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Michele Scotti

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Everything posted by Michele Scotti

  1. Following up on Posted September 19 regarding the scope bottom part. Under the azimuth table, there's a sub-assembly I'd call a flattened 'tripod'. CAD pic: Feet are not shown - along with a few other details... What I call tripod is a 3-leg structure that houses a central journal and secures in position the 3 sets of azimuth rollers found at the end of each leg. The central "hub" is in 3 pieces: a lower hexagon, a middle piece to slot the 3 legs in and an upper hexagon. The latter sustains the highest stress and can't be too thick. The middle piece is shown here: The central journal below in the schematics is made of a flange+bearing attached to the azimuth table and another flange with a fixed tube that matches the bearing. We want a bearing to achieve a robust radial constraint during the azimuth rotation so that the rollers are purely taking vertical forces. Also, we need a hollow axis to manage all cables coming from the mirror box and the UTA. After some considerations, the rear axle of a go-kart proved to tick all boxes. Its shaft is precise enough to match a bearing, it's hollow and we could make use of a matching sprocket flange. It's also dirt cheap. Here is the flange, made out of magnesium - more for the fun of machining Mg for the first time...been told it's sparky £18 on eBay. The shaft is a section of a warped one - free. In this picture the material for the 'legs' cut to length: Last but not least a pic of an inspirational design - don't have a clue where it's coming from, saved on my laptop for quite a while... Clear skies, Michele
  2. Once we started to deal with the main elements we were surprised - in a good way- by the dimension. Thanks for following!
  3. Follow-up on the azimuth table from #90 Surface preparation for bond adhesion - the strip needs to be 'normalized' as the CF layer was pretty heavy - 800gr/m^2- and left some pattern that we are concerned can cause a patchwork lack of bonding. So we poured some additional epoxy. Extreme flatness is not a concern/target in this phase. No fancier way to apply some load to the 3 stainless steel arches that make the ca.1200mm diameter - we want to get the best out of this but it's not the final surface. Removing some excess of epoxy - btw the one used is loaded with filler in order to control the flow and the fill under the metal strips. Semi-finished part under some sun - not (just) for a nice pic but rather for the final "high-temp" bake - a compulsory phase for most of the epoxy formulations. Some epoxies reach their peak performance after several days/weeks depending of environmental conditions. Next step: grinding the track
  4. The structure is bonded with epoxy. Welding would introduce distortion unless done very carefully. Also welding tends to require bigger wall thickness whereas all elements here are 1.5mm thick. Some gussets are riveted in critical joint. We used small L-shaped brackets to place the elements in a fairly precise way - the 2 main diagonals (corner to corner) were checked with a physical 'jig' to make sure the whole thing was 'squared'. The finished part needing only the holes for the mirror cell which dictated the position of the crossing elements. During the execution we added some six 45deg reinforcements - they were not part of the original FEA hence providing an additional safety margin to the results used so far to validate the structure. Given the small thickness some big aluminium 'washers' will be adopted to spread the pressure of the fixings on the pac-men.
  5. Material gathered from warehouse for the 'double H' mirror cell bearing structure that is an integrated part of the mirror box - the long and thin one is an inexpensive levelling bar (the one for concrete work) - amazingly straight, the beauty of extrusion All cut in a nearby shop - all 'paired' parts needs to be exact same length For the same reason holes are drilled together. This CAD screenshot is highlighting (maybe???) the structure for which we added some 45deg reinforcements during the execution - which were not part of the original FEA hence providing an additional safety margin to the results used so far to validate the structure. Assembly in place using the azimuth table just after the drum sanding so hopefully the flattest thing we had laying around. The diagonals dimension is checked to have everything squared and small L shaped brackets are used to keep everything in position before the final assembly Cheers, Michele
  6. Limit dripping of epoxy so there's a chance to have a sound bond between the stainless steel strip and the perimeter video.mov
  7. Next step on the pac-men build. Some 2mm stainless steel was laser cut at a nearby shop. Total for two cut and rolled 1800mm strips + 3 arches is 125Euros - I felt it wasn't much of a rip-off... still one of the highest amount of money spent on parts so far. . My line of thought is the following - the joining needs to be stay in place for as long as I can think of and should have a minimal settling along with not creating bumps. hence no way I'm screwing this to the pac-men.Also tensioning by the ends with springs or other mechanism - which look cool- I feel they don't provide the stability we need. We resorted once again to bonding with epoxy. Tricky bit is always exerting some sort of clamping pressure - truck-style ratchet band 50mm wide can do the trick. The overall operation was remarkably straightforward after sanding the surfaces and loading the epoxy with some filler to achieve higher viscosity.
  8. The casing for the 18650 is a great alternative to soldering. I'm in the process to convert an hand drill - this is an fairly informative YT https://www.youtube.com/watch?v=flNBEeG5KmQ&t=5s It adds a buzzer on top of the BMS. I'm planning to use a laptop charger instead of a 12v+step-up (who doesn't have a spare charger now!)
  9. Back from the shop for a final retouch with the belt sanding machine to have the face as parallel as possible Wrap of carbon fiber - no need for vac-bagging, only wet lay-up. More for protection than anything else - could be glass fiber Finding the CNC holes under the carbon skin and inserting rivnuts bonded in place. Holes are for the altitude bearing brackets
  10. Now on the Azimuth table: that's the last big subsystem to introduce is the bottom part of the structure. The original design comprised of 2 platforms - one to support the mirror box and the other -under the first one- to carry the the metal track for the azimuth movement on rollers. This area was identified from the very beginning as crucial to achieve the telescope modal performance - shouldn't be a surprise as it's the foundation of the overall structure. An exacerbated view of the two plates deformation proves that a serious improvement of the rigidity of such system was much needed. The following were intended to be GIFs to show the structural vibration modes: Without going into heavy or expensive solutions the way forward is to increase thickness. So from 40mm to almost 84mm by means of layers of plywood/foam/plywood/foam/plywood. With such an effort on the rotating table we wanted to simplify the manufacture of this system - big boards are a pain to move around!- so the track is moved to the bottom of the azimuth table and the rollers are placed on 3 legs of a sort of a flat tripod. Such tripod doesn't carry much of the load (bending momentum-wise) as long as the rollers and the ground feet are close enough. Here is the azimuth table with some wooden inserts for attaching parts later on and for reinforcement. Ready for height calibration through a drum sander:
  11. Now about the UTA brackets: truss beams connections to the upper cage and the pacmans are done with a variety of brackets. The 3 upper brackets are wacky-shaped L with an angle that ruled out a standard L-profile. They come from a slab of 8mm thick CF -leftover bought from eBay at a cheap price- that gives the nice opportunity to bond the 2 parts together. Again the material used was chiefly a matter of availability and convenience while the FEA is carried out assuming these parts being Aluminium. In hindsight I would bond Aluminum as well to avoid the warps of welding and the need for a milling operation afterward. Also, 8mm thickness is a total overkill - 6mm would be totally OK weigh some weight reduction. Each of the brackets weighs in at 153g which keep the forecast for the overall UTA at less than 15Kg. CAD snapshots: Al the sub-parts cut squared together: Finished parts:
  12. This is the focuser board to house a generous focuser/derotator, connect the upper and lower hexagons and provide attachment for one of the 4 spider vanes. Being part of the upper assembly this component has to be as light as possible. However we could not adopt a foam sandwich as there are too many cut-outs to expose the foam. The board has some depth to properly house the focuser outer diameter and to contribute to the overall assembly stiffness - can't be just a plate of few mm of aluminium. So a sandwich of plywood and paulownia is wrapped in carbon fiber - the latter can be glassfiber for protection against humidity. The plank itself (400x350mm) came out at 2Kg - exceeding the density assumed in the FEA by 20% - uhm..not looking on target! Cut-outs are carried-out on a CNC although it could be easily done with a router and some jigs. We just happened to have a good friend with a CNC - who doens't hav a friend with a CNC if you look hard enough! JK. The weight is now 1.2Kg, still room for improvements.... 166f0ecd-3d5c-481b-bb26-b891b756524c.mp4
  13. Follow-up#2 on pacmen: after lining the side with some veneer to protect the foam we dropped a layer of CF on the outer side whereas the inner side in glass fiber. No vacuum bagging - which takes time - just standard lay-up. There's really no need for anything more complicated than that. I'm pretty sure the wrap is not adding anything to the parts' stiffness but it definitely protects against humidity. We happened to have CF around, but enough only for 2 sides....so we went for the look! Downside? Have to find out the holes drilled with the CNC, laying under the CF - most of them blind
  14. Follow-up on 'pacmans' (or 'pacmen'??): we are lucky enough to have a friend with a CNC router for wood furniture. Popped in some quick coordinates and had the whole thing done. It took quite some time as the thickness was borderline with the maximum allowed by the milling bit so we did several slightly overdimensioned passages (by 1 or 2mm) and a final one with full depth. The holes are drilled on the same clamping position to preserve the overall accuracy and consistency - will be useful at the assembly stage. Next step: wrapping with glass fiber and lining with veneer on the sides to protect exposed foam. Main surfaces seems pretty flat - we might just give them a go in a drum sander after few days of settling. Haven't measured properly but each pacman weight it at around 10Kg - that's pretty much in line with the expectation.
  15. Thanks for the suggestion - you are spot on. The overall thickness is 54mm so it's pretty tricky....
  16. The mirror box is simplified into two main symmetrical parts for ease of manufacture - we started calling them 'pacman'. This design integrates the mirror box itself with the ‘bearing’. The target stiffness is achieved with 3 connecting members to minimize weight and complexity. In the lower part an aluminium H structure supports the mirror cell as well as contributing to the mirror box stiffness. In front, two crossing aluminium bars while in the back a single cross bar, bigger than the front ones as it carries the load from the upper cage through the beams. The material of the 'pacman' is crucial in this kind application. This is where the combination of lightweight, stiffness and low cost is coming together. A sandwich of plywood and foam provides remarkable stiffness to weight ratio - it is how cardboard boxes get their strength from, isn’t it? A Finnish amateur astronomer had a very nice implementation - https://uuki.kapsi.fi/cf16in.html - balsa&carbon/foam/honeycomb, pretty fascinating. The foam is an inexpensive 30mm sheet of polyurethane whereas the plywood is simply upgraded to marine grade to be less sensitive to long term deformation. All the material is available from a good warehouse - that's essential for the spirit of the project. The whole sandwich is then wrapped in fiberglass – more as a mean of protection to the environment than anything else. On the outer rim, a stainless steel strip will be glued to the pacman perimeter so that there are no bumps due to screws. After that the rails will be ground in place with a special homemade jig – hold on and I’ll shortly post something about that. Finally, the assembly is crucial to have everything squared and will touch based at a later stage – needs to be embedded in the design, not an afterthought. Pictures of the sandwich being made – foam glue to the first plywood, cut-outs for wood inserts, inserts in place. After that drum sanding machine to get everything flat for gluing the last plywood layer.
  17. Well, it doesn't really apply to our design, does it? I guess I put 50 just to allow some room. Or is it a parameter more important than I'm thinking?
  18. It's now time to ""explain"" the intended optical system: for simplicity and familiarity with its optics we choose a reflecting Newtonian configuration. Given the generous primary diameter it has to be fast to contain the overall scope dimensions due to focal length. Although the original project was born as f/3.75 to target 3000mm focal length we are changing our mind due course - CAD model update in progress. f/3.3 gives out a focal length of 2640mm with few benefits: - bigger field of view - lower sensitivity to tracking errors - the overall telescope height will be shorter with effect to weight, ease of transport, height of the eyepiece and higher modal frequency i.e. stiffer mount - last but not least at 2640mm the focal length is matching the FL of our ready available 500mm mirror allowing testing the mount capability before the 800mm mirror is coming to fruition Of course the mirror will be a bit more difficult to parabolize and the secondary mirror will be a bit bigger (180mm vs 170mm give or take). Depending on the usage and the imaging sensor size we might need to adopt a Wynne corrector - a pretty beefy one could set us off £1000 though - we'll evaluate that at a later stage. I'd appreciate any comment on the optical layout - specifically on the back focus. Please consider that we'll have a filter wheel and OAG. How would it change with a Wynne corrector? I guess what I'm looking for is the on-axis PM to secondary distance and the secondary to focal plane. Also I never quite developed the full travel of the focuser - when it comes to DSLR or CCDs I can figure that out but with the most common eyepieces I don't know exactly what would be the range. I suppose I need to consider 2" eyepieces? I've attached a couple of NewtWeb screenshots for reference.
  19. I think that over the years I've stumbled across all of these scopes. Always a source or inspiration
  20. Very good recurrent question. Not easy to answer properly. From a group pov we managed to build many telescopes up to 500mm - with 2 optics being sold. Of course you can do deep-sky with a small diameter and you may find that the major hurdle is tracking. I'm sure some guys here on CN can list out -better than I can do- the potential of a 800mm optics used as an imager. Somebody said that with 800mm you can see the colour of some DSO in visual. And that's what motivates some guys in our group. I like the challenge of project itself. I'm not an imager per se.
  21. Follow up on the upper cage element.Light-weighting operation at a friend's workshop....
  22. No worries - I like to double check my work. PLOP carried out some time ago -27 is the winner And done some FEA on the triangles:
  23. Following up on the upper cage hexagonal elements. It needs some detailing now....
  24. The attached pic of CAD model is the result of few months of work and the best of our engineering knowledge. We are currently working on a major update revision. The description below is about the new design so sorry for some mismatch. I reckon the structure is pretty typical for a Dob in this class. A simplified/integrated rocker box swivels on two ground bars - that's a smart design I've seen on few motorized Dobs. Those bars are supported with heavy-duty bearing on a rotating table that provides the azimuth axis. Under the Az table three 'legs' are housing the roller bearings to allow the azimuth rotation - this is like a big Lazy Susan bearing. I suppose one could look at the structure as two big 1200mm bearings. The upper cage is again a structure as simple as possible with 2 identical hexagonal elements connected through 3 pillars and a plate that serves as mounting support for the focuser/derotator. Pillars and plate provide the attachment for the spider that holds the housing of the secondary mirror assembly. To connect the upper cage and the rocker box we adopt a truss arrangement. 6 vs 8 beams configurations were compared eventually in favor of 6. The idea is to optimize the number mainly due to the cost of 1.5m poles. We use FEA to validate the structure. The performance of a telescope hinges on its ability to perform tracking to the specified purpose. That's where the structure itself plays a crucial role. Ideally, what you are looking for is stiffness, and that's to be insensitive to external perturbations and to be positively reactive to inputs such as tracking, PEC and autoguiding. In other words, a gust of wind shouldn't upset the tracking and if guiding corrections are imparted the reaction shouldn't over or undershoot. If you want to have something stiff but lightweight then your main engineering parameter is called structural natural frequency - or heigenmodes. In practical terms, if you tap on a mount you want the oscillation to be small and dampen out as quickly as possible. And the only way to predict it properly is Finite Element Analysis. We didn't want to spend any significant amount of money before knowing that the mount is fit for purpose. The gifs are showing exaggerated oscillations of structural frequency - the higher the frequency the stiffer the mount. The target is a bit tricky to set as there not much literature out there. There are definitely papers about big meters class telescopes but not for our purpose although the spirit is the same for any telescope. For our project I set the target to 25Hz. The first iteration showed 19Hz and 20Hz for the first 2 modes. The third mode -related to the mirror cell support- is over 30Hz so it's ok.
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