Michele Scotti
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Everything posted by Michele Scotti
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That kind of execution is always inspirational - the trick is to trickle down to DIY level of course with some limitation. DD motors are awesome but im not aware of any control unit available at amateur level. I reckon the motor itself is suitable for DIY. Wrt encoders I still need to figure out if anything is available at reasonable price
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There are some DRO systems out there that might be interesting with sub-micron resolution magnetic or optical strips that do not look outrageously expensive. It would need some DIY to integrate of course.
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This is just awesome!
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Well I see that you are making a good point for the absolute position - I still haven't figure out the implementation of this aspect completely though.
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great feedback - I've always been very worries about placing the absolute encoder anywhere else than solidly on the main axis. Out of curiosity how much a Gurley set you off? Is the 10k or 500k cpr?
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Addendum: last but not least a quick check on the herztian contact stress - just to make sure that the rail and roller will keep their integrity over time and won't have trail and indentations on the surface. Rollers are fairly small in diameter (22mm) just like the ground bar (20mm) to achieve an high ratio for the no-backlash last stage of the drivetrain - and the smaller the diameter of rollers the higher the stress. I felt important to check this tricky aspect as it can compromise the accuracy over-time. The altitude one sees an easier life as the track is at least 50mm wide and there are 4 contact points. The load on azimuth is spread on 3 set of rollers instead so the stresses are higher.
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Thanks for estimating the slewing speed - hadn't done it yet! 👍 Haven't said that 😉 The challenge is to implement something affordable which I haven't figured out yet. Clearly the encoder on the motor is good but what you need is really the one on the axis - Si-Tech can handle both, I suppose even together
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Let me quote an example from the Si-Tech manual which is somehow similar to my set-up: " With a Pittman servo that is labeled 10:1 gearbox, and 500 count encoder, and I attached it to the 360 tooth worm gear on my mount via a 1:2 cog belt drive. So what do I enter for the RA motor ticks/rev? 500 (encoder ticks) x 4 (quadrature) x 10 (gearbox ratio) x 2 (cog belt reduction) x 360 (worm gear drive) = 500 x 4 x 10 x 2 x 360 = 14,400,000 ticks per rev That‟s a lot, but it‟s actually about what you want to shoot for. You want to shoot for at least 10 ticks/arcsecond. This example has about 11.1 ticks/arcsecond. More ticks/arcsec is good, but you trade off slew speeds with a steeper gear ratio." Hope this helps?
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BBAstrodesign motors are brushed DC from Pittman GM8224D309-R1 rated at 19.1V. Here is it disassembled to take out the 2 gear pairs - pls don't tell it to Mel! I'm here to share, discuss and learn too
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Drive and tracking - I'd start stating that backlash is the foe. We are adopting a brushed motor system from BBAstrodesign. The overall recommended reduction sits between 3000:1 and 10000:1. It's a lot of reduction....But this is where the the dobson configuration nicely pairs with the friction drive concept. The set-up couples a stainless steel rail on a 1200mm diameter rocker with a 20mm h6 ground bar which offers a 60:1 no-backlash as a last stage. A compact low-backlash worm gear reducer for another 60:1 coupled with a 2.5:1 belt and pulley stage completes the drivetrain. Total ratio (60x60x2.5) is 9000:1. Please note that the motors come with a built in 10:1 gearbox that has few gear pairs - too much of backlash imho- conversely the belt will have a tensioner so that the main source of backlash is in the 60:1 worm-gear. Ideally I'd adopt an harmonic drive but it's pretty pricey and those I've found, come with a specced max 30' backlash which translate to 0.5arcsec on the scope axis - hopefully well manageable. I don't see adopting clutches. That's a limitation but if the slewing is 3deg/s then we'll use a bit of patience. Pic#1 of the mirror box with two main elements "pacman" - the rails is not present in this model version Pic#2 The actual rails - 2mm stainless steel. 2 pre-rolled arches for the Altitude and 3 sections for the Azimuth bearing. A full circle would have set us off a fortune - and there's a trick for a smooth section transition.
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The sketch is very explanatory - I have to say that it sounds very tricky if yo want to achieve decent level or run-out and concentricity. Working to precision big diameters on a lathe means the wall thickness can't be that small. Also Aluminium cannot be ground to achieve a good geometrical result. Btw what you sketched is a 3 ball bearing in the end. I suppose clocks are precise angle-wise but don't care much about the motion 'roundness' precision.
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You got exaclty the point. If you evaluate how good a mount is by it's Period Error you'll notice that it's all about making it smooth. If you don't have autoguiding then such error has to be taken down as much as possible i.e. toward perfect machining. With autoguiding what you need is a smooth drive i.e. without the high frequency errors that in the end are hard for the autoguiding system to correct.
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Hi Gina, let's put it this way - big telescopes (1m to 10m) adopt alt-az. whereas small (amateur) scopes hystorically adopted GEMs. It boils down to the engineering and mechanical aspects. For example a 50mm bearing is set you off £10. A 150mm bearing would be way more expensive.
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Exactly - we bought the system from Mel Bartels quite some time ago also to run some preliminary tests. It should handle de-rotation indeed. Thanks for the manual - I reckon I had a previous, less extensive, release
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Having set targets and goals what follows is drafting the type of mount. I suppose this part will be pretty arbitrary... Few types were casually taken into consideration with their pros and cons. Equatorial as GEM is ruled out as it would have to be huge and heavy - mainly driven by the overhang, big bearing, big shafts likely in steel to contain costs, last but not least the drive would require large worm gears. Too heavy and expensive. So we shift our thoughts to Alt-az. Weight load is inherently more balanced leaving room to more humble materials. Only drawback that I can really see? Field rotation. Will come back to the de-rotator at a later post. Drafting the scope we ended up in something I'd call a glorified Dobsonian. A Dob in its essence with few upgrades to turn it into a mount that can track for imaging. Big appeal is represented by the friction drive which perfectly fit this configuration. In hindsight, the horseshoe set-up earns a special mention - it's very palatable as is equatorial and it would adopt very similar material/manufacturing processes as the glorified Dob. How to turn a Dobson-like mount into a proper imager? What makes a mount an accurate tracking system? I reckon this boils down to 2 main elements - Low backlash and sound structure. How to achieve that? friction drive, stiff yet light structure, autoguiding capability. Quick math check to start with. A perfect structure with 1micron (0.00004") error on the friction rollers equals a drift on 0.23arcseconds on the focal plane. Now, if you consider a sensor with a generic 10micron pixel, every pixel equals to 0.79arcseconds (at least for this mount) Reality check: a commercially available ground bar or shaft has a run-out of 13 microns (ISO h6) - just to consider a relevant geometric error. If you are following me in this generic and very simplified calculation, long focal length optical systems cannot perform accurate tracking based solely on geometrical accuracy. It's just not robust enough. Hence we need autoguiding...how does a mount look like to achieve an effective autoguiding? Again low backlash via adopting friction drive paired to a stiff and light structure that promptly reacts to tracking adjustments. As you can see it's a whole package of traits that needs to be developed and implemented holistically. If one aspect is missed or poorly executed the entire system is not capable. Pics of parts I had a head start with: 2 driven azimuth roller assemblies . The third one will be the driving one. The wood preparation for the upper cage.
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I was in contact some time ago with the developers of Direct Drive - excellent system as it has zero backlash. Developing a specific de-rotator is part of the project - we considered a full frame imaging system. OAG and on-axis encoder are part of the project too - the challenge is to develop them at a "reasonable" cost. Reading my words it sounds like: Dear Santa, I want everything!
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You get exactly the point - the challenge of making, the budget challenge and the challenge to get something that has not been done so far - as far as I know. If I fail, well, it's going to be painful but we tried.
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I'm focusing on developing a system capable of imaging on deep-sky objects i.e. able to robustly track for several minutes with sub-diffraction pixel sensing system. Vlaiv, may I ask you what would you be taking imaging of if you had such a system?
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That's a great project Peter!
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At this point, I reckon I feel the need to clarify the vision. This project is aimed at science not just visual - all inspirational big apertures out there are mostly visual. The French T1000 is actually achieving some sub-minute integration as far as I know: http://www.astrosurf.com/altaz/T1000.html It needs to fill the gap between amateur and pro. Up to 500-600mm there are choices commercially available - then you jump almost directly into the meters-class from professional observatories. Success looks like other people will build a better version of this scope. Hence this is an open-source exercise where this specific project is just the execution of a sort of pilot project. Its realization will be just a reassurance of its capabilities. Affordable - being open source is not enough. The project is intended to utilize modern material in a smart way with a limitation to the use of fancy materials - rather we will develop new ad-hoc processes to extract the most from common materials. Being transportable is a plus but in all fairness not a must. As long as it can be put in a small van that's good enough before it finds its final home. Again very ambitious but that's what motivates us. If we fail well we won't regret to have tried at least! It'll be painful though! Attached is a strain energy analysis:
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Well we dont have that physically with us although we pre-checked the availability. In the design phase we are accounting for a 50mm thick mirror
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First of all thanks everybody for your warm welcome! Lots of hints and suggestions. I actually thought to be roasted! Anyway let me give you a bit more of background on this project. Over the years Charly -on the left- had achieved a mastery in working mirrors such as three 500mm objectives. One of these mirrors is dedicated to his personal telescope. The dream in the drawer was always to venture on higher diameters: 700mm, 800mm even 1 meter. Perhaps the very structure of the telescope beyond is the burden. In fact, his 500mm equatorial mount had required various improvements over the years and the feeling is that there's a lot of unexplored potential in that mirror. I hear many of you suggesting to sort the mirror out in first place. And that's a valid point. However the accent on this project is more on the mount that needs to be capable of tracking for several minutes. Don't get me wrong - a 800mm mirror is a huge challenge and we are not underestimating that. Sometimes passion leads to focus on one aspect and to overlook others. Making a big mirror inevitably unbalances the whole telescope project. As a result mounts suffer. To me, the bigger the mirror the more important the mount. From the beginning it was clear that the project was twofold. If on the one hand the mirror is in itself an exciting challenge -demanding and scary- it became clear that the telescope structure had to be in the same league. Cheers, Michele
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Good point Peter - although a basic Newtonian set-up is a stretch for such diameter and ration I reckon it's the only way for ATMs. We have some experience with parabolic mirrors - an;t got no clue about all other convex secondary mirrors. I'd prefer to spend some money on a Wynne and go through a careful design of the mount
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Holy cow - that's a monster telescope right there! What focal ratio are you planning? Keep it up - I'm sure there are some other places you can home your telescope.
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The overall weight target is 130kg mirror included. Don't mix r with D in your calculations
