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Michele Scotti

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About Michele Scotti

  • Rank
    Star Forming

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  • Interests
    Telescope making - Instagram #800mm_telescope
  • Location
    Woking, Surrey
  1. Kalimera! We considered this in the early stage of the project and that's one of the reason why the spider has a big cilindrical section to let the cone of light go thru it. However we decided to drop that - an additional add-on section and the cabling can be sorted out - no big deal. The showstopper is the focusing and de-rotator mechanism - they are part of the project in the standard configuration but in that position they'll add significant complexity. As a final note - we would really want to experience the magic of 'guessing' the colour of some DSOs - feature that only this kind of diamater can untap.
  2. New CAD model update, now version #13 with few improvements - mainly: - model aligned with actual built parts - new lower part sub-assembly Next step is another run of FEA for modal analysis and displacement. After that we are going to modify the structure from the original f/3.75 to the currently assumed f/3.3. Consequently the focal will be reduced from 3000mm to 2640mm.
  3. I always fancied giving it a go - I might actually need to do that for the derotator wheel. Ideally I think that lathe+tailstock -as you confirm- it's the must sturting point. I reckon that using the gear already on its on bearing i.e. in its final confirguration, will improve the result in terms of run-out. On top of that, running on the same set-up I'd do the lapp[ing after swapping the tap with the threaded shaft
  4. Next time make sure you shot a video - worm gear hobbing is fascianting. Would you reckon you had a similar set-up to any of these? https://www.youtube.com/watch?v=19jKlq8Ofd4&list=PLJtiP4DlrmJROzFYnk0rYQ9fnJPO9KyCn&index=16&t=0s https://www.youtube.com/watch?v=eKJe1JnvRc0 Hope not to detour the thread here....
  5. Rather than a video let's see some maths behind this - it applies to any modern smartphone with Mpixels. At the minimum focus distance - usually 50mm or something- this is a clear picture of a caliper. A crop-up shows that 5mm takes 77pixels. This equals to 65um/pxl By the way in a video you wold appreciate sub pixel movement for sure. Out of curiosity this is what happens using the zoom (digital in this case) Reoslution here is 14um/pxl Not maybe the most accurate tool but it detects tiny movements very efficiently - it might be handy when checking mounts for slack
  6. https://simplybearings.co.uk/shop/p88001/Budget-7000-Single-Row-Angular-Contact-Open-Ball-Bearing-10x26x8mm/product_info.html?backstep=1 2tons of axial load - i guess its enough
  7. I've check most of jig elements -and specifically the upper and lower bearings- is in a pretty funny and maybe novel way. I pointed the smatphone at the area where movements would manifest and shoot a video while applying load. As funny as it sounds you can detect way finer movment than naked eye. I was trying to put some maths behind it...maybe later.- you can see sub 0.1mm for sure. I have few videos where the check is OK but can't find the one that was clearly showing a minimal movement in a different area of the jig that was later fixed. Bottomline - to my surprise- bearings are ok. Anyway if they moved the laser pointer wouldn't move as the pipe would not bend but rather tilt.
  8. The quest for a stiff and reliable platform I've used some counterweights at the end of the board to evaluate the stiffness of the system. Not a good starting point... 0.9mm with something like 6Kg placed some 30cm outboard, just to exacerbate the flexure. I've then tighten all screws and added M6 bolts with inserts to cement the position of the wooden blocks. Better but still 0.6/0.7mm. Where's the flexing coming from?? Maybe from the element assumed to be the most rigid? Let's check the pipe... A cheap laser was clamped on the top of the pipe -see pic- pointing down. Some paper tape on the bottom to act as a 'screen' for the phone to record just under it. And there you go – the pipe flexes big time! Movement -subtle-is caused by pushing on the board and releaseing - laser dot moves 1 or 2mm. Smoking gun! So the pipe that fits perfectly in the bearings and we made all the wooden blocks around is not up to the task. Shall we throw it away? Nah! The pipe needs stiffening on the vertical plane where it meets the angle grinder – so next step is to insert a 5mm flat bar that is as wide as the ID of the pipe. Possibly a few welding points at the ends. For a structural application this would be gruesome borderline criminal – in this case it really needs to work under small stress.
  9. I’d suggest a different approach: fixed bearing housing after lapping. The way I see it, is that you have a run-out from the shaft to tollok, tollok to sleeve, sleeve to worm gear I and finally ID to the generated teeth. Actually it’s not all – your shaft has a nice press-fit brass thread. All of the mentioed interfaces might be very precise but they all come with few hundreths mm of run-out or clearance. Lapping will take care of all of them. I have to be honest – never did it myself just because I’ve recently stayed away from worm gears in my telescopes. Also I reckon it is only mandatory for Dec axis. It’s maybe a lengthy process (and messy?) but not that complicated. Let me know what you think about. I suppose the shaft material is not treated/hardened. You could tap an M5 and add an extension – belt&pulleys are pretty forgiving to run-out errors. I would never suggest that for a gear coupling. What is the ID/OD of the bearing and the width of housing? 12x30x8?
  10. my 2 cents - from previous picture the housing and bearing general dimension doens't look bad - I'd suggest to switch to tapered bearings with a (simple) system to pre-load one of the bearing. That will kill any lateral movement. Is the BH fixed or does it have a system to cope witht he inevitalbe run-out of the wormgear?
  11. I trust you realized that all the examples you brought are 'dynamic' at a certain extent - even your swivel chair. It totally makes sense to have more 'legs' to achieve a more stable set-up simply by increasing the area on which the CoG can fall on. One of the perks of a AltAz mount is that all loads are pretty well centered. The CoG of the 'OTA' rotates around on a 124mm circle - well within the 'triangle' set by the 3 rollers. I do not foresee any stability issue. To me one of the design aspect that I tried to optimized is related to the load transfer from the Alt bars thru the Az table then Az track to the rollers on the tripod and from there to the feet on the ground - which incidentally I haven't designed yet so they are not shown in any CAD so far. As a matter of fact the FEA we ran was on the worst case where one of the Alt bearing is placed half-way over 2 bearings
  12. To start by stating the obvious: the tripod has 3 legs and at the end of each leg there's a roller. So this set of 3 points defines a plane - adding any additional supporting point will introduce an unwanted constrain. Chairs have 4 legs and they are 'stable' because they deform under load allowing a distribution of weight on 4 points. So 3 is actually the only choice for such structure. Class-meter teelscopes uses a different approach - they usually have 2 very accurate flat rings that are part of a hydrostatic bearing where pressurized oil keeps the 2 rings apart. Hope this answers your question?
  13. I might actually resort to sand blocks, eventually! At least they'll provide smooth transitions although not going to do much for the overall run-out. "Using maths to analyze "a length of string" is a common fallacy." what do you mean with this specifically?
  14. I try to bundle up answers - in first place there's no such thing as a silly question. Only asnwers can be silly. Also, I totally share with both of you the fact that the likelyhood that all of this is going to be a big mess is very high - I'm not shooting for first time quality. I'll learn by mistakes along the way. There's a specific reason though why I'm going down this routet. And it's about the spirit of this project - this is a pilot project and an (ambitious) blueprint for a large, attainable imager that should be potentially built around the world without access to expensive/porfessional/dedicated machinery. A go-kart shaft and a couple of bearings are fairly common and inexpensive. Surely we might fail - then I'll reconsider what to do. Anyway - I took 2 measurements every 15deg - orange and grey lines. I applied a sinusoidal correction to simulate a tilted shaft being straighten-up - in blue. The result after such correction is the amber line.....which is more than I expected! We are looking at a ca. 1.3mm P-V or run-out. Quite some material to remove.... Worst scenario if (likely) the steel left is too thin I'll bond another set of arches like the first time - it probably costs less than 100Euros
  15. A lot of good points... Ref is the shaft and its bearing - the one that will stay on the Az table. I'd like to target a few tenths of microns of total vertical run-out with very smooth transitions - somehow the latter is more importand than the first. The pipe is a go-kart rear axle with 2mm wall thickness - its rigidity will be scrutinized due course. The idea is that it has to allow the generation of an ideal plane which is the new track surface. f it is slightly tilted - to a reasonable extent- compared to the table itself is not a big deal. What counts is that the track and the 2 Altitude ground bars are 'parallel' - this can be quite easilty checked and rectified with shimming under the Alitude bearing brackets. The Az table is fairly rigid - at least by design anf for the purpose- as it's the most critical element for the overall mount performance. The picture here is for the HUGE investment in a micrometer to test the accuracy of the surface and preliminary the stiffness of the jig.
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