Kutter 2" f/28.2 (finished)

[Chriske]

Hi,

In the past I made myself a few somewhat larger Kutters. Now I'm busy making a rather small version.
Almost all parts are printed. These parts are all drawn in Autodesk Inventor.
Mirrors are almost polished. I will be trepanning the optics out of 6" disks. But first I have to finish polishing these mirrors. Both radii are the same, so that makes it a bit easier.

Chris

[Stu]

Looks intriguing Chris, I'm assuming it is some form of folded path reflector?

[Chriske]

Indeed Stu it has a folded path. This drawing explains it all.
When the optics ar well made it should perform very well, as the spots fall well in the Airydisk.

 

[Stu]

Thanks Chris. Very clever  

I assume for visual, just handheld?

[Chriske]

No not at all Stu..!

50x28.2 so F=1410, that makes it a bit long to be used handheld. And I plan to use magnifications of about 100x (I think that's the most I can get out of this 50mm scope) that makes it even less suitable to be used handheld. Still I have a very good 4.8mm eyepiece I will try it out but I suppose that will be far to high magnification. But maybe, just maybe if I make myself good optics and with very good seeing it could work.

 

[Chriske]

Busy reworking this little guy.(Change of initial plan).

Installed was a 10" Kutter telescope in our public observatory Urania(Hove). This telescope(Invented by Anton Kutter) was a real masterpiece. It was intended for planetary work and did it very well to. But as so many observatories do these days : replace the 'smaller' and old scope with a larger one . So our Kutter was replaced by a 400mm Cass a few years back. Still I find it a shame, but maybe I'm to old to understand. And as a matter of fact there's a bit nostalgia involved I think...
So this little one I'm working(drawing for the moment) on will be a model scale 1:5.
Deadline mid-sept. for our open door.
All has to be reprinted in pristine white(like the original scope). Also a mount will be build.  The original Kutter was installed on a concrete pier 1m(40") diameter, 8m(26') high.
I'm thinking of installing a stepper for the RA axis. The original (installed in the early '60) had no motor on the Decl. axis.

This is an overview of my workspace this very moment.
Upper left is the wedge on which the fork will be installed. In real that steel wedge weigh about 90kg(about 180lbs)

[Chriske]

Meanwhile, mount almost done...
Inside view - Electronics done by my pal Guy

 

[Chriske]

Did some tests and it's not working as it should.
Initial idea was to drive the scope using just a contact-wheel. Seems that there's not enough weight to drive the fork properly. There also a second problem, due to the low mass onto the contactwheels, the fork/scope is very easy displaced even by a very gentle push.
So busy drawing plan 'B', worm and wormwheel will most shurely do the job. So busy making a worm/wormwheel set. Making the wormwheel is not done using professional gear. The milling tool to do this job is a simple tap M8.(could in fact be any tap). It' a very simple technique. The wheel itself is supported by two ball bearing, so it can rotate freely. Pushing the rotating tap very slow/gently into the perimeter of the wheel, the tap forces the wheel to rotate and at the same time it cuts the angled teeth into that perimeter. It takes only a few minutes to make an almost perfect wormwheel.

[Peter Drew]

Interesting project! I've made hundreds of wormwheels using the tap method albeit with the tap held between centres in a lathe. The big advantage of this method is that you can use standard screw sizes for the worm. However, stock threaded rod has threading that is rarely concentric to the core diameter and can easily produce severe periodic error. It's best if the worm shaft, the end bearing journals and the thread form are turned in a lathe in one setting. If being turned on a lathe it's worth making the outer diameter of the threads a little undersize, this produces a flat top profile which will not run on the troughs of he wormwheel but instead on the faces of he teeth.   

[Chriske]

 To clean up the worm(actually a M8 threaded rod) so to make it more concentric and removing burs I use a die. It really makes a big difference.
As a matter of fact you only need to clean up a very short part of that threaded rod.
Sometimes I cut the treads myself. Cutting thread on a brass rod gives me perfect threads. I know not a perfect combination to make both wormwheel and worm itself out of brass or bronze, but as it rotates incredibly slow there's no problem at all. And a drop of grease improves smoother action.

[Chriske]

Forgot to mention, the worm is spring loaded so whenever the bolt is very slightly off-center it will always make perfect contact.

[ZiggyNZ]

Any tips for getting the teeth on the gear to run smoothly around all 360 degrees of the gear? Or do you not worry about there being a bit of a "bump" once per rotation?

[Peter Drew]

You can only do your best with this method. Taps are not made for high precision purposes so the pitch can vary slightly, two taps of the same spec can give + or - a tooth from the required number even on the correct gear blank diameter. he number of teeth can be guaranteed by pre- gashing the blank on a dividing head.  Spring loading the worm is fine for keeping it in contact with the wormwheel and works well for visual but can still produce periodic error for imaging as the "push-pull" of the spring action away from the centre minutely varies the effective diameter. 

[Chriske]

Hi guys,

I'm pretty sure my worm will suffer from periodic errors, be it very little. As a matter of fact I'm not into serious photography (yet) so if there is any, I will deal with it later. But right now I want drive a scope for visual observing only.
I have to 'disappoint' both of you guys. During milling, at the end of one rotation of the wormwheel(to be) the teeth overlap without any errors..! You read this correctly. Do not ask me how I do this because I do not understand myself. The first time I tried this technique was to make a hobbed bolt for my 3d-Printers and it worked very well.  So with this knowledge in mind I tried it on a wormwheel to make myself a star-tracker. And it did work. So next logical step is to make a wormwheel to drive a real scope. I know, I know, thats something completely different compared to a startracker (to do some widefield photography).

Nevertheless milling a wormwheel with a tap works very well, and like I said, no sign of double teeth or whatever problem could(should?) arise. Believe it or not I inspected ALL teeth with a microscope. There was not even a hint were the teeth would meet. Only yesterday I made 4 different wormwheels, diameters 50, 45, 40 and 35mm. All of them were perfect, no overlapping or double teeth.

To be clear, I'm a mechanical engineer myself and what I'm telling you now is against all logic in a metal workshop, I'm aware of that..!. Still, you just try it, no need to do this with a mill. If you have a sturdy drillpress I think it would work to. The only problem would be advancing the wormwheel with a slow and steady pase toward the rotating tap. 

[Chriske]

Mount done.
The old stepper (I was planning to use) in combination with the wormwheel drive system is to large, it didn't fit in the wedged shaped housing. So I temporarily will drive this forkmount manually.  If there's enough time I will add a new (geared) stepper I ordered a while ago.
The steel bolt was not good enough, even after cleaning it up with a die the surface of threads was still rather rough. So I cut my own worm out of brass. I know not the best solution, but that's all I have at hand for the moment. I'm pretty sure there will not be a problem because that worm is running very slow.
I need two more knobs, one to manually drive the polar disk and the other one to fasten the wormwheel to the polarshaft with a little clamp in mounted inside the polardisk.

[Peter Drew]

There is no problem with making both the worm and wormwheel out of brass. As you mentioned, the turning rate is slow so wear is minimum. Surface finish is important and it is much easier to achieve this in brass. Steel, and particularly stainless, is difficult to machine to a good finish on the size lathes usually found in home workshops, professionally made steel worms are usually ground.  

[Chriske]

I know about SS,  don't like to machine it...

[Chriske]

Polishing the optics using pitch. Preliminary polishing is already done using polishing pads. This last session using pitch is just to remove the last irregularities like microrippel, small zones...

 

 

 

[Chriske]

Meanwhile electronics and stepper up and running.  That's all done by our electronics guru Guy.
Still remote for the speed-control to be made. Connection is not wireless but through UTP.
Two major tracking speeds will be involved, Moon speed and stellar speed.

[Chriske]

Using a diamond coringbit I drilled out a 25mm diameter secondary mirror for this scope.
It took me just under 4 min to drill the full 16mm thick glass disk.(scroll down to see (short)movie link)

25mm coring bit.mpg

[Chriske]

Just had 'firstlight' with this little scope.
I have three reflecting surfaces and no coatings yet, so I could safely observe the sun without filter.  Pity I had to hand held the eyepiece. Made a very small error during drawing in Inventor so during observing I had to remove (partly) the focuser and push the eyepiece by a few mm deeper as expected.
BUT..!  This little guy delivers .!
I just observed a few sunspots in full detail(even using a handheld eyepiece) I had the impression I could see(for a second or so) detail in the penumbra.
Still no contrast yet, because of  : no baffles yet, no internal blackening yet, and most of all no proper collimation. This first observingsession was just to find out whether the eyepieces were at the correct spot and there was enough travel.

A few specs :
Telescope type : Brachyt anastimatic (Often called Kutter telescope).
Diameter primary mirror 50mm (sec -convex- mirror diam. 26mm)
Telescope FL : 1440mm f/28.2
Magn. Erfle 28mm : 51x  (AFOV : 65° - TFOV : 1.2°)

[bluemaxroe]

Man, i need to learn Inventor!!!

this thread is great!....never seen a Kutter before.

what 3d printer are you using?

[Chriske]

Just do it...! Using Inventor is lots of fun.
There are only a few programs that can design parts and especially assemblies at this level, Inventor is one of them. I'm a lucky b*****d owning a legal version of this software(from my last employer).
If you are a student you can download it from Autodesk.. Im' 100% sure you are a student..!!!...

The printer I use is own design, designed in....Inventor...;-)) One of the most important features of my selfmade printers is the ability to swap hotends in a few seconds. So far I have a set of 6 hotends with nozzles starting from 0.4 to 0.9 mm. Next step is to make hotends with 1.0 and 1.2 mm nozzles.

Need more info about Kutters..?
These telescopes are superior instruments for observing the Moon, Sun and planets. If you're a 'deep-sky-man', forget about these.

[Chriske]

Electronics completely done now, 3 possible main speeds.  This is all Guy's work(our electronics Guru). He always completely make it himself. And I mean all of it, even the PCB,s for the drive, for the remote. Programming the Ic's for the different speeds is done by a simple push on just one button. Maybe overkill I know, but he even added solar speed...
I use three batteries in series from my Makita powerdrills. I have a bunch of these 7.2V powerpacks so why not use them for astronomical purposes to...? The holder for these batteries I designed and printed myself. Three of these Makita powerpacks in series gives me about  22V. I use this LM2596 DC-DC convertor to step down to 12V to power up Guy's stepper controller, but also to power the fan.

 

 

[bluemaxroe]

On 9/1/2017 at 16:43, Chriske said:

Just do it...! Using Inventor is lots of fun.
There are only a few programs that can design parts and especially assemblies at this level, Inventor is one of them. I'm a lucky b*****d owning a legal version of this software(from my last employer).
If you are a student you can download it from Autodesk.. Im' 100% sure you are a student..!!!...

The printer I use is own design, designed in....Inventor...;-)) One of the most important features of my selfmade printers is the ability to swap hotends in a few seconds. So far I have a set of 6 hotends with nozzles starting from 0.4 to 0.9 mm. Next step is to make hotends with 1.0 and 1.2 mm nozzles.

Need more info about Kutters..?
These telescopes are superior instruments for observing the Moon, Sun and planets. If you're a 'deep-sky-man', forget about these.

Not sure i am a student, but i can do most of what i need in 3D Studio max. and Autocad.  But i think you will agree, for assemblies nothing beats a parametric package, whether its solid edge, solid works or inventor and suchlike.  I take it you own you own lathe?  dont fancy making me a worm wheel set do you?  M8 tooth size 244 teeth?  lol

Looking to get a 3d printer with a 300 x 300 x 300mm work area soon....it will be my first, and might mod it from there.

 

thanks for the encouragement