am so happy with the results. I will send another pic once it’s fully finished. Thanks again for everyone’s help and input. I couldn’t have done it without you all.