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Equatorial Platform - New Build


Stub Mandrel

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17 hours ago, Chriske said:

I'm @ 51.8' so that is a design I could use...😉
Do you share that file..?

I will be putting all the STLs and the sketch on my website soon, I'll drop a link here when I have, but I want to do a proper test before I do in case any adjustments are needed.

I also need to draw the finished dimensions of the two wooden panels as accurate locations are important.

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I managed a test last night, this is an edit of my post in the 'what are you doing tonight' thread after sleeping on it.

Main conclusion- too much stiction on the dob part, I need to make it smoother! Not really an issue with the platform, I think I need to dose it with silicon polish at the very least. It moves in jumps and the minimum AZ move is often nearly a full FOV at ~30mm and getting a target centred at 5mm took forever! Alt is much better but not perfect, although if unbalanced it can move on its own.

As for the platform, PA was easy, at least to the suggested approach of getting Polaris in the finder and adjusting until it stayed still during a slew. The polar angle must be pretty accurate as I just set everything level and the task was setting it due north. Errors were rapidly apparent so a button to stop the slew early will be welcome. My finder's cross hairs come out at 45 degrees but that didn't make the process difficult. Better PA could be achieved with a polarscope in the finder holder and getting Polaris to travel around the circle.

Getting the track rate was hard. A first everything drifted off top left regardless of speed. This made me think it was running too slow. I reduced the delay down to about 15ms until the stars definitely went off the right side pretty sharpish. I then increased the delay, and eventually settled on about 64 milliseconds delay by the time clouds came in, which isn't bad given the estimate was ~68 and expected to need to speed up to allow for compression of the roller. I think the earlier problem was using Lyra which was very high and I suspect a combination of PA error and an unbalanced scope slowly moving in ALT when near vertical.

It was easiest to judge movement using small stars near the edge of the FOV rather than bright ones at the centre. Next time I will use a 25mm plossl with cross hairs.

Incidentally I used a Szentmartoni EP made according to a recipe on SGL that uses three small binocular objectives for about 33mm. As promised it was sharp to the edge and comfortable to use!

I changed to a Skywatcher UWA 5mm to 'fine tune' the tracking rate. the sky wasn't very dark, but I went back and (just) split the double double at 300X - seeing wasn't brilliant, but it seemed to be keeping in place even at that magnification, certainly long enough for planetary imaging.

Changes needed:

I will wire the disabled reset button on the shield across the end limit switch. This will provide a way of interrupting a polar alignment slew, as initial drift is obvious in a couple of seconds, you don't need to wait for a full slew.

I will round the default track speeds 0.1ms and keep the adjustment step of 0.1ms. The 0.1ms seems fine enough and it's annoying that all the readouts are not round numbers - and no point reading the delay to finer resolution than the adjustment. I may change the step to 0.05ms, at the expense of slowing down the tuning process a little.

Although up to speed up tracking and down to slow it seemed logical, as up increases the delay and vice versa. It's confusing to press up and see a number decrease.

The debounce period for the buttons is too long, as are some of the 'confirmation delays' of 1s. I will reduce both.

The 'nudge RA' increment was WAY too aggressive, the briefest of presses and the view changes completely.

A remote handset might be a useful addition, perhaps with a reduced set of controls.

Mechanically, everything worked fine with the platform. Balance is great and I didn't even bother with a nut on the pivot bolt for the dob, so worth considering a plain pin instead. There is some wobble when moving the scope, but it isn't excessive and I didn't notice any when hands-off.

Main beef is with the dob itself, I must find a way of reducing 'stiction' and counter-intuitively locking the alt movement. But my brain is already thinking of ways to add steppers and belt drive to ALT and AZ - perhaps for 2021!

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  • 1 month later...

Here's an update, I tried the setup a second time with a camera for planetary.

Two problems - I need to move the mirror up 13-20mm for use with my ADC as with it I can't get enough in-focus.

The other was something that was intermittent and confusing on my first test - sometimes the scope was just not tracking fast enough.

The problem turned out to be the drive roller, this is a section of 13mm bore rubber pipe on a 3D printed core. I thought the roller was slipping on the sector but this wasn't the issue.

It turned out that under the weight of the scope the pipe was crushing slightly, making the wall thinner and slightly greater in circumference. This looosened its grip on the inner core which at low speeds was prone to slipping inside the pipe. The cure was obvious - I glued the pipe to the core instead of relying  on friction. A full test with the scope on the platform is running now and after 30 minutes  the movement is exactly as expected.

Phew!

Hopefully we will get a clear hour or two over the weekend for a proper test.

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  • 4 months later...

Also.. why is the polar axis aligned with the C of G rather than the centre of the OTA?  I get that it mustn't tip over at extremes, but is that the only reason?  I would've though aligning with the optical centre would be more accurate (although accept this is probably a negligible error)?

Edited by wobblewing
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3 hours ago, wobblewing said:

I was wondering if a 'rack and pinion' approach may help with the sector control?  That is something that could be 3d printed quite easily into the 'sector' parts.

My thought exactly. It is a nice project indeed, but the 3D printed design capabilities aren't unfolded their true potential here. It's just a way to avoid building a jig or tediously shaving and lining sectors surfaces for the otherwise totally standard platform design.

To answer your question, this particular platform type is not compatible with the rack-and-pinion (actually long rack sector-and-) driving natively, because its sectors are sliding and rocking on the bearing surface as the HA changing (they are not following the ideal circle). It will be hard to implement a cog following them. 

However, you can add such a mechanism on a separate "row" of the virtual cone instead, e.g. right behind the sectors assembly. The sectors row will take all the scope weight load, while the actual driving will be done behind them.

Moreover, you can use a trivial 1/4"x20 metal rod and have a venerable worm drive for that 3D printed cog sector with an enormous gear ratio and miniscule torque required, so even a 5V DC motor on PWM controller will do the tracking just fine, just add some lithium grease to delay wearing (when worn out, just print that little rack part again). You will lose the slewing feature that way (which is fun to have indeed), but you will get the rock-solid slipping guard, while adding a simple 3D printed rocking mechanism to disengage the worm (e.g. with a side or front pedal) would allow to move the platform manually for resetting (even $1000+ platforms doing just that or even less). The PA slewing idea is good, but I would instead just incorporate a cheap laser pointer module to hit the Polaris directly before the scope (or even its rocking top board) is on the platform yet. The sub-degree PA can be skipped in favor of the convenient speed control, as instead of the Arduino and the dedicated display screen (another unnecessary complication scaring many folks not skillful in electronics), which is hard to operate at your footsteps, I'd use an ESP8266 or ESP32 chip and have all controls on the smartphone (no need to learn smartphone coding, as these controllers can host a Web server and providing a dedicated standalone WiFi AP, so you can control your platform from a simple web page in the phone web-browser, E.g. see my recent Ultimate Blinker project).

Even more than that, with the 3D printer at hands you can experiment with removing ALL metal bearings from your construction, e.g. from making it on friction bearings exclusively all the way to making the FULL conical-needle-bearing-with-slide-stopper for the entire front and rear sectors... The sky is the limit!

PS: Such a platform is indeed a good thing for the automatic imaging of planets, however for the visual enjoyment I would rather invest the creative energy into your telescope mount fluidity. I'm personally having no single problem or issue pointing (Telrad and lately the QuInsight) and tracking (all the way to my maximum of 810x) with my 12" 16 years old classic Dobson. It's rock stable and smooth moving in all directions thanks to the 100% classical Dobsonian scheme (well, with a slight deviation of spring loaded altitude axis) and the trivial sliding magnetic counterweight (that explains why I'm still on the fence with my 3D printed platform, even though the dedicated Baltic Birch ply piece is waiting in my man's cave corner already :) ).

Edited by AlexK
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16 hours ago, wobblewing said:

This is great.  I was thinking about making one of these for a small heritage 150p, using my 3d printer as you have.

Did you have any problems with roller grip on the sectors in the end?   I was wondering if a 'rack and pinion' approach may help with the sector control?  That is something that could be 3d printed quite easily into the 'sector' parts.

 

Hi,

Just make sure the weight is concentrated near the rollers

I had to superglue the rubber tube onto the rollers to stop them slipping, but once that was done it worked fine. I took it along to a club observing session and also used it for some planetary imaging, even with only a rough polar alignment it was great for these uses. I don't think you could use it for long-exposure imaging without more sophisticated alignment and control.

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16 hours ago, wobblewing said:

This is great.  I was thinking about making one of these for a small heritage 150p, using my 3d printer as you have.

Did you have any problems with roller grip on the sectors in the end?   I was wondering if a 'rack and pinion' approach may help with the sector control?  That is something that could be 3d printed quite easily into the 'sector' parts.

 

Hi,

Just make sure the weight is concentrated near the rollers.

I had to superglue the rubber tube onto the rollers to stop them slipping, but once that was done it worked fine. I took it along to a club observing session and also used it for some planetary imaging, even with only a rough polar alignment it was great for these uses. I don't think you could use it for long-exposure imaging without more sophisticated alignment and control.

 

13 hours ago, AlexK said:

To answer your question, this particular platform type is not compatible with the rack-and-pinion (actually long rack sector-and-) driving natively, because its sectors are sliding and rocking on the bearing surface as the HA changing (they are not following the ideal circle). It will be hard to implement a cog following them. 

However, you can add such a mechanism on a separate "row" of the virtual cone instead, e.g. right behind the sectors assembly. The sectors row will take all the scope weight load, while the actual driving will be done behind them.

This is true, by making an 'all force straight down' sector makes them unsuitable for easily being a rack, but it would be easy to print a suitable sector of a circular rack and fit it between the roller sectors.

 

Edited by Stub Mandrel
clumsiness...
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I was thinking of trying that,  might give that a go in CAD and see how it looks. That's one big bearing!

Will have to think through the geometry to make sure it's equivalent, I suppose it should be.  The C of G will be way off, but that might not matter if driven this way.  Say one massive belt that goes around one whole diameter of the bearing (limited to 25cm diameter due to 3d printer bed size). 

One really good benefit would be that it would track all the way around without having to reset after an hour to so. 

https://simplybearings.co.uk/shop/Bearings-Lazy-Susans/c3_22/p42341/12-Inch-Round-Lazy-Susan-Turntable-Bearing/product_info.html

 

Edited by wobblewing
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On 07/01/2021 at 00:46, AlexK said:

Moreover, you can use a trivial 1/4"x20 metal rod and have a venerable worm drive for that 3D printed cog sector with an enormous gear ratio and miniscule torque required,

Yes,  I've got an old 3D printer that is no good anymore, but a supply of NEMO17 style steppers and really long worm drives (used on a 3D printer Z axis).  I could mount that on a rail underneath the platform and use the worm to drag it from side to side.. with a printed part used to deal with the 'tipping' of the angle between the worm and the platform.  That way the front sectors can be fully supported on freewheeling rollers, without the need to be concerned with a drive.    A good thought.  I'd have to figure out what the sidereal resolution will be - maybe it's possible to use it straight off the old 3d printer without adding extra gearing.

The South bearing is so far a friction bearing, based upon making the 'layers' of the FDM process parallel on each part, so they slide against each other nicely... not sure how successful that will be, but could try it!  I have some metal bearings if that doesn't work out.

image.png.7f1d219beee6d7a9cb369f6339d8ddef.png

If I do it like this I could go back to proper circular section North supports instead of vertical North sections.

Edited by wobblewing
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wobblewing, perhaps, you should rather start a new thread on your project instead of hijacking Neil's (standard forum rules). Just post a link to it here and in your's to this one for the useful cross-reference. I'll comment on your design progress there then (there are a couple of options on the driving scheme which you might want to consider).

Re the lazy suzan. In short: DON'T!
The Dobsonian mount is not a simplified fork mount, it just looks like it for a not mechanic. You will ruin half of Dobsonian principles replacing 3 friction pads fine-tuned system with any ball bearing, creating a finger-spinner toy from your telescope mount. Again, I don't want to hijack Neil's thread, so if you are curious to know the details, start the new thread on that bad idea :) and link here, I'll explain what's the issue.

Edited by AlexK
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Yes, you are quite right.. I should start another thread... I just began playing around to see if I had any appetite to do it.  the experience on this thread is very useful, gaining Neil's thoughts from building one earlier this year.  I can take some of these findings and progress it a little.

Unless I was not understanding the suggestion, I could not see how a lazy susan could work, as it would ruin the geometries of the cone angle.. Maybe something else was intended?

Anyway, have managed to 3D print a decent ish ball joint for the coupling...

 

 

Edited by wobblewing
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48 minutes ago, Stub Mandrel said:

Yep, you can get them 300mm diameter. This one has a capacity of half a ton - search 'lazy susan bearing':

https://simplybearings.co.uk/shop/product_info.php?products_id=42341&vat_inc=true

lsr.jpg

Yes, but did you work out the geometries?  I don't think it'll work if you follow the axis of rotation at 52degrees, the Dob will fall off the platform!  Unless I've completely missed your point 🙂

Here's a drawing to explain my reasoning.  Based upon my Dob with CofG about 24cms and 52.2deg latitude.  It'll fall off the platform.

image.png.3d0856d12494b879e9e73d25c0a4c4bd.png

 

 

Edited by wobblewing
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wobblewing, that's right, you've missed the point. Neil has been complaining earlier that his dob is too sticky, so he's got an idea to put that bearing (LS) on top of the platform to replace the baseboard of the original dob mount. It has nothing to do with the platform's cone mechanics, but people often just eliminating that baseboard moving PTFE pads from it to the top board of the platform and making a hole for the center pin. Just to reduce the bulk and weight. 

OK, as Neil seems to be OK having the lazy susan (LS) on this thread: Dobson mount was never intended to mimic or replace a real fork mount because the trivial particle-board or even solid wood construction will never match the real compact fork mount due to the increased flex and softness of the wood degrading such a fork mount's performance over time with just normal use. In fact, when you are operating the Dobsonian you don't actually rotate it on its axes, you are repositioning the OTA over 7 teflon pads which are providing perfect stability of the OTA in any possible position and at the same time minimising the effort required to reposition it any moment by a simple push, which is also precisely conditioned to eliminate any sudden jump (that stickiness Neil is suffering from) of the OTA from a static state to moving and back. The size of the pads, their shape and locations, as well as materials of mating surfaces are important and matching the weight of the OTA and OTA + rocker. E.g. for the latter folks are using even vinyl records disks! As even the texture of teflon-mating surface matters.

Adding a ball bearing anywhere (like that LS) will require addition of a brake. The brake is usually a dumb friction slab on the side, which will shift the view on engage/disengage. The LS will produce vibrations, will get clogged and have sticky spots, and on a platform will for sure spin on its own as with time people will not engage the brake fully for convenience. On some recent dob models from China, their engineers tried to use needle and roller bearings in tandem with PTFE, but the result is so miserable that owners tearing off that abomination and installing classic pads when their multi-months hunt for the specific starbound-like textured laminate succeeds.

So, my advice: don't do that. Ever. Instead, look for the actual root cause of your dobson system's issues. They are usually trivial and easy to fix for no considerable cost.

Edited by AlexK
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N/P!

On a side note, an EQ platform will interfere with core Dobsonian principles no doubt, as a dob is not intended to work on a significant slope (that's why a trivial EQ wedge you have sketched above would not work well with it if at all, you need an EQ platform). So, many folks has figured that for the visual tracking it is better to have two of the pads on the N-S line and on the West side of the platform (the third is tangent to that line on the East) as eventually they will get more load as the EQP tracks over the meridian.

Also, it's beneficial to point at your targets having the platform more or less leveled (insignificant slope, and Alt/Az operation is more natural when working with digital star charts), and track from there for 20 min (half of a typical platform tracking range), which is enough for most objects/projects, even for the sketching as you practice more.

Considering the latter, I'm poking an idea of repeating this barn door design instead:
Box1s.jpg 

Much more 3D printable! Can naturally utilize 3D printer parts you've been talking about. And It's also a more feasible tracker for Northern-ish latitudes (>45) like those in UK.

Edited by AlexK
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I tried to make a plywood/standard Dob bearing equatorial for my DIY 5" f:15 refractor over 30 years ago.
Far too much friction! A couple of lazy susans might have been the answer.
Or, just replace the PTFE pads with journal bearing rollers, set on edge.

There are other platforms. I made a Poncet Platform 50 year ago. Easier to design and make.
Though stability with larger instruments might be an issue.

 

Telescope 5in 6in plytube equatorial rsz 500 brighter.jpg

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On 05/07/2020 at 18:23, Stub Mandrel said:

That unshielded bearing is annoying me...

That unshielded battery is annoying me.....

Its amazing what a battery can do when shorted out. Cheap enough to 3D print a battery box or cover. Why not add a fuse and isolating switch while you are at it.

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