Last updated: 23 January 2000

Focus Knob mod

From: James Chambers

I have enclosed picture of a simple add on focussing motor for the ETX telescope. The main features are:-

Focus Mod 1. It can be assembled from standard parts available in model shops or mail order.
2. It enables one to focus locally or sit indoors and remotely focus.
3. It can be connected by cable or without cables via a radio link.
4. Only one hole has to be drilled and threaded (in the focus knob)
5. The normal focus can still be used
6. The scope can still swing around fully in Declination.
7. Reasonably low cost.

Parts you will need:-

One Micro servo type S133 or 3101 by Futaba or similar Hi-Tec, size around 28mm by 13 by 29mm.
Two Snap on ball links, often used on model helicopters
One Short length of threaded rod usually around 2mm .
One Brass extender to fit the unthreaded end of the threaded rod.
Two Black plastic arms to fit the servo. These usually are free with the servo.
One Small screw and spring washer around 2.5 mm or 3/32 inch. 3/8 long

The picture hopefully says it all, but the following notes may help.

1. The focus knob is removed with the correct key, 1.25mm or 0.050 inch
2. Make an indent with a punch in the centre, drill and tap a thread for the screw, if you have access to a lathe so much the better. I have not tried it but you could drill a plain hole and use a small nut and bolt ( again use Loctite)
3. Replace the focus knob and tighten grub screw.
4. Cut down the 2 black servo arms leaving 3 holes for the ball links.
5. If the ball link threads won,t enter, open the holes with a hot wire or drill
6. Mount one arm on the focus knob with a spring washer under the head, (if you cannot find a spring washer bent a flat one) tighten the screw to enable one to still turn the knob even when you hold the black arm stationary. This is a simple slipping clutch. Put some Loctite or similar thread lock on the thread of the screw or it will come loose.
7. Screw the thread rod into one of the ball links and the extender into the other ball link.
8. With both black arms horizontal enter the threaded rod into the extender, cutting the length down until the rod enters the extender 1/4 inch. Solder or epoxy.
9. The servo can be attached by 3 methods, a. Double sided sticky foam tape. b. a complex metal bracket bolted to one of the 1/4 inch threaded holes in the OTA. c. Use a thick cyanacrylic instant glue. Use a very small amount or you will weld up your fingers or spoil the telescope with a run. The more usual cyan is very thin and goes mad travelling all over, I have used this latter method. It has the advantage that the servo can be removed with a sharp tap with hardly any disfiguration on the ETX black plastic molding. On some ball links finger pressure can ease the link off, then you can manually focus more easily.

There are only two ways of moving the servo, one is to plug it into socket 3 of a small model aircraft receiver. Then use the throttle stick to focus. If you use a very cheap remote control transmitter with 2 channels then take the back off and unhook the spring on the stick of your choice. The stick will then stay where you leave it instead of springing back to center. Note that most cheap outfits come with 2 servos but they are normally far to large to fit on the ETX. If you are a model flyer then all the above will be familiar. If two astronomers live close then make sure you use different frequencies otherwise it could be fun de-focussing your buddies scope!

The second way to move the servo is to purchase what is called a servo tester. This is a small box with a socket for a battery and one servo. A knob on the box will turn the servo to any position. Of course this time there must be a physical copper connection between the scope and the control for focus.

Note that servos do not normally move more than half a turn, but this is ample on my ETX to focus 3 different eyepieces. In use move the servo until the arms are horizontal then move the original silver focus knob until the image is sharp. From then on use the remote lever or knob. The whole thing took less than an hour and works like a charm. I don't know the USA prices of servos and radios but you will be surprised at the bargains you can find. I think Towers Models is one of you mail order houses if you can't find a hobbies shop.

If you cannot find a servo tester in the USA a small company here in the UK can supply at a total cost of $ 13.50 (thirteen dollars fifty cents) Their address is
MK Services, 18 Orchard Way, Cranfield,Bedfordshire, Code MK 43 OHU UK.
Terry takes cards. Tel. 01234 751 095 Fax is the same .

All the best wishes from..........jim.chambers@tesco.net North London England.

Could you add an appendage to the article as I have just found a company for USA readers who have a good site for servo motors and low cost transmitter receiver kits. They are http://www.towerhobbies.com/ (they can also supply the ball joints but a quick look did not reveal a servo tester,probably worth a ring but the UK price is good) The servo mentioned (S3101 Futaba) is $29.99, but there are cheaper micro servos of other makes like Hobbico at $19.99, just make sure its a micro size similar to the dimensions in the article. The tester which drives the servo without radio remote control is only $13.50 from here, which is a lot less than the scandalous price here of $180 for the Meade focusser.

Thank you for taking the article for your spendid site.

Cheers..........Jim Chambers

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