The standard Meade LX200GPS
microfocuser is not accurate enough for precision astrophotography. I
had a RoboFocus that I used to operate the primary focus knob, but this
precludes the use of the mirror lock. To get the best of both worlds, I
mated the RoboFocus unit to the microfocuser.
As you can see, this is a
very clean system. It also works very well. The step size, using the
minimum microstep setting on the RoboFocus, is approximately 7.5
microns. This is accurate enough for astrophotography down to at least
f/2 focal ratio.
The basic procedure for the
modification follows. In addition to screwdrivers and a selection of
tiny allen wrenches, you will need a drill motor and a 1/2" bit, a pair
of pliers, some silver solder, and a small torch or sizeable soldering
iron. Also, I did this with the LX200 version of the RoboFocus. I'm not
sure how or if other versions will work. RoboFocus is available from
Technical Innovations, Inc.
DISCLAIMER: Don't attempt this modification unless you are
reasonably mechanically inclined. You are fully responsible for what
happens to your RoboFocus, your microfocuser, and/or your person as a
result of your attempt to make these modifications.
Okay. With that out of the
way, here is what the completed unit looks like with one half of the
cover removed. Refer to this picture as you read the instructions.
- Remove the bottom bracket
from the RoboFocus unit. This is the part that you normally
sticky-tape to the rear cell of your scope. It is attached with two
screws and nuts.
- Remove the sprocket. You
will see that it is secured with a couple of pieces of wire that are
soldered into a brass coupler on the output shaft. What I did was to
break the wires off at the coupler and pull the sprocket off with
pliers. You might have to work it a bit to get it off.
- Heat up the brass coupler
with the torch or soldering iron to melt the solder that holds in the
remainders of the wires. Once the solder liquefies, turn the RoboFocus
over and shake the solder out onto the floor (or something else that
won't get damaged). Don't burn yourself. Note: I don't know if the
gears in the RoboFocus gearbox are metal or nylon or what. To avoid
melting anything, avoid heating things too much or for very long.
- You will notice that the
coupler is slotted at the now-emptied end. With a screwdriver or
similar prying-type tool, spread the ends open a bit. This is to allow
insertion of the microfocuser shaft later.
- Remove the cover of the
microfocuser. It is in two halves. The halves are joined to each other
with four phillips-head screws, and each half is attached to the
microfocuser itself with four allen bolts.
- Remove the gears from the
two shafts by loosening the set screws in their collars.
- Remove the little aluminum
bracket from the big aluminum piece. Note: don't remove the big
aluminum piece from the microfocuser body. The big aluminum piece is
pictured above and should stay put. The little aluminum bracket, that
is to be removed, isn't pictured.
- You now need to drill a
1/2" hole in the microfocuser cover to accept the RoboFocus output
shaft and the flange on its reduction box. The flange is what is
almost 1/2"; the shaft itself is much narrower. To correctly position
this hole, temporarily assemble one half of the microfocuser cover
back onto the big aluminum piece, as in the picture above. Point the
microfocuser shaft at your eye and figure out where the center of the
hole should be. You will find that the hole mostly needs to be in one
half-piece, but will extend slightly into the other half-piece. To
drill the hole, I found it helpful to drill a smaller pilot hole at
the desired center, screw the two plastic cover halves together, and
then drill the hole to the final 1/2" size. Without breaking it, clamp
the cover assembly while doing this, or the drill bit will grab when
it breaks through and mess up your hole.
- Slide the coupler of the
RoboFocus output shaft over the end of the microfocuser shaft. You may
need to widen the end of the coupler and/or try different angles to
get it to fit.
- Test out the positioning by
seeing if the plastic cover will still fit with the RoboFocus in
place. You may need to adjust how much of the microfocuser shaft is
inside the brass coupler. You can see in the picture above that the
flange on the Robofocus gearbox just protrudes into the hole in the
- Make sure the two shafts
are co-linear (i.e., straight). You don't want any binding when the
RoboFocus tries to turn.
- Set the plastic cover
halves aside so you don't melt them during the following step.
- Solder the coupler to the
- When things have cooled
down, check your alignment and positioning again. Re-heat and
re-position as necessary.
- Assemble the plastic halves
to the microfocuser aluminum piece and to each other. (You may want to
only use a couple of screws until you have tested things out.)
- Somehow get a piece of
sticky tape between the bottom of the RoboFocus gearbox and the
plastic cover. You can see it in the picture above. It's a bit tricky
to get it in there and get the RoboFocus positioned over it. See the
first picture above for a final position reference.
That's it! You should now
have a working unit. Hook it up to the computer and run it through its
paces. I found the following settings to be workable:
- Microstep pause: 3
- StepSize: 1
- Max travel: 3400
The way the microfocuser
works, it is its own clutch; the shaft will slip on the inner tube if
you try to drive it beyond its limits. I found that "in" on the
RoboFocus is indeed "in" on the microfocuser, so I define the minimum
RoboFocus setting to correspond to the max "in" position.
Also, I noticed that the
telescope tends to vibrate a bit after a move of the focuser, especially
when a heavy camera is attached to it. This was causing the star images
to be blurred a bit during autofocus. I think this happens because the
RoboFocus starts and stops so abruptly. You can either enter a longer
microstep pause to make the motion slower, or set a delay in your
autofocus algorithm so it waits a couple of seconds after a focuser move
(CCDSoft V5 has a setting for this). I did the latter and it seems to
Good luck! Feel free to
me if you have any questions.