Converting a 300P into a Dob

[badgerchap]

I have one of the older blue SW 300P Newts. It’s excessively large, the only mount I have for it is an NEQ6 which understandably hates it, and as a result it barely gets used. In fact it’s almost killed my Astro interest.

I was going to try and sell it to fund the purchase of a small cat (Bresser 127/1900?) but I don’t think anyone is going to want to buy it. Instead then I’m considering converting it into a collapsible truss tube Dob. This should solve some of the unportability issues and give me something much more satisfying for observation, leaving the NEQ6 free for my imaging scopes. 

I intend then to preserve a little of the original blue tube on the primary end and some near the EP and secondary, whilst swapping out the middle portion for either a telescopic collapsing rod arrangement or a fully removable set of angled rods, either plan hopefully allowing me to transport the scope in a small car.

Has anyone done anything similar or have any advice/criticism I might find useful? I’m obviously concerned about preserving the rigidity along the optical axis. Also balancing the scope will be important.

Any insight gratefully received! I’ll try and post some updates as I go along.

[badgerchap]

OK I have the first putative design. It's very simple, which appeals to me for a few reasons:

1. There's a chance that I, with my ham fisted approach to DIY, can actually build it without too many issues.

2. I won't have to destroy the original OTA and so would still be able to re-mount it equitorially, in theory at least.

3. This design will be cheap to build and cheap to repair if it breaks.

4. There isn't too much that can go wrong, and almost nothing which can go wrong irreversibly (see points 1 and 2!).

 

There are definitely some caveats though:

 

1. This achieves no improvement in portability; in fact it makes the whole problem worse as this will be very heavy.

2. I am only able to 3D resin print the bearings - these will wear, may crack and will generally be substandard. Having them machined will be expensive.

3. This will not fit in the car at all.

4. I've had a very similar Dobsonian mount before. It was fine until it got damp. Then it was a clunky, sticky disaster.

 

Next I'm going to design the truss tube-esque version. I'll report back here with the plans.

[Peter Drew]

The side bearing ideally should be around 12" diameter, small ones as shown will present balance  and movement issues.   Some rings cut from large plastic soil pipe  and fitted on to plywood discs is a good DIY option.      🙂     

[badgerchap]

7 minutes ago, Peter Drew said:

The side bearing ideally should be around 12" diameter, small ones as shown will present balance  and movement issues.   Some rings cut from large plastic soil pipe  and fitted on to plywood discs is a good DIY option.      🙂     

Why 12"?

Good idea

[johninderby]

Or something like the alt bearings used on a Bresser dob.

[badgerchap]

3 minutes ago, johninderby said:

Or something like the alt bearings used on a Bresser dob.

I was thinking about something like that, which I'll be incorporating into the next design. I do question my ability to produce it though! XD 

[Peter Drew]

9 minutes ago, badgerchap said:

Why 12"?

Good idea

General "rule of thumb"  should equal the aperture.  Bigger the better within reason.      🙂

[faulksy]

orion optics uk sell a dob mount kit

moonshane converted a 16 ob to collapsing truss design

[johninderby]

Wonder if Bresser would sell the bearings on their own? 

[badgerchap]

6 minutes ago, Peter Drew said:

General "rule of thumb"  should equal the aperture.  Bigger the better within reason.      🙂

That makes sense, I suppose. I seem to remember seeing a Dob with bearings the diameter of the tube length!

[badgerchap]

5 minutes ago, faulksy said:

orion optics uk sell a dob mount kit

Definitely too easy. I'm trying to kill the lockdown boredom here 🤣

[badgerchap]

6 minutes ago, johninderby said:

Wonder if Bresser would sell the bearings on their own? 

That's a thought - if not, I'm sure I can find something similar elsewhere.

[johninderby]

Ouch!!!!

[badgerchap]

1 hour ago, johninderby said:

Ouch!!!!

 

Ouch indeed! Somewhat defeats the object in my (thrifty) case!

 

[badgerchap]

And here's plan number two. 

Pros:

1. Collapsible design means this should fit in a Ford Fiesta if needs be.
2. Bearings are definitely a better design.
3. Easier access to the mirror for cleaning 
4. Despite the added complexity, setup time shouldn't be excessive
5. Cooldown time should be very rapid
6. It just looks ten times sexier.

Cons:

1. Really not convinced I have the DIY skills for this
2. Will need some kind of jacket for stray light
3. Design needs a bit of work to add access for collimation of the primary
4. Still relies on some resin printed parts (rollers under bearings)
5. As the design stands the original OTA will be destroyed irreversibly.

The bearing cradle can be seen in the third photo. The little grey bits are resin printed collars, but I think they might be a bit rubbish against the plastic track surrounding the bearing wheels. I can probably find some bits of thin nylon tube to do the same job, but the idea is to decrease friction with the wheels.

The secondary mirror housing just holds a slice of the original OTA held in by screws. Minimal reworking is the plan.

Go easy on me, it's late and the kids have been driving me to distraction tonight hehe.

 

[johninderby]

Wouldn’t use rollers. Teflon blocks are better and if you can find some FORMICA stardust HGP F1782 edge trim or offcuts for the disks. The laminate needs to be “bumpy” to work properly.

There are places that sell plywood disks that might suit depending on price of course.

https://one-stop-diy.co.uk/pages/plywood-circle-cutting/

Edited May 19, 2020 by johninderby

[badgerchap]

Yeah I think pre-cut disks are going to be essential. Cutting with a jigsaw would just be a mess!

Edited May 19, 2020 by badgerchap

[niallk]

You may have seen this - there's a great book by Dave Kriege:

https://www.amazon.co.uk/Dobsonian-Telescope-Practical-Building-Telescopes/dp/0943396557

Worth reading - engaging conversational tone you might pick up a cheap used copy on the US site.  Lots of practical advice and calculations for balance etc.

[Mick J]

We mounted a 11" very much like your first sketch, used something like pipe ends screwed to the box running on teflon blocks (ebay).  Truss looks more fun. 

Perhaps pinch some ideas from @Dave_D  seems to find interesting solutions : 

 

[badgerchap]

1 minute ago, Mick J said:

Perhaps pinch some ideas from @Dave_D  seems to find interesting solutions : 

 

Ugh, that's just made me jealous about his CNC!

[John]

My 12 inch newtonian was bought as an optical tube with tube rings and I then had a dobsonian mount made for it from plywood based on the Orion Optics design:

 

 

[barkis]

8 hours ago, badgerchap said:

And here's plan number two. 

Pros:

1. Collapsible design means this should fit in a Ford Fiesta if needs be.
2. Bearings are definitely a better design.
3. Easier access to the mirror for cleaning 
4. Despite the added complexity, setup time shouldn't be excessive
5. Cooldown time should be very rapid
6. It just looks ten times sexier.

Cons:

1. Really not convinced I have the DIY skills for this
2. Will need some kind of jacket for stray light
3. Design needs a bit of work to add access for collimation of the primary
4. Still relies on some resin printed parts (rollers under bearings)
5. As the design stands the original OTA will be destroyed irreversibly.

The bearing cradle can be seen in the third photo. The little grey bits are resin printed collars, but I think they might be a bit rubbish against the plastic track surrounding the bearing wheels. I can probably find some bits of thin nylon tube to do the same job, but the idea is to decrease friction with the wheels.

The secondary mirror housing just holds a slice of the original OTA held in by screws. Minimal reworking is the plan.

Go easy on me, it's late and the kids have been driving me to distraction tonight hehe.

 

Re con No.2 strike it from your list, this is no time to harbour negative thoughts. From my pov you are adequately equipped to complete this project, and with flying colours.

Ron.

[Mick J]

11 minutes ago, badgerchap said:

Ugh, that's just made me jealous about his CNC!

Yes, seems to have to many toys !   some of his earlier posts had good finds for dob making.

[badgerchap]

16 minutes ago, John said:

My 12 inch newtonian was bought as an optical tube with tube rings and I then had a dobsonian mount made for it from plywood based on the Orion Optics design:

Nice, simple and elegant!

[badgerchap]

11 minutes ago, barkis said:

Re con No.2 strike it from your list, this is no time to harbour negative thoughts. From my pov you are adequately equipped to complete this project, and with flying colours.

Ron.

Thanks for the vote of confidence! I've pretty much decided to go with this (rough) design now, so in at the deep end it is!