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Old 05-11-2020, 08:34 PM
BKruger (Bert)
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My 16” conversion for EAA

I have purchased the 16” Meade Dob (Starfinder?) in 1999 but when we moved to Australia there was no room in the container to bring it with in its original (bulky) format. So I disassembled it and just kept the essentials to be used at a later stage for a future project. I also had a 9.25” Ultima which was de-forked to put on an AVX mount. The Ultima’s mount is equipped with the well-known Byers precision worm gear and has a good slip clutch. I initially planned to remove the Byers gear from the Ultima mount to build a lightweight Dobsonian. However, after a lot of deliberation I decided to follow a completely different route with the conversion.

The project is still a work in progress and as will be evident in many of the photos the job is far from complete. I first wanted to prove to myself that the different approach is workable and I think I have achieved that. I have been through many iterations and with my crude skills and poor quality tools it is going to take some effort to get it in a much better shape. The initial aims that I had in mind with this project was to build:
  • a telescope with goto and tracking capability by using Onstep as control system
  • a telescope that would be easy to setup or pack up and take up minimum storage and transport space
  • a telescope that will be used for EAA purposes only by using a CMOS camera mounted at prime focus position (this means there is no need for a secondary mirror or a secondary cage)
  • a telescope that can easily be moved around (up/down stairs for instance)
  • a telescope without a viewfinder by using plate solving with the goto capability of the mount
  • a telescope with an automated focuser and rotator/de-rotator (so that there is no need to touch the telescope after initial setup and alignment)
The current state of the project is depicted in the first photo. What follows is a basic desciption of the conversion.

The first step was to widen the distance between the existing fork uprights to accommodate the 16” mirror box (500mm wide) plus two bearings (about 17mm wide). This was done by inserting four pieces of 50x50x1.6mm (each 106mm long – two each side) between the mount body (central part) and the fork arms. The original bolts that held the arms against the body were replaced with threaded rods long enough to sandwich the 50x50 extrusions. The top of the arms were shortened as well and with a bit of surgery the mirror box with its attached altitude bearings basically lowered into slots and is fixed to the arms by using only two 6mm thumb nuts. Very easy to remove or assemble. The second photo shows the aluminium used to widen the space between the forks.

The Ultima mount was converted into a goto mount by using the Teensy 3.2 mini PCB V1.2.4 build with TMC2130 drivers for the Alt and Az axes. These boards (10 of them) have already been purchased in 2018 but I just never got the time to start with the project. So in the end I decided I've got the parts and therefore I must just use it (I also like the samll footprint of this board). I have modified the normal Teensy 3.2 build by adding a focuser driver and a rotator/de-rotator driver (more detail of how to do it is available on the ONSTEP website. The 4th photo shows the board built into the Ultima mount.

For the AZ axis a Nema 17, 400 step per rotation stepper motor is used with 32 micro steps, belt reduction of 1.5 plus the Byers worm drive of 359:1 is used (total steps per degree = 19146.7).

For the ALT axis a Nema 17, 200 step per rotation with a 99.05 planetary gearbox and a belt reduction of 338:36 is used (total steps per degree = 16532.8). For the altitude bearings a “better” quality Lazy Susan bearing was used (with spacers between the ball bearings to prevent the ball bearings from locking up). These bearings are 200mm in diameter but you can get much larger ones in the same format. I have used my 3D printer to print 4 sections of a GT2 pulley (to form an outer sleeve) that were screwed onto the outside of the one Lazy Susan bearing. A 9mm GT2 belt is used from the slip clutch at the bottom to drive the Lazy Susan pulley. I am very proud of the 3D printed slip clutch that I have designed for this scope – very small with a lot of oomph – Only a half turn is needed to lock it up as well. The slip clutch attached to the Nema 17 motor and planetary gearbox is shown in the 5th photo

Fixed to the mount are two wheels with a telescopic handle that was salvaged from a large toolbox. The draw handle is fixed to the wheel base with swivel brackets so that the handle can be laid flat on the ground when the scope is in use (a slight tripping hazard but because the scope is not used for visual purposes there is minimal movement around the scope once setup is completed). The handle is fixed to the one side of the mirror box with two 3D printed wing nuts when the telescope is stored or moved around. When the handles are fixed onto the mirror box it is very easy to move the telescope in its collapsed state or erected state (through doorways or even up and down stairs). The fact that there is a slip clutch on both axes makes it easy to position the mirror box so that the handle could quickly be secured onto the mirror box to form rigid unit when it is time to pack up and go. I must still add a third (swivel) wheel to the base so that the scope could be pulled around as well (at this stage I am just using a 2”x3” piece of timber as a leg as a quick and dirty solution).

The current mirror box size is 500mmx500mmx260mm high built with 20mm plywood. It has a fixed lid at the bottom (to give strength to the box). The bottom lid is only 12mm thick with a 110mm hole in the centre that is closed with a PVC cover when the scope is not in use (the hole is visible in the second photo). The hole is opened when the fan is switched on (the fan is not connected or used yet). I will have to rebuild the mirror box because it is not possible to fix or patch the current one (too many holes were drilled in places and later moved as I went along making modifications) and it looks really ugly right now. An 18 point mirror cell with wiffletree support is used to carry the mirror- I may also have to revisit this part of the scope at a later stage (shown in the 3rd photo)

The folding trellis system makes it possible to collapse the scope into the mirror box without having to take it apart (the folded trellis truss is shown in the second last photo). It comprises four sections that folds into each other plus the focuser section and the vanes that holds the rotator/de-rotator onto which the camera is mounted. The trellis system was made with 25.4mmx1.2mm square tubing, 3D printed brackets for the pivot ends and a 10mm x 1.6mm round tubing as cross members between the pivot points (this assist with stiffening up the truss in the vertical direction – the truss is very stiff in the vertical direction when everything is tightened up). Wing nuts (3D printed) are used to tighten up the trellis system. Three 1mm cables are used to stabilize the trellis truss. The steel cables are easily attached or detached at the top end and once they are hook up there is enough support to hold up the camera in the middle and to minimise lateral movement. I think there would definitely be better systems than what I have used here but for now I am sticking to this system otherwise I may easily end up in a never-ending improvement project – trust me this was not the first version. In hindsight I think that a single 40x40x2 mm pole with three support cables would have done the job just as well and would have been much easier and simpler solution but then it would not have been collapsible (maybe a single telescopic pole would do it).

The last part of the trellis upright is the automated focuser (the bottom of the focuser is a 40x40x4 square tubing that was cut into shape. The top part of the focuser is a 25x20x2.5mm channel extrusion that is used as a slide. The focuser is driven by a Nema 11 (200 steps) stepper motor (no belt or gear reduction is used). A 6mm threaded rod attached to the Nema 11 motor drives the focuser block up or down and the focuser 's outer body slides on 10 small bearings. It should be noted that final collimation (to optically align the camera with the mirror) is done on the focuser unit. The focuser unit can easily be detached from the trellis by undoing four wing nuts. The focuser and rotator is shown in the 6th photo.

The final component of the scope is the rotator/de-rotator that sits in the position where you would normally expect the secondary mirror. It is actually smaller than the original secondary mirror of the scope and more or less the same weight (I don’t have an accurate small scale to weight it). The rotator/de-rotator is driven by a small 28BYJ-48 stepper motor with a 90:20 belt reduction. The 90 tooth GT2 pulley and the housing of the rotator was 3D printed. The camera is directly screw onto the M42 tube (45mm outer diameter) that is supported in the housing with 2x 58x45x7mm bearings. More detail about the focuser and rotator/de-rotator can be provided if required.

I am not sure what to call this build? It is definitely an ALTAZ mount but it is not a Newtonian or a Dobsonian anymore. For now I will just call it my 16” EAA telescope? At this stage I am using it with an ASI178/224 camera (the last photo of the Trifid Nebula was taken with the ASI224 - total exposure of 68 Secs stacked and stretched with Sharpcap Pro). Not the right cameras for this scope (I know) but I can still see much better and more detail that what I could have by using it as a visual scope. I am also using these cameras because they are very light at the top end. I am yet to test it with a cheap 0.5 GSO focal reducer in front of the ASI178 – I think that will give me a fairly wide field. The last photo show what I could capture with the ASI224 during a very small window of opportunity on the first night out. Collimation is clearly out and there was drift but the latter is mainly because of poor alignment and levelling that could be improved.
Attached Thumbnails
Click for full-size image (16 Inch - Full scope-small.jpg)
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Click for full-size image (16 Inch - Widening of the mount small.jpg)
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Click for full-size image (16 Inch Mirror Cell Small.jpg)
38.2 KB66 views
Click for full-size image (16 Inch - Teensy Board small.jpg)
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Click for full-size image (16 Inch - Slip Clutch Small.jpg)
31.9 KB64 views
Click for full-size image (16 Inch - Focuser and Derotator small.jpg)
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Click for full-size image (16Inch - Collapsed scope small.jpg)
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Click for full-size image (16 Inch - Trifid Nebula 78 Secs with Displ Stretch small.jpg)
21.3 KB71 views
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  #2  
Old 06-11-2020, 07:55 AM
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GUS.K (Ivan)
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Interesting project Bert, thanks for sharing the details and pics, will be following with interest.
Ivan
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Old 06-11-2020, 11:07 AM
sfarndell (Scott)
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I'll be following too Bert! Nice project.

I've recently built an Onstep controller for my 16", but the STM module as I don't need as many auxiliary outputs (focuser, etc). It makes a big difference to the useability of a scope.
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Old 06-11-2020, 11:29 AM
croweater (Richard)
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Good stuff Bert. Will also be watching. Love these home build projects.
Cheers, Richard
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Old 06-11-2020, 06:56 PM
BKruger (Bert)
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3D Printed Clutch

I know that owners of Dobsonians that want to convert their mounts to a Goto mount often have a need for a slip clutch. As mentioned in my previous post I have designed and 3D printed my own version of a slip clutch with a very small footprint.

The following Sketchup drawing shows the different components of the slip clutch. The pulley sizes can be changed depending on the drive ratios that you want to achieve. Please note that these type of clutches are available if you search for it – I was just not prepared to pay a ridiculous price for it – hence I have made my own.

I designed the GT2 pulleys by using tools available on OPENSCAD. I normally copy the STL file to Sketchup (which I know well), modify it there and export an STL file again for 3D printing. The clutch pulley in the drawing below needs to be screwed onto the Outer Shell of the clutch. You can also strengthen the Outer Shell with 4 small screws or bolts (use the small holes for that purpose). That would prevent the outer shell from shearing when the forces are applied. At this stage I am not using it as such but have been able to drive the scope without any problem (so far). I must however still buy long enough bolts/screws and fit them because this is definitely a weak area.

If you use more Inner/Outer Disks more force will be transferred to the Outer Shell and ultimately the Clutch Pulley (Which means the footprint can be small). I have only used three of each and they are only 2mm thick. It is VERY important for a very tight fit of the Inner Disks over the Nuts and likewise for the Outer Disk’s octagon shape to fit very tightly into the Outer Shell of the clutch. This is to rule out any play or backlash. I had to print a couple of sizes to ensure a tight fit because the dimensions of the 3D printed version is almost never exactly the same as the drawing measurement. Also note that in my application the Clutch Pulley buts up against the inner bearing part of the KP08 pillow block.

The clutch that I have used for this scope is very small (if you compare it to the size of the Nema 17 motor). That is because I had limited space in the area where it had to fit in. You can increase the sizes of the Inner/Outer Disks and the Outer Shell (to increase the area onto which pressure will be exerted). It is also important to slightly sand the flat surfaces of Inner/Outer disks - this will prevent slippage.

The drawing below shows my current configuration. However it is possible to re-configure the layout of the different components - it all depends on what you require in your specific application. If have printed the components by just using ABS with 70% print density. In my case I have also just used an 8mm threaded rod and not a rod with a shaft as shown in the drawing. If you can get an 8mm shaft and tap the thread onto the one end it will probably be the best solution.

Hope the above and the drawing makes sense but let me know.
Attached Thumbnails
Click for full-size image (Slip Clutch drawing.jpg)
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Last edited by BKruger; 09-11-2020 at 01:23 PM. Reason: Clutch spelled incorrectly
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Old 09-11-2020, 10:05 AM
sfarndell (Scott)
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That is a great, elegant design. Are you willing to share / sell your STL files? I'd love to use this on my binodob
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Old 09-11-2020, 01:21 PM
BKruger (Bert)
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Thanks Scott!

I just have to search through all the different STLs (my configuration control with them was not too great) and then I will either upload it here (not sure if we can upload STL's here) or PM it to you. Will probably do it later tonight.

I am using a Davinci Pro - not sure what you are using but hopefully the dimensions would work out right for you.

Cheers
Bert
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Old 09-11-2020, 05:54 PM
BKruger (Bert)
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Scott

Okay got the files. I see that we can't attach STL files on IIS. Send me a PM with your email address then I will forward it to you.

Bert
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Old 16-11-2020, 09:16 PM
sfarndell (Scott)
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PM sent. Thanks Bert
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Old 17-11-2020, 11:34 AM
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bojan
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Quote:
Originally Posted by BKruger View Post
... I see that we can't attach STL files on IIS. ...
You can attach anything in zip archive.
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Old 17-11-2020, 03:41 PM
BKruger (Bert)
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Thanks Bojan - didn't know.

Zip file with the STL files for the clutch attached.
Quote:
Originally Posted by bojan View Post
You can attach anything in zip archive.
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