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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. LTA assembly dry-run. After trimming some beams we pulled together the entire sub-assembly. That was mainly to check the robustness of all joints but also for the sake of seeing how it looks like. The structure feels sound and light enough to be carried by a single individual if you stay inside the structure - otherwise it's very bulky. Overall it's should be around 28kg/60lbs. Next up is grinding the Altitude bearing surfaces. Despite the SS strips are bonded to a CNC machined surface and there are no screws or discontinuities, they do not provide at all the accuracy required by this project. During grinding I'll take some before/after run-outs measurements. What's following from that is a specific assembly procedure This is critical to ensure that the two rolling surfaces sit on an ideal cylinder. At this point we have all the 4 main assemblies that took shape: UTA, "tripod", Azimith table and now the Lower "Tube" Assembly. Still a lot to do though. Cheers
  2. Had the chance to do a couple of solid sessions on the hardware. It clearly was about time.... Anyway, focusing on the Lower Tube Assembly I've finished installing all the stud-screws. They are all pre-drilled and epoxied in place. This is a crucial element, just like any joint, the connections work with friction. And friction is proportional to the amount of load that you can torque the nut with. That is quite a big pull the stud-screws have to sustain. I've used M8 and M10 To assemble the H- frame I wanted to have all holes aligned on both sides. They whee pre-drilled on the CNC but in the meantime I had to change the position of one of those. Anyway - the 2 pac-men are aligned on the circumference and bolted together. Only then I drilled the hole - 3 per side., through holes. Here is how the Aluminium H-frame is going to be secured to the internal side on the pacman - which is covered in fiberglass whereas the outer one is CF covered. Layed up on the pacman you can see the M8 bolts - I actually going to use shanked ones, a big washer and another plate made of 3mm Al to spread the contact pressure even more and they fit snug inside the H-frame side elements. So the screw heads, washer/plate is inside and in close contact with the inner wall. This is a design decision to maximise the joint strength. Last but not least, with the help of a specific cutting wheel for Al - it really does a good job!- I'm preparing all the beams and crossing elements to complete the LTA.
  3. Hi Nigel, good question – it is probably the most popular one. The few telescopes I made in the past indeed hinged around the optics so the mirror was the first one to focus on and to complete. Then the rest of the telescope. I agree that this is the most difficult part to sort out. With a caveat – the size. And the purpose. This project aims at a robotized imager eventually been remotely controlled in a small observatory. Let’s start with a consideration – if you pool the amateur telescope in the 700mm+ category there are very few scopes being used as imagers – actually, none as far as I know at a purely amateur level. And for good reasons – the engineering of the mounts gets more complicated. And focal lengths are getting bigger, making tracking more challenging thus demanding more sophisticated mounts. That’s why we started with the mount. If the mirror is not up to the task it will be disappointing. But it will be even more if the mount is not capable to be an imager – it would fail the entire project, leaving us with a big light bucket. Still enjoyable but not serving the ultimate purpose of this project And if the quality of the first mirror is not adequate we’ll keep trying as we did in the past. Lastly, we have a 500mm mirror that can be adapted to mount as it has a similar FL – not the perfect situation but it will serve to debug the mount while the mirror progresses. Not trying to convince anybody this is the "right" approach however I agree that we are adopting the same principle here which is: start witht the most critical part of the project.
  4. Quite some time from the last post on this. Hoever not being able to physically progress the telescope we used some time to update the cad model and run some FEA – this is actually the second run after modifying a number of ‘details’. Modal analysis, deformation and strain energy were carried out. Here is a check on the deformation of the telescope at 45deg -just to get the feeling at an arbitrary altitude. A mass of 2.5Kg is applied on the focuser axis 200mm outboard to simulate camera/filter-wheel/etc… Specifically, I was interested in re-evaluating the deflection of the light beams compared to the focuser center-line. To properly evaluate the light beam I’m evaluating where the primary is now pointing and where the secondary is aiming at. The secondary concerned me as it’s an overhanging mass held in place by tenso-cross. And it wasn't extensivly analysed in the previous FEA run. Not really a big idea how would it react at 45deg. Would it twist? Twist and translate?? Twist, rotate and translate??? Here is the primary mirror: Predictably, it rotates. >>>> Disclaimer here is that I didn’t include any whiffle-tree or lateral constraint – more on that later on <<<< So with a bit of math, the rotation is 0.017deg. Now where the primary is pointing now? At the distance of the secondary mirror, it means that the beam is now landing 0.55mm lower with negligible skew on (my) Z-axis. How about the secondary? Looking at 3 axis it translates by 0.29mm on the altitude plane and again negligibly on the Z-axis. Overall: the primary tilts by 0.017deg, the secondary translates with the UTA by 0.29mm and doesn’t (appreciably) sag relative to the UTA (that’s good and slightly surprising). So effectively half of the deviation of the beam due to the primary rotation is taken by the secondary that sags with the UTA. This leaves an off-center light path of ca. 0.26mm but (more importantly) paralell to the focuser axis. To me it's OK although -important remark - all of this is what I can do now waiting for the build to complete and correlate this analysis.
  5. C'mon, just a lil sketch I suppose you want to do astrophotography - if so the Dec has to be pretty accurate too. It won't carry as much load as the RA but it's crucial for corrections either guiding manually or with an autoguider. Try and fit the larger worm gear you can - thats the secret ...my 2 cents. I saw you techinque for gears and it's quite inspirational.
  6. Hi Tomasz, great project and I'm looking forward to following the progress. Do you have a skectch of the entire mount to share? Are you intending to use the HD on the AR axis and input that with a smaller worm gear? I suppose you want to do astro-photography with the mount? I'm sure you are aware that HD are virtually backlash-free but that comes at the cost of accuracy - almost inherently in their design. Having say that on the filed it might perform great
  7. Thanks John, very appreciated. This is line with the feedback from the Bath group - difficult but doable https://groups.io/g/Interferometry/topic/bath_for_a_800mm_f_3_3/74609578?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,74609578 We'll start with Ronchi and do the optimization wwith the BAth if we can make it work. I always had thought that Bath gives that topographic feedback that makes addressing errors effectively.
  8. What I'd look fo ris a support that mimick a flat turning table of a M-o-M. In that case you have a decently flat surface (sometimes is made of wood) and a layer of "softer" material to compensate for small irregulatiries. With the difference and complication that here I'd have a curved back. I woulnd't rotate if I can make it match properly or alternatively I'd properly rough-ground the back - again, isn't this what it is done for the back of a standard flat-black glass? The accuracu targegt is a tricky question. Lambda/4 at least - anything les than that and it would be more of a light buck than anything
  9. John, I'd agree that the volume/surface ratio on big thin menisci provides an opportunity to stick to float glass - I just wonder about using borosilicate. It costs more - for a 800mm, 25mm it'si around £800. vs float that I suppose would be £250(?). Borosilicate should behave better during figuring as well making it a bit easier (??). Possibly the slumping temeprature is higher - 800degC vs 650degC (???). You see - a lot of question marks.... On the support I had a lot of thought s for quite long time - my idea would be to provide the same support as if it would be on a flat-back. What I'm trying to say is that I'm thinking about a solid support that is shaped as the back of the meniscus -like carved-out wood- and a layer of compressible material - the same that you would put under the mirror of a M-o-M . Again I'd replicate the 120deg holder on the side. Stacking some (several) boards like well seasoneds pallets would be quite inexpensive. The a curved jig/rail to guide a router to carve-out the 'pit'. I woun't be quick but I feel it's inevitable. IF the meniscus back face is irregular - whcih is usually the case- I suppose one can use the meniscus itself to mould a layer of any material (plaster/sawdust+glue/etc) to have a cozy rest. I reckon I'd need to sketch it out to make sense of it. Oh btw, I've read that Mel Bartels uses towels as a support.... "I should probably go back and have another go at it" -----> Oh yeah!
  10. Fire bricks sound like an excellent idea - did it last all runs in the kiln? We used some dental plaster - worked fine but was expensive although I know there are suitable alternatives now. So were you able to complete the mirror and test it? Did you use float glass or borosilicate? The last experiment it is indeed what Fullum and Hubble Optics are doing - very interestinf - you weren't happy with the quality? Again thanks for sharing experience and pics - it's really appreciated. Annd thanks for your patience in getting back to all my questions...
  11. Hi John, thanks for taking the time to write - by no means I take this as negative. It's rather a realistic approach. As we speak I'm going through the resources you shared - same new, some already known. To start with I agree on the testing - I'm not the 'optician' of the group but I pushed some glass in the past and at least I appreciate where your concern is coming from. May I ask you why you reckon Bath is not suitable for a big+fast mirror? I recall some discussions in the Bath Interpehromertry group but I not sure it was conclusive. About slumping: so far our group has slumped up to 400mm, thin mirrors - it was quite some time ago and it was dotted with quite some shattered glass.... We are investigating where to go 'nearby' to get an oven big enough - and available for such things. I'd really like to avoid building one from scratch though. I agree on menisci (aww, that looks and sounds terrible!) haven't proven yet to be a fully viable way - it's nonetheless pretty attractive because for instance in our case the alternative is a more standard but hefty 4k blank. Btw, ...Thompson....isn't he the guy that built some 800mm in Liverpool years ago? That group had a radical approach to using the Bath as far as I remember. Lastly, thanking you again for pitching in, have you wrote anything about that slumped 20"/25mm? That would be a golden experience. Regards, Michele
  12. I had a quotation from Stathis some couple of months ago - it seems that he isn't able to provide an 800mm borofloat disk more than 25mm thick, Yeah, Stathis is top bloke
  13. Thanks for openly sharing! You separate frit pump system seems very clever. I'm very familiar with Gordon's videos - among my favorites! I like the concept of fixed post as it simplifies the constrcution and/or make it more robust. However I do not get what is the disadvantage of that
  14. Peter, thanks for sharing the details of your optics - it sounds like a monster of a telescope! Are you able to do some tracking or is it just visual? 35mm thickness is really the minium I would dare to go - ending up with 20ish mm in the center. I agree - if we have to slump then Borosilicate would be better and at a marginally higher cost. Btw I've just enquired today Galvoptics in Basildon - they have borosilicate 25mm...they need to check the inventory for thicker material.. On testing I have all the bits to pull together a Bath - however I reckon our mirror maker memeber is going to prefer his Ronchi test.
  15. We like challenges, don't we? The mirror making machine is something I'm curious about - how does yours look like? Can you recall a thread were various kind of machines are discussed? That topic is something I just know I need to dive in sooner rather than later..
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