800mm Telescope Project

[Michele Scotti]

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?  

[vlaiv]

42 minutes ago, Michele Scotti said:

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?  

I'm sort of struggling to see justification in such a project on science side.

I understand the appeal of such a large aperture for visual, so that is legitimate requirement, but on the science side, especially if we are looking at DSO imaging, I would look at other options.

Don't know what is expected budget for this project, but such a large aperture is unlikely to give you significant advantage over smaller aperture in terms of resolution unless you construct very precise tracking platform. It is large telescope and it needs to be tracking very well to have edge over for example 16" RC on a good mount.

There is of course huge light grasp of such aperture, but let's do a comparison against alternative that can be used for "quick" acquisition of DSO images - like small galaxies for photometric / astrometric  measurements for example.

You expect to be tracking for at least couple of minutes, so I presume that you will be doing multiple exposures and stacking anyway, and not working with fast transient phenomena that would benefit from short exposures and "concentrated" light gathering.

Here is a quick calculation:

16" RC with x0.75 reducer / field flattener will provide you with very large corrected field at ~2400mm FL. It costs about 6K? Put that on Mesu 200 mount (another 5-6K), attach suitable camera / filterwheel whatever you like and repeat 4 times. That is about 50K of investment and you will have total of 800mm aperture, 2400mm FL and very multi hour tracking / guiding.

I'm not sure if there will be any significant resolution advantage in 32" vs 16" aperture, given even very good seeing and tracking (there will be some, but not sure exactly how much, we just had a discussion on seeing impact vs aperture size in another thread, and concluded that that topic is out of reach for our level of understanding as is). Best you can hope to achieve in my estimate is about 0.75"/px practical sampling rate (FWHM of about 1.2").

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.

As for me, and my usage of such system, well I would use it the way I use smaller scope - to do what ever comes to mind, with addition of crazy imaging speed - or rather large SNR for given imaging time. I do have couple of ideas for some processing algorithms that require higher SNR than usually achievable, and would like to opportunity to test those, so such scope would be good for that purpose.

[DaveS]

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.

[Rusted]

If one expects to have copiests of this project then it assumes you are pushing the boundaries and greatly reducing costs.
The first requires that you manage to go well beyond the bleeding edge of current best practice.
Which requires a thorough knowledge of the latest PROVEN trends of well heeled and highly skilled builders.
Reducing optical costs must include obtaining the same very high quality for much less money than existing commercial sources.
Making short focal length optics is very hard compared with average focal lengths.

To expect others to follow you must offer repeatable excellence in all areas available to all who can match, undercut or exceed your own budget.
Otherwise, why bother? Do you have a mechanical genius with a gift for really innovative design? Access to organic AI?

It is now decades since Dobson popularized crude but workable, large aperture telescopes.
The biggest were a real struggle or dangerously tall for a wobbly stepladder ladder in the dark.
He bypassed centuries of heavy engineering but at the huge cost of poor drives for photography and imaging.

Nobody {as far as I am aware] has yet pulled off a similarly affordable and repeatable equatorial mounting for large apertures built from dumpster diving.
Though there is certainly plenty of rusting scrap metal hiding away in amateur's back gardens.

[Peter Drew]

Equatorial platforms are the affordable and repeatable mounts currently available even for large apertures but the makers of the very best ones would not claim them to be suitable for research grade imaging. I can't see any way past the £££ route to achieve this.    ☹️

[Rusted]

Back in the 1980s I needed an equatorial mount for my newly finished 16" f/5 mirror.
I thought of railway wheels on steel rails. So I tried a 24"Ø x 1" thick machined iron disk.
Resting on 4" steel edge rollers fitted with ball bearings and a stumpy PA. Too heavy to slew by hand! Grr!

Then I made an 18" diameter PVC tube into a right angle joint and another joint at 50° for the PA. Think giant plumbing joints.
I fitted the straight sections with 3/4" ply disk and glassed [GRP] the joints inside for extra rigidity.
I imagined I'd use [Dobsonian] PTFE/Formica disk bearings but the friction was far too high.

The idea of using very large diameter tubing for a mounting may have practical applications these day with foam cored, carbon fiber.
It just needs a low friction bearing at the disk contact faces. Multiple journal bearings on edge set in a ring?
Skate bearings are cheap and cheerful these days. Not back then.
Precision of axes and surfaces could be arranged by longitudinal threaded rods like simple mirror cells.

The finished items was hugely [sic] impressive and the accuracy of the fit between the tubes  was truly remarkable.
I thought myself a bit of a genius for a while [or a **** artist] until the bearing friction problem reared its ugly head.
Cantilevered  structures require high linear pressures to retain bearing contact.

No doubt a lightweight equatorial fork could be assembled using foam cored carbon fiber.
Just as we built stressed skin, lightweight plywood forks back then.
No need for heavy steel sections and welding and solid shafts as thick as your arm.
They can be built one handed by those impoverished enough. Or daft enough to try.

[Michele Scotti]

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. 

[whizzbang]

if you never try you never know ...... go for it  

[Michele Scotti]

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!

[Michele Scotti]

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.

[vlaiv]

Having thought about this, I'm worried

If you are going to go for something like 0.79"/px, and aim to properly sample at this resolution, you will be wanting something like 0.1-0.2" RMS error in tracking/guiding.

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+.

I believe this calls for exceptional machining precision and knowledge of materials. You would not want your mechanics seizing due to temperature change or becoming jerky or whatever, and there is also potential for forming a slack in hotter conditions - at this scales, with such large parts - it can easily happen. Things can even get out of shape if load is not spread evenly ....

Another thing to consider is that you are going to need custom software written for this. No guiding software, as far as I'm aware, guides in alt-az mode. Software needs to know exact pointing of the scope to properly calculate needed shift depending on guide command. Same goes for tracking software - it needs to know precisely where the scope is pointing at any given time - that means either full precision absolute encoders on both axis ($$$) or some sort of split configuration with calibration - meaning lower resolution encoder on both axis and lower resolution encoders on drive shafts.

With Ra/Dec system it is fairly easy to determine tracking rate and needed "resolution" of the motor (provided you are using steppers) to keep things within certain limits. For Alt-Az, this is not so easy calculation, as rotation rate will change with respect to where the scope is pointing.

You will also be giving up best position in the sky for imaging - near zenith, as alt-az has trouble properly tracking in this region of the sky.

Just some things to consider if high resolution imaging is one of your goals.

[Horwig]

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 😀

1 hour ago, vlaiv said:

Another thing to consider is that you are going to need custom software written for this. No guiding software, as far as I'm aware, guides in alt-az mode.

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

[Michele Scotti]

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 

[Gina]

Why use an Alt-Az mount and then have to de-rotate?  What's wrong with an Eq mount?  What am I missing here?

Edited August 29, 2019 by Gina

[Davey-T]

33 minutes ago, Gina said:

Why use an Alt-Az mount and then have to de-rotate?  What's wrong with an Eq mount?  What am I missing here?

Anything that heavy is better served by an Alt / Az mount I think, better for the big heavy mirror than  "whizzing" around in the sky especially if doing a meridian flip

Dave

Edited August 29, 2019 by Davey-T

[Gina]

Is it???

[Davey-T]

2 minutes ago, Gina said:

Is it???

Dunno I'm no engineer just seems more chocolaty, I should think collimating it and keeping it collimated will be hard enough without flipping it upside down 😂

Dave

[Michele Scotti]

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.

[Gina]

I guess it's to do with gravity acting along the axis with Alt-Az whereas it acts at an angle to the RA axis with an Equatorial mount.

[Rusted]

Er-um-er..  "upside down" is not considered best practice for telescopes around these parts.

Getting back to basics: One should never be afraid to borrow proven methods from other disciplines. 
Amateur astronomers always tend to think of equatorials in terms of heavy axles in bearings in solid castings.
An engineering cul-de-sac inappropriate to the larger task in question here.

Clock makers have used axles restrained by two or three radial bearings in a cage-like ring for centuries.
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 shaft rotates very freely. Which is why clockmakers used the device in both ultra heavy and in high precision clocks.
The shaft is tightly constrained radially by the wheels.  I use the term "wheels" deliberately to show that they need not be small rollers.
Small rollers assume very high levels of precision in roundness and hardened surfaces. Wheels can roll over rougher surfaces without hindrance.
In some of the heaviest and largest public clocks ever built, once per century between services was achieved. 
Plain bearings [bronze or brass on steel] were simply not up to the task because of very heavy, highly cantilevered loads.
Literally tons dangling on the far end of the shaft and subject to weather, icing and massive wind loads.

Now imagine your lightweight but bulkier equatorial mounting, so built.
Large diameter, concentric, tubular, structural forms with radial wheel bearings separating the two at distinct points along the length.
But, not nearly so bulky as a horseshoe or fork mounting. Nor with the problematic, high friction of Formica & PTFE in larger Dobsonian sizes.
Both inner and outer structures can be open and cage-like for lightness but employing rings and wheels as radial bearings at critical points.

Apologies for rushed image not having external rollers set 120° apart.

Edited August 29, 2019 by Rusted
Added image

[Michele Scotti]

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. 

[Michele Scotti]

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.

[Michele Scotti]

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.

[vlaiv]

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.

 

[Rusted]

8 hours ago, Michele Scotti said:

I suppose clocks are precise angle-wise but don't care much about the motion 'roundness' precision.

A world authority on the highest precision ever achieved in astronomical horology too?
Including the genius of "Woody" Harrison, of Longitude fame? No. Not the US, comic actor of a similar name, but close.
A multi-train, precision drive system, including serial spur gears bolted onto a plywood, altaz construction?
Reversible synchronous or DC motor? Please keep those pictures coming! We are all here to learn!