Michele Scotti

5 hours ago, DaveS said:

This is what the professionals buy when they want a 800mm 'scope.

Just throwing this up, no I don't suggest you need all of this, just fillet it to your needs. If you have anyone who can make you DD motors for both axes I would seriously consider it.

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 

1 hour ago, markse68 said:

Yes- I was just questioning the need for absolute encoders - nice if the scope never gets moved but not really a necessity. Also resolution doesn't equal accuracy and like Horwig mentioned mounting accuracy can throw everything. Heidenhain (and others I'm sure) make some huge encoders for CNC machines etc which benefit from their diameter and very thick accurate glass grating  

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.

19 hours ago, Chriske said:

Thanks Robert.
Need to speed up things a bit. Mid Sept. is our open door at the observatory.

This is how far I got today. Tomorrow I'll take it all apart again and paint it all(except for the 'tripod' of course).
A few things need to be done before I hang the scope in it's fork.

This is just awesome!

20 hours ago, vlaiv said:

I'm going to expand a bit why absolute encoders are necessary for Alt-Az type mount if you want accurate tracking - it is not the same as with GEM mounts where you increase precision of tracking - it is for accurate tracking with AltAz.

Maybe diagram is going to explain it better - I'm going to exaggerate curvature of things so you can see more easily what is going on.

Imagine scope can't precisely determine Az position - there is certain error. Scope is just a tad past meridian, but "thinks" it is pre meridian in position. Software will tell scope that Alt should increase a bit, but in reality needed Alt position will decrease a bit - tracking will create error in Alt because it is going "the wrong way", and guiding will combat this instead of only correcting things it should correct.

Similar thing applies to guiding as well - it needs to know direction of vector - where Alt is pointing and where Az is pointing in order to give proper corrections - if orientation of scope's Alt/Az is different from that of guide system - wrong corrections will be given and guiding will not work to optimum - it is what happens in GEM mounts when you have wrong guider calibration.

Difference between GEM guide system and Alt/Az system is that Gem guide system in principle needs calibration only once (although people do it every time when changing target / part of the sky - good practice because of cone errors and fact that RA/DEC axis are not always perfectly at 90 degrees to each other). With Alt/Az - you need constantly changing calibration, and software needs to track this and change things, but in order to do so accurately it must know exact pointing of the scope.

This is why you need absolute encoders. Not sure what is needed precision of such encoders, but we might be able to calculate drift rates depending on guide command cycle length and position in the sky.

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. 

21 hours ago, Horwig said:

If I may add my experience here with trying to add an absolute encoder to my Sitech controlled DIY fork, I bought a Gurley encoder as specd for Sitech (not cheap), and mounted it via a flexible shaft coupling to the RA axis, the body of the encoder was solidly mounted by bracket to the bearing block. I must have some axial offset in the centering of the encoder, it gives differing RA speed, dependant on pointing position, so will have to go back to the drawing board in its implementation.

 

Huw

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? 

20 hours ago, Michele Scotti said:

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.

 

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.

4 hours ago, vlaiv said:

That is 166.66 revolutions per second. One revolution is 1.6666 arcminutes, so it's 277.777777 arcminutes per second, or 4.63 degrees per second. Ok, that is actually quite fine (if my calculation is correct).

Thanks for estimating the slewing speed - hadn't done it yet! 👍

4 hours ago, vlaiv said:

and lack of absolute encoders.

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

10 hours ago, vlaiv said:

Could you go a bit more into detail regarding precision of motors.

9000:1 is total reduction of motor spin right?

Let's say that you need to move thru 90 degrees in altitude. You also want at least 0.1" precision in altitude position (to be able to guide properly, and possibly you will want more precision).

This gives you 100 revs per degree, or 1.6666 revs per arc minute, or 0.027777 motor revolutions per arc second, so we are looking at 0.002 revolution of a motor per "step".  That is something like 0.72 degree precision on motor shaft, or 1/500 accuracy. I guess that should be doable with 10 bit encoder on shaft if we are talking about servo motors.

 

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?

3 hours ago, Rusted said:

Reversible synchronous or DC motor? Please keep those pictures coming! We are all here to learn! 

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

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.

3 hours ago, Rusted said:

So you can use very large diameter, lightweight, hollow shafts  like tubular aluminium] or even local rings.
Supported by three wheels set 120° apart around the circumference.

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.

9 hours ago, vlaiv said:

I'm not so much worried about the drive (although that is also major concern), I'm worried about smoothness of Alt-Az mechanism. It needs to be extremely rigid, yet so smooth in motion that it does not have even slightest "stiction / jerk" anywhere along the arc of motion. All of that holding something like 200kg+.

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. 

13 minutes ago, Gina said:

Is it???

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.

11 minutes ago, Horwig said:

Best of luck with this project, I've been fighting my equatorial fork mounted 400mm f3.5 Newtonian for more time than I care to remember 😀

Not sure, but I think the Sitech system can handle Alt-Az mounts, as well as guiding and de-rotation, see page 22 of this manual

http://pgrasc.org/wp-content/uploads/2014/03/SetupManual1-1-a.pdf

 

Huw

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 

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.

6 hours ago, DaveS said:

If you plan multi minute (I think you should plan for multi-hour) exposures then you will need *very* stable tracking on both axes. Do you have anyone who can make Direct Drive motors? I have seen it done for "amateur" telescopes but it takes serious skill. What about bearings, again for both axes. Forget about the usual dob bearings, they're ok for mere visual, but imaging is a whole different level.

What about field rotation? Big field de-rotators are expensive, and you will want a big aperture to pass the light cone. How about cameras? For something at this level I'd be looking at a 16803 camera, or even a FLI Kepler 4040. Not cheap. Plus, of course you're putting all this stuff on the end of the truss tubes (Another reason why big 'scopes in the 800mm and up class go for Nasmyth systems.

Guiding? I assume an OAG since encoder guiding will be even more expensive and you could end up with something like This.

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!

3 hours ago, vlaiv said:

If you can get acceptable tracking, field correction at F/3 with some CC in this custom solution for less money then yes, it's worth it. It's worth anyways if you like the challenge of custom making and see it as open source project to be repeated by others, and of course to be used as awesome visual scope.

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. 

3 hours ago, vlaiv said:

What sort of science are you looking at with this scope?

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?  

22 minutes ago, Peter Drew said:

Search www.astronomycentre.org.uk and click on "instrument special feature" to see our 750mm F4.1 Dobsonian to give you some idea of overall sizes.    😀

That's a great project Peter!

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:

57 minutes ago, Peter Drew said:

Do you already have a suitable mirror blank. I really do advise that this is imperative before other considerations as it could significantly determine the mounting  design.   😀

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 

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

22 minutes ago, Peter Drew said:

I have been through all the ramifications of such a project, believe me!. The Nasmyth configuration looks good on paper but the convex secondary imparts an amplification to the focal length as with traditional Cassegrains, increasing the already long focal length of a  large primary mirror, not ideal for imaging. The extra weight  (and expense) of the additional optics and their support  requires an even more robust construction. I've been looking into the possibility of re-imaging via a Mersenne arrangement or a flat circular secondary and transfer system directing the focus through the primary as a relatively non amplified focal length, again expensive.  😀

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

9 hours ago, Peter Drew said:

We have a similar project, currently "on hold" due to problems caused by being ring fenced by wind turbines that create bad seeing turbulence. The scheme was initiated before the turbines were erected or we would not have started the project in the first place. A first priority, if you don't already have one is to source a mirror blank.  An 800mm diameter thin blank and fast focal ratio would be a challenge for a top professional and would probably need to be a slumped meniscus configuration to provide adequate stiffness. Our mirror blank is 1070mm diameter and 76mm thick fused ceramic, pre-generated to curve and a 76mm bored core, weight is 160k.

An alt-az mount would be relatively easier and cheaper to make than most other mounts  and best made of metal for long term stability. I went ahead at the time and built the all aluminium tube assembly once the mirror blank was secured, as mentioned in an earlier post, a concrete dummy mirror was used for handling experimentation. Despite heavier weight, I think a traditional 8 component truss would be needed for stiffness if longish exposure imaging is planned.  😀 

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.

4 hours ago, Dr_Ju_ju said:

An 800mm diameter mirror is going to weigh a LOT (simple calculation based on 800mm x 25mm = 125 Kg) [https://www.glass-ts.com/glass-weight-calculator] so I'd forget timber laminates & instead go for Aluminium.... 

The overall weight target is 130kg mirror included. Don't mix r with D in your calculations