DIY Focuser Design

[Mav359]

Hi guys

wanted to get some thoughts and opinions on  the following, "don't do that because" "Why not do this instead" etc.

I have been looking at as many DIY focusers to give me an idea as how to proceed, not all these are ideas are mine but i think i have cobbeled together some of them to best fit me.

The plan is to make a Easy Stepper Driver/Arduino controlled auto focuser for use with the PC, i intent to extend an arm from the stock focuser using the screw points around the orange ring surrounding the focuser  as the first mounting point and extend out past the edge of the SCT. Part of the backet will return against the side of the scope and use one of the rear cell mounting screws as another mounting point. I also intend once all the wiring is done to connect the power and connection from the ESD to the Arduino using a single RJ11 cable down to another box close to the USB Hub.

Any thoughts

[Zakalwe]

No reason why it wouldn't work, though a Crayford might be easier and provide more accurate focusing? There'd be a whole lot less slop in the focusing solution by moving the Crayford tube rather than the mirror?

[Mav359]

Can't use a crayford as i have a hyperstar so my only way to focus the scope is at this point, i can upgrade the stock focuser to a feather touch but changing to that would change this design & i'm not sure if it would be worth it if this works

[keithatrochdale]

Hi

Am I correct in saying you are intending to use a large pulley on the motor?

If so and the stock focuser is smaller, you are gearing it the wrong way. You want a smaller pulley on the motor and a large one on the focuser to give down gearing and fine adjustment.

Just measured the clearance on my C11 and you could use a 4" one on the focuser and maybe a 1/2" one on the stepper motor. 

I would also suggest you use a toothed belt.

Keep us posted on how you decide, with a few pictures please.

[Mav359]

Ok cool didn't know that.

it had been my intention to leave the rubber grip on the focuser and use it as is.

I had wanted a proper pully on the focuser knob however If i pull the grip of the focuser knob and expose the brass shaft thats there it has a diameter of 15mm & i cant find anything (i don't if im looking for the right thing or in the right place) that has a 15mm+ bore on it.

[keithatrochdale]

Hi

Download this .pdf

http://www.cross-morse.co.uk/timing_belt.asp

Try 60XL037 for the focuser, you can open the hole out with a drill

And 10XL037 for the motor, you may need to make a sleeve (do not know the diameter of your motor shaft).

There are some stepper motor drive boards on e-bay and motors.

http://www.ebay.co.uk/itm/5V-Stepper-Motor-28BYJ-48-With-Drive-Test-Module-Board-ULN2003-4-Phase-v-h9-/321227759077?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4acaaa9de5

[Merlin66]

Hmmmm

I took the easy way out on my C11 and just used a SW Accura focuser motor with a timing belt pully and the timing belt just ran on the existing focus knob. A small aluminium bracket held it on the OTA.

Worked very well - no issues...

(I've only recently replaced it with a JMI focuser motor - because it came up cheap and just "clipped" on.....)

[perfrej]

You need to lose the rubber thing for sure as it is not perfectly round. Also, I am actually of the opinion that this may be a cause in vain as the internals of the focusing system is a rather loose affair. You will need to apply mirror locks for it to keep focus and the locks themself are not perfect in any way (may cause small shifts as well).

Ii used to have an EdgeHD 925 with Hyperstar, and although the system was great in terms of optics, the focusing was a real problem - with a Microtouch focuser installed!

Somebody should come up with a focuser that sits up by the Hyperstar instead, maybe like an FLI Atlas...

/per

[Peter Drew]

The main advantage of motorising the primary mirror focusing system is not needing to touch the telescope. Not a complete solution but not to be "sniffed at", the Celestron system currently seems to have minimum image shift. 

[Mav359]

Opinions on the mirror shift with these i have found seem to vary, i have to say thats the one thing i haven't ever had any issue's with. Seems to be pot luck.  The quality of the focuser assemble is what it is, i can't change that so the way i figure it i loose nothing by doing this just gain the advantage of motorised focuser

Hi

Download this .pdf

http://www.cross-morse.co.uk/timing_belt.asp

Try 60XL037 for the focuser, you can open the hole out with a drill

And 10XL037 for the motor, you may need to make a sleeve (do not know the diameter of your motor shaft).

There are some stepper motor drive boards on e-bay and motors.

http://www.ebay.co.uk/itm/5V-Stepper-Motor-28BYJ-48-With-Drive-Test-Module-Board-ULN2003-4-Phase-v-h9-/321227759077?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4acaaa9de5

Cheers for that Keith

I'm looking at the specs for the 60xL037, on the catalogue spec sheet it has

Min -   6mm

Max - 29mm

Am i reading that right, in my head that means the bore holeis already 29mm so without drilling it it would fit over the focuser shaft (15mm) and tighten up to it. Does that sound right?

The shaft on the motor i bought is 5mm so the 10xL037 should be fine

Min - 4mm

Max - 6mm

This is the motor i bought, hasn'r arrived yet. any thoughts? i know about the 6 wire config.

http://www.ebay.co.uk/itm/121145827395?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

[Mav359]

Hey Keith i just rang them and they are at the min bore diamteter and would need to be drilled, that is a service they do so i can send them my requirments and they'll arrive ready to use.

[Gina]

Looks like that stepper motor hasn't got a gearbox built in so you might find it a bit coarse depending on how fine the focussing is with the knob.  I have never used this type of scope.

[Mav359]

Any suggestions for better motors that will work in conjunction with the Easy Stepper Driver/Arduino config?

[keithatrochdale]

Hi

Don't forget this little motor/gearbox;

http://www.ebay.co.u...=item4acaaa9de5

Get one to have a play with, you may be surprised with it's power and it is geared down well. Cheap enough to get a couple of spares too. IIRC I have driven mine from  an arduino using stock code.

[Mav359]

Oh yeah sorry keith

[keithatrochdale]

You may well need to consider pulley sizes with this combination, as the output is a lot slower.

Good news about the pulleys, do they offer belts too?

With a toothed belt you should not need to have it too tight as it will not slip unless very slack.

[Mav359]

Keith i thought the motors had to be at least 1.8 degrees / 200 steps

also do you know the shaft size?

Lastly i am only on the edge of understanding the electronics involed in all this, ive seen the 4 & 6 wire configs of the motors, this has 5, how doe that work.

Ive also sent Cross Morse all my specs for the pulley on the focuser shaft so i should by the looks of it i'll get a proper pulley secured to the focuser, the largets diameter i can go to is 64mm against a smaller pulley on the motor of 16mm. are those sized pulleys good enough for it to be geared right?

[Mav359]

ive asked about the belts

[Mav359]

So now i'm looking at something like this

[keithatrochdale]

Hi

This was a project I was looking at a while ago, so is not fresh in my mind.

The output shaft is 5mm dia.

You may need to experiment with pulley sizes to give you the max speed of focus you need. The stepper code can be written to drive the motors at a 'fast speed' to get to rough focus and then at a lower speed to give 'fine' focus. It may well require a bit of experimentation to get things as you like them.

I was looking, initially, at a stand alone system using something like this;

http://www.ebay.co.uk/itm/Stepper-Motor-Driver-Controller-Board-Speed-Adjustable-with-Remote-Control-new-/281170834994?pt=UK_Gadgets&hash=item4177167632

The motor, control boards and battery would be on the scope and the remote would control it.

[Mav359]

the reason i went with the easy stepper driver & Arduino is because the whole assemble can be controlled via the PC, i have had a look at the standalone ASCOM compiant software and that appears to have speed control as part of the software.

[keithatrochdale]

Hi completely understand that. My thoughts were, get it working alone first and later integrate it into a bigger system. All it would mean is unplugging (or switching) the stepper motor and plugging into another board connected to say a bigger arduino system.

That way it can be completely portable or part of a system, also I would learn what speeds for focusing I like and fine tune the gearing.

And once developed, it could be applied to other scopes with different types of focuser; for instance the output shaft could be coupled in place of the focus knob on many scopes.

[Mav359]

As usual i've jumped in not knowing all the facts.

The Easy Stepper Motor is already here and the Arduino board is on the way as is that motor. I think i'll proceed first in just gettin the boards soldered where they need to be connect the whole thing to the PC and see if i can control the motor. Once thats all done i'll assemble it all and determin the pulley size and distance.

[NEM]

Lastly i am only on the edge of understanding the electronics involed in all this, ive seen the 4 & 6 wire configs of the motors, this has 5, how doe that work.

4 wire motors have 2 separate windings and are known as 'bipolar' motors because the current has to flow through the coils in both directions at different parts of the cycle.  They have to be driven by a 'dual H bridge' circuit.

5 wire motors have four coils with a common return line. They are known as 'unipolar' motors and can be driven by a simpler circuit using 4 transistors (or quite often a ULN2003A driver IC).

Finally, 6 wire motors have two coils, each with a centre tap.  They are the most versatile as they can be wired for either bipolar or unipolar use.  If you search for 'unipolar stepper motor' you'll find lots of info and circuits.

Noel

[keithatrochdale]

Sounds like a plan, you might think it worthwhile getting a breadboard and some jumper wires to work on your layouts;

http://www.ebay.co.uk/itm/Breadboard-Power-Supply-Kits-with-65pcs-Jumper-Wires-Cables-For-Arduino-Project-/261295920966?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3cd6735746

and maybe a PSU for it;

http://www.ebay.co.uk/itm/Breadboard-3-3V-5V-Power-Supply-Module-MB102-For-Solderless-Bread-board-Projects-/390684644737?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item5af69e9581

or

http://www.ebay.co.uk/itm/Dual-Output-Breadboard-PSU-with-7805-Perfect-for-Arduino-Self-Build-Kit-/221275565308?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item33850d2cfc

Have a look at some of the other kits.

You can start off by flashing LEDs, use 4 and flash them in series, much like driving a stepper motor!

And I am only just learning this too, there are probably many others who know what they are doing.

Which arduino board are you getting?