Truss Dobsonian project

[Stu]

A little while ago I bought a very nice Truss dobsonian for an indecently cheap price off ABS. One reason it was so cheap was that it does not have a mirror set in it! It was designed for a Zambuto 12.5” f5, and ultimately that is what I would like to replace it with, but as a stop gap I have sourced, from a very fine gentleman on the forum (who seems to wear warm socks ) a 12” f5 mirror set, secondary and spider plus a focuser.

The dob came with part of the primary mirror cell, and I have plans to combine this with the support from the Revelation cell to get a functioning scope. There is a small degree of DIY needed, enough to challenge me with my normal chaotic life, limited time available and the fact that I am also supposed to be building my daughter a bed at the same time!

Basic steps required are to fit the focuser, fit the secondary, sort the primary cell and fit that in the correct position (slightly higher) to allow for the shorter focal length. I may need to do some slight balance adjustment to allow for the change, not sure yet.

Pics of the dob attached, I will post up some detail shots of what I need to do as I move on and suggestions are welcomed!

My intention is to have this setup semi permanently in the garden but obviously able to be transportable if needed. It is not particularly lightweight.I have just made another purchase which to a degree duplicates this scope but one the other hand it is a very different proposition, will post up about it shortly elsewhere.

Looking forward to getting this lovely scope up and running again.

[Adreneline]

Wow! I am seriously impressed. I am also impressed this feat of construction appears to be taking place in the lounge! But to be serious that is some undertaking and I only wish I felt I could do something similar. I'm currently trying to pluck up the courage to drill two holes in a Vixen bar - maybe tomorrow.

Good luck and I look forward to seeing the finished item.

Thank you for sharing and inspiring.

Adrian

[Stu]

3 minutes ago, Adreneline said:

Wow! I am seriously impressed. I am also impressed this feat of construction appears to be taking place in the lounge! But to be serious that is some undertaking and I only wish I felt I could do something similar. I'm currently trying to pluck up the courage to drill two holes in a Vixen bar - maybe tomorrow.

Good luck and I look forward to seeing the finished item.

Thank you for sharing and inspiring.

Adrian

Thanks Adrian, although for clarity I purchased the scope in this condition so so far I can take zero credit for it! The lounge is the original owner’s  

Hopefully I can do it justice though and get it back to working condition.

[Alan White]

I can see why this will have tempted you Stu, a very nice looking truss tube indeed.

Looking forward to your postings on progress and the first light.

[Rob]

Very nice fella!.. super project to have, and shall provide nice views to boot!. I'm after a dob now also.. I've been taken by the Meade lightbridge ever since they were released some years back. Going to keep an eye on AB for mirror less flex tube also.

Anyway.. very nice bit of kit indeed Sir!

Edited August 22, 2018 by Rob
typo

[Stu]

Thanks Rob. I would do a bit more research on the Lightbridges, I think they are pretty heavy and I’ve somehow never really been a Meade Fan. I think @John had one for a while.

One reason for posting this is to force me to crack on with it!!! So please do keep asking  

[Rob]

Cheers for info Stu.. humm, yes the weight is important, research is a defo then. I really do like your example there.. I think the materials really make it a super nice item. I look forward to your first light report when ready

Rob

[PeterW]

@Stu if you’re getting all ATM, I could do with some help with a mirror set and focusser I have hiding under a bed.... living in a flat results in no space for tools or tinkering. 

PEterW

[Stu]

30 minutes ago, PeterW said:

@Stu if you’re getting all ATM, I could do with some help with a mirror set and focusser I have hiding under a bed.... living in a flat results in no space for tools or tinkering. 

PEterW

Let’s get something sorted then Peter! A bit of moral won’t go amiss from my end either  

[PeterW]

Any help @Stul? 

PEter

[Stu]

On 22/08/2018 at 21:45, PeterW said:

Any help @Stul? 

PEter

Thanks Peter. You’ve reminded me, I think I have the one on the right but the other might be worth a look next time we meet. Thanks again

[Stu]

I thought I would kick start this again as a motivator for getting on with this scope finally. Having finally finished Lorna's bed, and getting into the final stages of my pallet built garden seat, I might finally get a chance to pull it all together.

Last night I drilled the holes necessary to attach the two parts of the primary cell together, I just need a couple more nuts to finalise this. The bit I'm struggling with is how to work out where to position the primary so that the light path is correct with the secondary etc. I guess I need to sort the focuser out first, then position the primary accordingly. Reading that book I've got might make sense too!

 

[PeterW]

Looks good, where did you find it?  I think the plan starts at the top end, then the rocker and then the base, so you have the balance numbers. Keep us posted!

 

peter

[Stu]

54 minutes ago, PeterW said:

Looks good, where did you find it?  I think the plan starts at the top end, then the rocker and then the base, so you have the balance numbers. Keep us posted!

 

peter

It was on ABS, was just lucky and managed to get in first.

I've fitted the focuser I have for it, nothing special but functional. I guess I'll then do the spider and secondary and go from there?

[markse68]

I like the spring balance assist- haven’t seen that before and was thinking of using springs on my 12” design to enable use of smaller rockers and have some adjustability for heavy eps etc. Is that something unique to that dob? I haven’t come across it before. Didn’t see you mention it (sorry if you did) but do you know who made it?

[PeterW]

Several people sell spiders, or you can get creative with some bent metal or wire. To balance stuff just make the scope bottom heavy and use trim weights at the top end. You can have a low profile mirror box and use oversize rockers for compactness.

Peter

[Stu]

2 minutes ago, markse68 said:

I like the spring balance assist- haven’t seen that before and was thinking of using springs on my 12” design to enable use of smaller rockers and have some adjustability for heavy eps etc. Is that something unique to that dob? I haven’t come across it before. Didn’t see you mention it (sorry if you did) but do you know who made it?

Thanks Mark. The spring system is not the most beautiful part of the scope but it seems effective enough.

I believe it was made by a carpenter to a design produced by the guy that had it built, and who I bought it from. It is a lovely, solid piece of kit. I hope to keep it assembled and under cover as a bit of aperture to use from home and for club nights. My 14" can do the dark site trips when we do manage them.

I've managed to fit the focuser ok, just need some more suitable bolts for it. Next job the secondary. The vanes on the one I have are too short so I need to find suitable material for replacements. Any suggestions out there?

[Stu]

A few pics. The cell is basically ready to bolt in once I can work out where to put it. Focuser fitted now, simple but will do for now. I don't have much to spend on this currently but just want to start using it.

[jetstream]

Great Stu!

Can you use a square on the top of the mirror box with the hole to define the hole edges below? once done you can locate the cell so the mirror is lined up with the mirror box opening(?). Maybe using a cardboard "mirror?

Edited June 26, 2019 by jetstream

[Stu]

4 hours ago, jetstream said:

Great Stu!

Can you use a square on the top of the mirror box with the hole to define the hole edges below? once done you can locate the cell so the mirror is lined up with the mirror box opening(?). Maybe using a cardboard "mirror?

Thanks Gerry. Yes I hadn't thought of that but think I could do.

Any advice on working out where I need to place the primary vertically? I guess I need to work out where my focal point needs to be relative to the focuser then work backwards as Peter suggests?

[jetstream]

50 minutes ago, Stu said:

Thanks Gerry. Yes I hadn't thought of that but think I could do.

Any advice on working out where I need to place the primary vertically? I guess I need to work out where my focal point needs to be relative to the focuser then work backwards as Peter suggests?

Holy cow Stu! I just got in -1:30am here- what a night! M17 stole the show... unbelievable.

Do you have the focal length of the mirror?

If so calculate "L"- set the focuser in the middle of its travel and measure the distance from the top of it to the center of the secondary. Subtract this off of the true primary focal length and then set the cell triangles this distance (from center of sec) plus the thickness of the mirror, should work. When doing this set the collimation bolts in the middle of their travel too.

Eagerly waiting reports Stu!

[JOC]

Did you find out why the scope, that otherwise looks mostly complete to my uneducated eye, didn't have a mirror fitted - was it someone's abandoned project?

Edited June 26, 2019 by JOC

[chiltonstar]

What will the weight of this one be when finished Stu? (I'm thinking of your back, and more significantly mine as I'm wondering about a large truss tube Dob)

Great project!!

Chris

[Stu]

2 hours ago, jetstream said:

Holy cow Stu! I just got in -1:30am here- what a night! M17 stole the show... unbelievable.

Do you have the focal length of the mirror?

If so calculate "L"- set the focuser in the middle of its travel and measure the distance from the top of it to the center of the secondary. Subtract this off of the true primary focal length and then set the cell triangles this distance (from center of sec) plus the thickness of the mirror, should work. When doing this set the collimation bolts in the middle of their travel too.

Eagerly waiting reports Stu!

Sounds amazing Gerry, you must be right up there in terms of big aperture under some of the best skies in the world! Look forward to reading all about it!

Thanks for the advice. I have a focal length spec but it might be worth checking this for accuracy. I guess this is doable?

So, now I have a method I can work through this step by step. Initial challenge is to find some decent material to make the vanes from. I had been tempted to try a curved vane approach, but will probably keep things simple to start with.

[Stu]

2 hours ago, JOC said:

Did you find out why the scope, that otherwise looks mostly complete to my uneducated eye, didn't have a mirror fitted - was it someone's abandoned project?

The scope was originally equipped with a 12.5" Zambuto mirror which the guys wanted to keep to use in a different scope. My guilty secret is that I have a direct replacement on order, just not sure when it will be delivered. I need to save up for a decent mirror support for it as it is quite thin for better cooling but it needs better support to avoid astigmatism. I have a 12" mirror set and the cobbled together support which I will use as a learning process, then finally fit the Zambuto when I can afford it.

2 hours ago, chiltonstar said:

What will the weight of this one be when finished Stu? (I'm thinking of your back, and more significantly mine as I'm wondering about a large truss tube Dob)

Great project!!

Chris

Not sure Chris, but it's not light. My plan is to use this from home and will probably make some wheelbarrow handles for it so I can easily move it around the garden. Despite being a truss, I think I will mostly keep it fully built.

I also have an EQ platform so I can have tracking at high powers, useful when those planets get a bit higher!