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Roel's 16" f/4 ultralight traveldob (Propdob Mega)

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#1 Roel

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Posted 03 December 2020 - 02:54 AM

After building several "Propdobs" (200mm, 250mm, 300mm and even a 100mm) in the past years, I am currently building my biggest one yet: a 400mm f/4.

 

This autumn I would have made another observing trip to the island of La Palma, but unfortunately I had to skip this one (for obvious reasons). However: this, together with selling one of my earlier Propdobs, did create some financial space to purchase a high quality 16" mirror.

 

The design is mostly just a scaled up version of my 12", with two big differences: it's going to be a lowrider (which means the focuser is pointed a bit upward to lower the eyepiece height), and it will need a 6-point mirror cell instead of the 4-point cell as used in my 300mm.

 

I started with the secondary cage, which is actually a ring made of two partially "hollowed" rings glued together. It's almost half the weight of a solid ring, but just as rigid. The focuser plate is removable and features a KineOptics HC-2 pointing upwards at 15 degrees. This brings the eyepiece height 4 inches down! For people unfamiliar with this so-called "lowrider"-concept, German ATM Reiner Vogel explaines it in detail on his website: http://www.reinervog...lowrider_e.html

 

With my length I will be able to look through the eyepiece sitting on a normal chair, which is very convenient for a traveldob. No need to bring an observing chair on the plane, a chair on site (or a beer crate for that matter laugh.gif ) will work just fine.

 

DSC_2080CN.jpg

 

DSC_2082CN.jpg

 

DSC_2105CN.jpg

 

DSC_2106CN.jpg

 

(By the way, I'm not trying to build the smallest or lightest 16" ever. Of course everything could be build just a litter more compact, ore a little lighter. The goal for me is to keep it under a certain maximum weight (20 kg, or 45 pounds all in), without compromising stiffness, ease of use or ease of transport). Furthermore I don't use heavyweight eyepieces or a comacorrector, just a set of DeLites, so keeping the thing collimated will not be an issue.


Edited by Roel, 03 December 2020 - 03:01 AM.

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#2 Roel

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Posted 03 December 2020 - 03:40 AM

The primary mirror cell is made out of three layers of 18mm birch plywood, glued together. The two upper layers are cut to "rings", the bottom layer is closed, except for some ventilation holes. The corners of the upper ring are drilled and cut to classic truss clamps.

 

The bottom plate is in fact the mirror cell. Not being able to use a simple 4-point cell due to the thickness of the 16" mirror, I wanted to design something that is just as compact and just as easy to make, but now with six points. I had a hard time explaining to my fellow Dutch ATM's how this 6-point design works, so please forgive me if my explanation in English is even more unclear...

 

The mirror is supported by six points, positions calculated with PLOP. Three levers (each with two support points) rotate freely in V-grooves in the bottom plate. Two of the support points are nylon screws, which screws through the levers - these are the collimation adjusment screws. The screws can be reached through larger holes in the back plate.This means that not the whole lever is adjusted, but only one support point per lever. The levers maintain their function at all times, rotating freely in the V-grooves, so the weight distribution over the six points remains even.
The range of collimation is a bit limited with this method (no more than a few millimeters, but that is not really a problem).

 

The pictures below shows the mirror box, front and backside. The blue circles indicate the two collimation points. The view of the backside shows the two chrome plated adjusment screws. You can see the aluminium levers through the holes.

 

DSC_2130CN.jpg

 

DSC_2131CN.jpg

 

The big black knob in the center is just a "footrest" for the OTA and has no function for collimation whatsoever. More on the edge supports later on, I have not made them yet.

 


Edited by Roel, 03 December 2020 - 03:50 AM.

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#3 coinboy1

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Posted 03 December 2020 - 10:14 AM

Awesome engineering! Great build!



#4 Praise.M42

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Posted 03 December 2020 - 11:03 AM

Wow, it has everything I'd do for a ultra-compact dob! Nice job!

A question (as I'm a bit short myself). Is it comfortable to view at zenith with a low rider configuration? Is it easy any different to collimate?

#5 Roel

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Posted 03 December 2020 - 12:07 PM

Well, I was a bit sceptical about that too. But after looking through the eyepiece of friend's lowrider, I was convinced. Looking upward a few degrees isn't uncomfortable at all. Compare it to watching the moon at 15 degrees altitude with the naked eye. It feels quite natural.

Collimation is exactly the same as with any Newtonian, exept that some circles will appear as ellipses. (Not through the eyepiece by the way. Stars remain pinpoints, diffraction rings are round, etc. No differences there)

Edited by Roel, 03 December 2020 - 12:17 PM.


#6 Sergey Stern

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Posted 04 December 2020 - 01:30 PM

Roel, what a great design. I am impressed! Do you have more pics of your telescope? Very interesting!

 

Sergey



#7 Pierre Lemay

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Posted 04 December 2020 - 02:53 PM

Roel,

Congratulations. Impressive work, as usual. Your collimation feature is not only innovative and simple it is also well executed. Looking forward to seeing the completed instrument.

 

I just noticed you live in Deventer. I had a meeting in Epe just last year, a few kilometers away! Next time I'm in your part of the world I might look you up if time allows and you are available. I would love to see some of your creations up close.

 

Keep up the good work and keep us posted.


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#8 ad701xx

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Posted 04 December 2020 - 06:23 PM

Very nice! I'm ashamed to admit it took me a while to figure out how adjusting those two point actually changed collimation.



#9 Roel

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Posted 05 December 2020 - 02:53 AM

 

I just noticed you live in Deventer. I had a meeting in Epe just last year, a few kilometers away! Next time I'm in your part of the world I might look you up if time allows and you are available. I would love to see some of your creations up close.

@Pierre Lemay, That would be great, you are of course very welcome!

 

Below is a picture of the removable secondary mirror holder. It simply clicks on a round metal disk with three magnets. Only this disk is collimated (by the classic push-pull system). This way remounting of the secondary requires just some very minor collimation tweaks (in theory just a rotation).

 

DSC_4733.jpg

 

 

 

 

 

 

 

 


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#10 Roel

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Posted 05 December 2020 - 03:01 AM

Although far from completed, the telescope is starting to take shape. The first layer of black paint is applied, and the trusses are covered with heat shrink tubing. Overall the thing is still very rough on the edges, I still need to smooth every part with a router and sanding paper etc. etc. etc. And most importantly, the primary mirror won't be finished until spring!

 

DSC_2121CN.jpg

DSC_2129CN.jpg


Edited by Roel, 05 December 2020 - 03:01 AM.

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#11 Augustus

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Posted 05 December 2020 - 11:18 AM

@Pierre Lemay, That would be great, you are of course very welcome!

 

Below is a picture of the removable secondary mirror holder. It simply clicks on a round metal disk with three magnets. Only this disk is collimated (by the classic push-pull system). This way remounting of the secondary requires just some very minor collimation tweaks (in theory just a rotation).

 

attachicon.gifDSC_4733.jpg

Wonder how big of a secondary you could mount with a system like this? 



#12 Kudeyar

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Posted 05 December 2020 - 01:28 PM

Your scopes are very elegant and inspiring, Roel ))

And this collimation idea is really cool; however, I'm not sure about its safety in a travel scope- any accidental kick on a screw will pass directly into the primary. The 'footrest' knob only offers protection against flat surfaces.

I also wonder how you fix the vanes inside the spider's body. I've never seen any bolts whatsoever.


Edited by Kudeyar, 05 December 2020 - 01:31 PM.


#13 mikymike

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Posted 05 December 2020 - 01:35 PM

So simple and elegant !

Nice work !



#14 Roel

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Posted 05 December 2020 - 02:15 PM


I also wonder how you fix the vanes inside the spider's body. I've never seen any bolts whatsoever.

The spidervanes are simply bent at the tip, covered with a bit of glue, and then hammered into a wooden hub (with pre cut slits) from the top. It's extremely rigid and I could not pull one out using a vice and pliers. I use this method with every Dob now and I can put as much tension on the spider as I want.

 

Here is a picture of the spide hub from one of my other scopes:

 

spider1.jpg

I used to make an thin extra cover above and below the hub to prevent the vanes vertical movement, but this later showed to be unnecessary; the force on the vanes is in one direction only (outwards).


Edited by Roel, 05 December 2020 - 02:29 PM.

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#15 coinboy1

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Posted 05 December 2020 - 02:28 PM

Great job! That is sharp!

#16 Kudeyar

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Posted 06 December 2020 - 10:37 PM

The spidervanes are simply bent at the tip, covered with a bit of glue, and then hammered into a wooden hub (with pre cut slits) from the top. It's extremely rigid and I could not pull one out using a vice and pliers. I use this method with every Dob now and I can put as much tension on the spider as I want.

Here is a picture of the spide hub from one of my other scopes:

spider1.jpg

I used to make an thin extra cover above and below the hub to prevent the vanes vertical movement, but this later showed to be unnecessary; the force on the vanes is in one direction only (outwards).


Nice, thanks!

#17 Roel

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Posted 15 December 2020 - 06:05 AM

Look, I made a video! :)

 

Checking the collimation witch a laser but no primary mirror: I filled the mirror box with 7,5 kg of concrete (the weight of the future mirror) and drew a 3mm circle on piece of paper:

 

https://www.youtube....h?v=7eweZtmMHIo


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#18 Sergey Stern

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Posted 15 December 2020 - 06:28 AM

Roel, what are dimensions and total weight of your telescope?

Sergey

#19 Roel

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Posted 15 December 2020 - 06:33 AM

The packed telescope (without trusses and focuser) is 18,5 x 19 x 5,7 inches. The total weight (everything included) wil be 42 pounds (19kg). This is the (empty) transport package:

 

DSC_2155.jpg


Edited by Roel, 15 December 2020 - 06:34 AM.

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#20 Sergey Stern

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Posted 15 December 2020 - 07:18 AM

Thank you, Roel.

That is a really compact scope!

 

I see you are using the rubber band for balance. So the weight of the bottom cage with this big mirror seem still not be high enough for not using the rubber band. 

 

How long are the trusses? Do you intend to make them divisible for transport?

 

Sergey


Edited by Sergey Stern, 15 December 2020 - 02:05 PM.


#21 Sincos

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Posted 15 December 2020 - 02:01 PM

Multiple lines of evolution culminating in the beautiful instrument you created, well done! More than one ingenious divergent ideas that made me go, why didn’t I think of that. Spider vane hub for instance. Brilliant!



#22 YeloSub

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Posted 15 December 2020 - 05:43 PM

This has to be my favorite design yet! Love how simple and elegant it is. Can you build me one? bow.gif



#23 coinboy1

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Posted 15 December 2020 - 08:39 PM

That is a superb build! I love the minimalist yet strong looking design!

#24 gordtulloch

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Posted 16 December 2020 - 10:46 AM

The first layer of black paint is applied, and the trusses are covered with heat shrink tubing.

Gorgeous scope! I wish I'd seen it before I embarked on my project :)

 

Did you find a cost effective source for shrink wrap? Anything I've found is pretty pricy. I tried plastikoat paint but its far too fragile for trusses!



#25 Benach

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Posted 17 December 2020 - 03:11 AM

Roel: Nice one. Can you put more detailed building plans online?

Less than a month ago I mailed you about your question about glue, secondaries and modeling of the glue dots since you asked some guys on Astroforum about that. You did not respond to that. Did it land in your spam box?


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