16" Dob remake

[Sfarndell]

3 years ago, I was in the process of making a 18"x1" F4.1 mirror which was in the figuring stage when I came across an advert for a 16"x2" F 4.5 mirror complete with an aluminium 18pt mirror cell for around GBP250 which was being sold as part of an attic cleanout. Who could resist!

Since most of the basic construction for the 1st mirror was done for star-testing, all I needed to do was make a smaller mirror box and hey-presto, I had a large working dob within a week (Rather than 2-3 months based on my eta for the 18"). Turns out the new mirror was fairly decent too (using star and ronchi testing) so I didn't have to refigure it.

Before you go blind looking at my handywork, woodworking is really not my strong suit. My aim was to I was playing with some interesting design concepts (Ultralight design, wire spider, front-collimation screws) and the result is a rather ugly, but functional scope (see below).

The issue with the ultralight contruction is that the poles are sooooo long and don't fit in the boot so SWMBO insists I leave it at home when we go on holiday.

After looking at all the amazing scopes under construction/recently built by Swampthing, Pablito and Rustysplit, combined with a significant discount at my local hardware store on pine plywood, Inspiration struck!!

I intend to keep some of the designs elements from the original that worked really well like the wire spider and the front-collimation screws and probably 'borrow' some ideas from great scope makers , but the two most important elements are that the whole scope must fit in the boot, and it must look nicer than it currently is.

Some imagination is required, but here is my new Dob.

...

[michael.h.f.wilkinson]

Sounds like a great project

[SnakeyJ]

Looks like a great project too.

How much do you need to shorten the truss poles to make them fit the boot? Are you thinking of using two section poles, or extending the mirror box to meet shorter poles?

[swamp thing]

Great stuff.

I do so like following these projects.

Please keep us posted.

[Sfarndell]

Thanks all for the encouragement.

I couldn't wait for the weekend so started last night within an hour of buying the wood. The mirror box panels and upper cage rings are cut out already , but I forgot to take pics last night. Will do so tonight before I start on the next section to keep a more accurate timeline. I'll try to post regularly too...I also love following other's builds - its so additive and annoying when they dont post enough to satisfy one's need for another "fix".

How much do you need to shorten the truss poles to make them fit the boot? Are you thinking of using two section poles, or extending the mirror box to meet shorter poles?

Jake - my plan is to follow a more traditional route and have a secondary cage to protect the mirror more effectively as well as extend the mirror box. Length of the poles are +-1.5 metres so I'm looking to shave 30-40cm off the length. The old mirror box outer dimension for height is just 17cm (7") so there is room to play with. Weight of sections is not an issue really for me right now, but once the OTA is complete and depending on the balance point, I'll look at weight-saving techniques

[SnakeyJ]

Jake - my plan is to follow a more traditional route and have a secondary cage to protect the mirror more effectively as well as extend the mirror box. Length of the poles are +-1.5 metres so I'm looking to shave 30-40cm off the length. The old mirror box outer dimension for height is just 17cm (7") so there is room to play with. Weight of sections is not an issue really for me right now, but once the OTA is complete and depending on the balance point, I'll look at weight-saving techniques

Sometimes I can be so dull, but in my defence it was another late night battling the seeing to capture Saturn again! Absolutely agree with extending the cage, effectively shortening the truss rods from both ends. Other than a slight increase in weight, the cage should offer considerably more scope for mounting finder/telrad. At F4.1 (1870ish fl) what's the EP height like on your original build?

[Sfarndell]

I guestimate it's about 1.75 metres (1870-300 eyepiece to secondary + 200 from floor to top of mirror for rockerbox/base). The perspective in the pic is deceptive and I'm not standing staight, but I'm 6'3" and don't need a foot-stool at the zenith.

On modifying the scope to accommodate the 16", the height of the focusser didn't change by much due to the FL differences and I only needed to replace the mirror box.

[SnakeyJ]

Ah the joys of being tall - it is quite useful with these big dobs. Though not that much trouble to bring a step up for the more vertically challenged, and there are some nice step up tool boxes that would make great EP/Accessory cases to take out in to the field.

[Sfarndell]

Here is the progress after day 1. Next on the list is to finish the cage supports and to screw the mirror box together so the edges can be neatened up before fitting the mirror cell.

[swamp thing]

I see you've gone for octagonal upper tube rings. Any particular reason for this, or s it just personal preference?

[Sfarndell]

It's just personal preference. I considered circular, but the purpose of the scope is to be able to travel. The secondary cell is to slot into the top of the mirror box and i wanted more surface ontact between the side of the cell and the mirror box to prevent it rattling around.

[swamp thing]

Makes perfect sense to me. I had thought of this "UTA fitting into mirror box" myself, but on my 20" I reckon it would have made the mirror box too big to go through any regular doorway. Unless I left off the light baffle, but this would have weakened the mirror box considerably (not a good idea on such a big scope). On a 16" one can simply increase the size of the mirror box, as doorway access is straightforward enough.

[Sfarndell]

Today was mostly spent gluing the inner base of the mirror box to the outer base (essentially a strengthening layer) and the UTA supports together. I miss not having a table saw as getting the supports the same length and square after glueing 2 layers of ply together has been very time consuming.

Herewith the UTA with no glue or screws yet

here's the mirror cell

[Moonshane]

starting to look very nice!

[Sfarndell]

Hit a snag in that the support struts aren't as stable as I hoped so will need reinforcement. Without a table saw it was difficult to get the ends flush and parallel which might be the root cause so I'll have to make a plan otherwise a re-think on design may be in order.

[swamp thing]

Why not get yourself (or make) a simple mitre box. This enables you to make nice accurate square cuts.

like this

http://www.homebase....rid:18800659112

[Sfarndell]

good suggestion - have one buried in a box and didn't think about it.

I did some more work this evening and using the mitre box helped to add some more rigidity, it didn't work well enough for comfort so I used additional supports of the same dimension as the focuser bracket with 9mm ply. After coring the supports, the additional weight is not significant and the UTA is now rock solid without applying glue.

[swamp thing]

Nice solution.

Glad you've sorted it.

[Moonshane]

I agree, looks a lot more solid and attractive too. looks like you work like I do with every tool you have used during a session on the floor with bits of wood and everything else! my father in law pulls his hair out as he's a put every tool back in the box every time you use it kind of a guy! I think the quality of ply would make quite a difference to rigidity. yours looks like a softwood centre with not many plys? my UTA rings have 11 hardwood plys and are 15mm thick. that said, your solution gets around this nicely!

[Sfarndell]

LOL. It drives my wife crazy too! I do pack up after each session though.

I'm not sure if i'll be painting or varnishing afterwards (depending on my woodworking skills!), so opted for pine ply. The UTA is 9mm with 5 plys to reduce weight at the end of the trusses, and the mirror and rocker boxes are going to be 12mm, double layered where extra support is needed.

[Moonshane]

same here but it does (in support of my father in law's protesations) take me ages to clear up.

I'm sure you will be fine with what you are using given the recent changes. it's surprising how much stability corner brace / gusset pieces make to a box too.

[Sfarndell]

It's been a few days and I've made some progress, but the embarrassment factor that my home-made scope still looks very much home-made has prevented me posting some progress photos until I could show something looking semi-decent. This is very much a trial and error process, with loads of trials and plenty of errors before things worked. My only power tools are a jig-saw, router, drill, drill press & angle-grinder and my skill is not what it could be - evident in the neatness of the finishes. The overall engineering worked well so I'm happy with the progress and most of the aesthetic issues will be addressed later on in the project.

Lessons learned from my 1st build are evident - positives include the front collimation bolts and wire spider as well as negatives like spray-painiting aluminium doesn't work (I used black insulation tape to cover the truss poles in build 1, now removed, but it will be redone again once the poles are cut to length).

New additions are the mirror-box truss clamps which I poached from a much better carpenter/engineer (http://www.roelblog....5-truss-dobson/) and they are rock-solid.

It's not pretty yet as I want to finish the complete construction before doing all the sanding/varnishing/painting. It does work though - I tested the focus on the moon last night and the poles are 3cm too long (I added 2 cm for error, so am chuffed by this at least). Next phase is to find the CoG and make bearings, followed by the rocker box.

Below: Wire spider made from circular wood cutouts, washers and some threaded rod. The length of the rods will be adjusted when I cut the truss poles to length and the spider will be painted blackboard black. Wire fishing trace cable (the flexible kind) was used for the spider (straight off build 1 which had a bigger secondary cage). I originally tried 50lb single-strand trace wire that I had lying around first, but it was too stiff and difficult to work with when adjusting so the cable is highly recommended. The current arragement I found to be incredibly stable, with little/no vibration on manual tracking and fast dampening times.

Below: mirror box. Cover is from my original build and is to be cut to size & painted matt black. Wingnuts on the collimation knobs are to be replaced with wooden knobs similar to the clamp bolts, but after refinement of the design.

Below: Full build in its current state to test focus point/fine-tune truss length.

Any suggestions/improvement options are welcome!

Scott

[Moonshane]

hi Scott

I think you have done a sterling job so far despite what you say about your skills. I'd try to make things tool free where you can but otherwise it's looking great!

[Sfarndell]

Thanks Shane!

On home-builds, one always sees the problems that need fixing. Telescopes are meant to work in the dark where nobody can see them so as long as the engineering and useability is good, I'm happy. Ecstatic would come if it looked great.

I'll spend some time on neatness once the scope is fully functional and should be able to fix most of my issues.

I agree on the tool-free setup - pics of the UTA truss clamps were before I put knobs on the bolts (similar to the mirror box clamp bolts) so everything can be tightened/fastened by hand.

[swamp thing]

Coming together Scott.

Are you going to use the same primary cell in this one?