ETX TECHNICAL TIPS Last updated: 15 September 2003

Homemade Focus Cable

Since you posted my comments on the ETX 90 RA at Sam's Warehouse the other day, I have received several requests for a parts list and instructions for making a low cost flexible focus control adapter. Since there seems to be some interest in this and I have already written it up, I thought I would pass it along with a picture for possible inclusion on your Tech Tips page.

Thanks again for all of your hard work in making your site available. It is absolutely killer!!!

Regards,

Dooner

Low Cost Flexible Focus Control Adapter for ETX 90

All parts excluding original ETX focus control knob were obtained at my local Ace Hardware store (nuts and bolts isle).

PARTS:

(1) 5/16" X 1" aluminum spacer (55 cents)
(1) 5 to 6" long piece of 3/32" plastic covered wire cable (29 cents per foot)
(2) 6 - 40 X 1/8" allen head set screws (20 cents each)
(1) original ETX focus control knob

TOOLS:

drill
3/32" and 1/8" drill bits
center punch
hammer
small vise grips
wire cutters
180 grit sandpaper
small and medium allen wrenches that come with the ETX 90

ASSEMBLY INSTRUCTIONS:

(1) cut a piece of the wire to length (5 to 6" or what ever length you want)

(2) clamp vise grips on the spacer to hold it in place and make a small dent with the center punch and hammer about 1/4 to 5/16" from each end on the outside surface of the spacer. (be careful to strike the center punch only hard enough to make an indention for starting the drill bit and not so hard that it flattens or affects the roundness of the hole through the center of the spacer)

(3) with spacer still in the vise grips, drill a 3/32" hole (through one side of the spacer) in each dent made with the center punch and then re-drill the holes again with the 1/8" bit. (you may be able to drill the holes only once using just the 1/8" bit but not having a drill press, I found it easier to start the smaller bit first)

(4) using the medium allen wrench that comes with the ETX you can tap the holes drilled in step #3 above simply by threading one of the set screws into each hole. (take care to thread the set screws in straight and not at an angle)

(5) once the holes are tapped, remove the set screws and roll up a small piece of the sandpaper and run it through the lengthwise hole in the spacer to remove any burs or metal filings from drilling the holes so that it will fit on to the focus control shaft.

(6) using the small allen wrench, remove the stock ETX focus control knob and attach it to one end of the wire cable.

(7) insert and attach the other end of the wire cable to one end of the spacer allowing the wire to enter about half way through the spacer.

(8) attach the open end of the spacer to the ETX's focus control shaft.

(9) enjoy a more easily accessible ETX focusing mechanism. (adding a clothes pin to the ETX's focus control knob greatly improves ability to "fine tune" the focus)

Subject:	 Homemade Focus Cable -- Modifications to Dooner's design
From:	stantstk@pacbell.net (Stan Glaser)
First, my flexible focus cable modification is in no way an attempt to
lessen the original write-up or design sent by Dooner (GJMIII@aol.com)
and posted on this page. He is very happy with his design and it works
well for him. But I had e-mailed Dooner and discussed two of the three
"problems" listed below and discovered that he has his cable attached to
an older ETX-90RA model which was removed from its original base and
then mounted on to a DS mount so he could gain GOTO capability.

Dooner's design costs about $1.25. Mine costs a few dollars more. I
still think that although the Meade Electronic Focuser is probably the
-best- choice, at $119 it is still way too much for a tiny motor, some
gears, and a plastic housing. Scopetronix Flexi-Focus seemed the other
better (-cheaper-) alternative, but it, too, is way overpriced at $35
for a simple 6" cable and knob. There had to be a less expensive way.

There are 3 aspects of Dooner's design that I had a problem with: (1)
the 3/32" plastic-covered cable wire, (2) reusing the original ETX-90
focus knob, and (3) trying to self-tap threads for the 6-40 x 1/8" allen
head set screws.

(1) The 3/32" plastic-covered cable wire is just too stiff to make the
sharp radial curve required when the scope is aimed high. It is forced
to bend against the ETX-90EC base, putting stress on the mirror focus
shaft, and possibly causing stress on the Dec motor as well (or causing
Dec clutch slippage). Since Dooner mounted his OTA on a DS base, there
is nothin to interfere with the cable dangling off the back at any
angle. But on an unmodified -RA or -EC, the problem exists.

Instead, I purchased a Speedometer Cable Repair Kit from my local auto
parts house (~$4.00). It is extremely flexible and comes in a length of
120" (plenty to spare in case you make a mistake!!). It has a diameter
of 1/8" which leads to other design changes. It's missing the plastic
coating to protect the ETX base (just a nitpicky observation), but it's
that plastic that prevents the 3/32" cable from bending easily. I cut my
cable to a length of 4-1/4".

(2) Dooner reused the original ETX focus knob, which saves some money,
but doesn't gain any real degree of finer focusing control as does the
Scopetronix Flexi-Focus.

I found a knurled rim aluminum instrument knob at a local electronics
supply house (NO, NOT Radio Shack!). It is an Augat(r) Alcoswitch Stock
No. 92002, with a 3/4" diameter, a .630" height, and fits onto a 1/4"
shaft, leading to another design change. Cost was $3.87.

To accommodate the new 1/8" cable, I found Augat(r) Alcoswitch Stock No.
92015 "Shaft Hole Reducers" (2/pkg) which are perfect. Cost was $.94.
(Note: It was extremely difficult to find any knob (I never did) at
local stores which DID NOT have a 1/4" shaft hole -- this seems to be
the standard size -- the ETX knob fits the 3/32" plastic-covered cable
wire that Dooner picked, which is probably why he stuck with the
original knob).

(3) I have tried to self-tap threads using the male thread before;
sometimes you win, sometimes you lose. 6-40 x 1/8" allen head screws are
very small, and tapping through the 5/16" spacer in Dooner's design
should be easy because it is quite thin. (I also decided to use a 3/4"
long spacer, not the 1" length that Dooner used.)

But I prefer to do a real tap rather than trying to force one. Only
problem -- it was difficult finding a 6-40 tap at any local store, so I
decided to switch from the 6-40 x 1/8" allen head set screws to 6-32 x
1/8" set screws. Cost is the same at $.20 each. Besides, the 6-32 tap
can be found easily. Also, I now needed a way to fit the 1/8" cable into
the spacer, which had a 3/32" inner diameter. I found a Teflon bushing
with a flared end which fit perfectly inside the spacer and stopped it
down to 1/8" to fit the cable. Cost about $.15. The spacer, the allen
head screws, and the bushing were all found at Ace Hardware in their
Nuts and Bolts section. They also carry the 6-32 tap.

I followed Dooner's original assembly instructions, but on the end of
the spacer with the Teflon bushing, I inserted the bushing all the way
into the spacer and then drilled the 1/8" hole through the spacer AND
the bushing. Then I tapped both holes on the spacer; one through the
spacer alone, and the other through the spacer and bushing. The 1/8"
cable fits into the bushing and is held with the allen head screw, and
the other end of the cable goes into the 1/4"-to-1/8" shaft hole reducer
which fits into the aluminum knob and held in place with its own
supplied allen head screws. The entire assembly fits onto the ETX focus
shaft.

I found that with the focus cable installed, there really is very little
vibration from the cable when it is just hanging off the back. If there
is any oscillation, it's a simple matter of cradling the knob with two
fingers to stop it from jiggling, and it damps down almost immediately.
Even when the ETX is performing a slew at high speed, when it reaches
its stopping point the cable barely wiggles. And when the OTA begins to
aim high, the cable bends easily under the focus shaft between the rear
of the tube and the base of the scope. The larger diameter knob also
allows for a finer focusing adjustment.

Subject:	Homemade Focus Cable
Sent:	Sunday, September 23, 2001 13:25:03
From:	ken.thorpe@surewest.net (Ken Thorpe)
First of all, thanks for a great site!!!  I haven't had a chance to go
through all of it, but have really liked what I've seen so far.

In the Modifications Section of your Telescope Tech Tips, I read with
interest the inputs on the Homemade Focus Cable for my ETX.

I built one similar to the one described by GJMIII.  I didn't think I'd
be that happy with using regular wire so I checked around my hardware
store and found some large (0.130" diameter) weedeater replacement line.
It fits snugly into the existing focus knob and the aluminum spacer. 
They had a card of ten eight-inch pieces in dayglow orange.  Not only is
it quite "torque resistant", it is easy to see in the dark!

Ken Thorpe

Subject:	Easy homemade $2 ETX-70AT focus knob extension
Sent:	Sunday, April 14, 2002 17:45:47
From:	Stribula@aol.com
I just made a good focus knob extension for my ETX-70AT from a few
inches of clear vinyl tubing from Home Depot (3/8" inner diameter works
perfect, at 19 cents a foot).  It's snug enough to stay on, yet not so
much that it's a hassle.  The tubing comes with a pronounced curve in
it; it put it over some wood doweling and ran a blow dryer over it to
soften it up & then let it cool off overnight.

I also got a plastic "T" connector ($1.45) to put on the end of the
tube, to allow for finer control.  Compare less than $2 with the $20-30
models on the market.

Subject:	Flexible Focus for ETX - cable hint
Sent:	Thursday, January 9, 2003 9:18:03
From:	edelmann@hcst.net (Edelmann)
In the posting :  

Subject:	 Homemade Focus Cable -- Modifications to Dooner's design [above]

article, Stan mentions the fact that the speedometer cable that he used
isn't coated.  However, if you have some vinyl spray laying around
(blue, red, black, etc.) ... you can simply spray coat the cable, as
thinly as necessary, and be able to accomplish the same goal... and even
make it black....!

john.

From:	stantstk@pacbell.net (Stan Glaser)
Hi, John --

I had thought of doing something like that, too -- but figured that
spraying anything on a flexible cable would probably either end up
"cracking" from the flexure or wearing off on the ETX base if it rubbed
after a period of time. Another option I had thought of was using
heat-shrink tubing (used for soldering) along the entire length of the
exposed cable, but having used that before, I noticed that it becomes
stiff. When I pack up the ETX in its case for traveling, I just flex the
cable and tuck it up into the space between the rear of the OTA and the
RA lock handle in the base. I've had it on a long time now, and haven't
noticed any damage to my ETX base over and above any regular bumps and
scratches that might occur during regular use.

But thanks for the suggestion, and hey, if it works for you (and
anothers that make their own cable), then go with it!!

Stan Glaser

Subject:	photos for home-made flexi-focus cable...
Sent:	Tuesday, August 26, 2003 13:53:53
From:	stantstk@pacbell.net (Stan Glaser)
Just saw a recent post where someone's asking about the Flexi-focus
cable, again -- attached are 3 photos that show detail closeups of what
I made. Would you add them at the end of my original write-up describing
how I changed Dooner's design? (in the Tech Tips section) It's been a
long time since I sent you THAT e-mail, but a picture is worth a
thousand words, and in this case it might help beginners get a better
idea of how to build one.

Thanks!!

Stan Glaser
stantstk@pacbell.net

Subject:	re: ETX TECHNICAL TIPS - Homemade Focus Cable
Sent:	Thursday, September 4, 2003 12:04:27
From:	mpkboss@yahoo.com (t.s. lee)
Here's more info for the recent thread at
http://www.weasner.com/etx/techtips/focus_cable.html

There's an off the shelf alternative to threading a aluminum spacer. 
Shaft couplers are exactly what we're all trying to make here.  They
come in various inside diameters, commonly 1/4" but found in other sizes
too.  They're used to connect a small electric motor shaft to another
mechanism.

I called around here in Southern Calif, and I found them at a surplus
shop (motors, gears, etc), and a regular hardware store.  Cost is around
$2.  I was told that hobby shops (RC car and airplane shops) carry this
too, but more expensively at $5-20.

As for the shaft, maybe one of those flexible magnetic pickup tools can
be used.  Not sure if they are more or less flexible than a speedo
cable.

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