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Electric Focuser for Astromaster 130

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A few people have asked me for details of the focuser I made for my Celestron Astromaster 130.


All the details are on this site which I used to make it, so all the credit goes to the many people who worked it out before me.

I thought it would be easier if I put all the details in one thread instead of lots of links.

I've listed all the components I used and their part codes from Maplins.

RN41 Panel Mounted Speed Regulator Module

HF28 PP3 Type Battery Snap

N70BQ ABS Enclosure (box) 112x62x27

FJ89 Phono Plug

YW06G Phono Chassis Socket

FH07H Miniture Toggle Switch On/Off/On non locking both ways

FH98G SPDT Ultra Mini Toggle Switch Off/On

A length of speaker wire, 2mtrs should be enough.

First of all you need to remove the focuser wheel towards the base of the scope. This is held in by a small philips screw.

I managed to find a similar screw, but much longer in my spares. You will need one of at least 25mm long that will fit into the focuser rod and still be long enough to attach to the servo.

I'm sorry but I don't know the exact thread size, maybe someone reading this may be able to tell us.

Next you need to take the R/C servo apart. Different makes etc have different internals so basically all you need are the motor and gearing. All other parts ie wiring need to be removed.

On the main gear will be a raised stop, this basically stops the gear from revolving 360 degrees. This needs to be cut off with a sharp knife.


The next part needs to be taken slowly, have a drill bit the same size as your screw and drill out the centre of the gear where the original servo screw went, you will need to check how your servo gear is supported, you may only be able to drill down a short way. On mine I went all the way through. If you can't you'll need to cut the screw head off.

You can now connect the screw to the gear so the other end will screw into the focuser rod.

It should be Araldited into the gear once you are sure it is long enough.

The next stage is to drill a small hole in the servo base to allow the speaker wire through. Once done both cables at one end need to be soldered to the motor terminals. The servo is now complete and ready to be put back together.

The other end of the cable needs to be connected to the male Phono Plug.



It is better to screw the servo into the focusing rod so you can get the exact measurements for your bracket.

These are the measurements that I used for mine.


I made my bracket from a piece of aluminium I had spare and then sprayed it black.


If you look under the focuser there is a metal plate which holds it all together with 2 screws in. Once you make your bracket you can drill 2 holes in your bracket and use these screws for extra support if you wish, I didn't on mine and it is stable enough.

I also added a small piece of rubber sleeve to cover the screw thread.


The next stage is the controller. All the parts listed fit into the box, just!


You can see more or less where everything fits.

The hardest part was wiring and soldering the main switch as it is quite small and fiddly.

I have included a very basic wiring diagram and you can see how the switch is wired. It must be done like this as you need forward and reverse for the motor.


And here it is in the flesh.


Once everything is done and you are happy screw the thread into the focuser with a tiny bit of thread lock and do up tight. Attach the bracket and then fix the servo to it.

I included an on/off switch as in another thread as it was noted the controller would run the battery down even when not in use.

When I hold the box in my hand right hand I can use the focusing switch which is forwards for in and back for out. If this isn't right for you just swap the wires in the phono plug around.

Your left hand can adjust the speed.

Well there it is, any questions just ask. No more wobbles at high mag, using a webcam you can get the focus spot on, and you can still use the left side for manual.


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Nice one :)

I couldn't see mention of which servo you used and where from. What speed is the output? You'll want a nice slow motion drive, as you probably appreciate.

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Hi Gina

The servo I used was one I've had for at least 30yrs! It's an old Sanwa, but I'd of thought most servo's would do the job, they'll be slightly different inside though.

You just have to be able to mount a thread into the main gear.

I'm not sure what the speed output is for the controller but it can go very slow for pinpoint focusing and as fast as you would like, to fast really! It was just an easy option as it was ready to use straight from the box.


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