Meade LX200 Classic and GPS Focuser

Transcription

Installing Albrecht Digi*FocusTM on Meade LX200/
LX200GPS / LX200R telescopes
The Albrecht Digi*Focus (patent pending) provides a means for precisely determining coarse focuser
position on Meade 7” through 12” LX200, LX200GPS, and LX200R telescopes.
Albrecht Digi*Focus advantages:
• Numerical representation of focus position permits repeatable setting of focus, instead of tedious trialand-error turning of an un-indexed knob
• Ability to quickly and confidently achieve coarse focus when changing various attached apparatus,
without actually observing or imaging to monitor focusing progress
• Focus position is displayed throughout the entire focus travel range (20 or more turns of the focus
knob)
• Simple installation using ordinary hand tools
• No permanent telescope modification required; installation is completely reversible
• Ability to display “absolute” or “relative” focus position (i.e. absolute from one extreme of focus
travel, or relative from an arbitrary focus position by mechanical zero adjustment
• Elimination of ambiguity in determining which direction to turn focus knob
• Ability to return to preset focus in situations where several observers may re-adjust or mis-adjust focus
(i.e. public viewing)
• Ability to compensate for thermal effects on focus position
Although the Digi*Focus can be installed with an unmodified standard Meade focuser assembly, to reduce
focus knob turning effort the installation kit includes roller thrust bearings, hardened races to prevent
premature wear, and a shorter focuser bushing to accommodate the bearings and provide a larger thrust
surface to match the bearings.
1. Know thine enemy: How the focuser works.
On the Meade LX200 family of telescopes, the coarse focus knob works by moving the primary mirror
forward or backward within the telescope tube. It does this by pushing or pulling on a thin (1/8”) rod – the
focus arm – which sticks out, radially, from the mirror mount. Pushing or pulling on this rod transfers the
motion to the primary mirror, which is fitted with a central tube that is free to slide on a slightly smallerdiameter baffle tube. This is very easy to see if you look into the front of an LX200 telescope while you (or
an assistant) operate the focusing knob. You will see the sliding action of the mirror along the length of the
longer baffle tube. You will also note how little the mirror actually moves to achieve a large range of focus.
This brings up one important point about improving focusing action on the LX200 family, but it has
nothing to do with the focuser assembly itself. These tubes slide on a layer of grease. In addition to its
lubricating function, the grease acts as a “packing” between the tubes. If the grease is not evenly
distributed, the primary mirror has greater freedom to “rock” as focus direction is reversed. This gives rise
to the notorious phenomenon of “image shift” as one is moving the focus knob back and forth – an object,
especially at high power, will be seen to jump sideways in the field of view as focus direction is changed.
The solution for this is simple: redistribute the grease occasionally. The procedure will be given below, as
part of Section II, Disassembly.
The focuser assembly consists of several precisely machined parts. A black-anodized aluminum focuser
bushing (sometimes called the “top hat” for obvious reasons) is attached to the rear of the telescope by
three stainless steel button head socket capscrews. This focuser bushing is bored to accept a precisely
machined brass focuser shaft. The shaft is internally threaded with ¼”–28 threads to match the stainless
steel focuser screw, and its outside end is externally threaded with ½”–20 threads for the focus knob. The
outer end of the focuser screw has a small socket head capscrew, Loc-tited [cemented] into place, and a
Digi*Focus installation – page 1
loosely trapped washer; these serve as focus travel stops. The inner end of the focuser screw has a drilled
cross-hole which slips over the rod of the focus arm that extends from the mirror mount. In its factory-stock
form, the focuser assembly uses stacks of plastic washers as thrust bearings, between the focus knob and
focuser bushing, and between the focuser bushing and a flange on the inner end of the brass focuser shaft.
A picture is worth a thousand words.
Note (from left to right) aluminum focus knob, black focuser bushing (small end goes inside the telescope),
stack of thin plastic washers, flange of brass focuser shaft, stainless focuser screw with cross-hole in end.
Here’s an exploded view.
Note plastic washers, some of which fit into a machined “pocket” in the focus knob, the focuser shaft with
its focuser screw, stop screw and washer. Note also the necked-down area of brass focuser shaft, on which
the two setscrews from the focus knob are intended to bear (to avoid damaging the threads). Brass is soft;
note divots left by setscrews.
2. Disassembly
This procedure may be performed with the telescope mounted in alt-azimuth or polar mode, or with the
telescope dismounted and suitably supported, for example, by resting it in its carrying case, or lying on a
couch or workbench.
1.
Support the telescope in an approximately horizontal position.
2.
Make sure the focus lock is unlocked.
3.
Turn the focus knob counterclockwise to its stop. This will extend the focus screw and move the
primary mirror as far forward as possible.
4.
Using a 5/64 or 2mm hex key (“allen wrench”), remove the three stainless steel button-head cap
screws holding the focuser assembly to the telescope.
If you should lose or damage any of these, replacements may be found at any good hardware store: #632 x 5/16” button-head cap screws.
5.
With the black-anodized focuser bushing now disconnected from the back of the telescope, gently pull
the entire assembly back. As long as the focus screw is still hooked onto the focus arm, it will not come
off the telescope yet. So don’t pull too hard. You should have about an inch of travel.
Remember the explanation of how
the mirror slides on a greased tube?
(Section I, above). This is a good
time to make sure the grease is well
distributed on that tube. Now, if the
focuser were still bolted to the
telescope, this would mean cranking
the knob back and forth between its
limits of travel, several times. This is
tedious and time-consuming. But
since we have disconnected the
focuser bushing, we can just
manually move the focus arm by
using the focuser assembly as a
convenient push-pull handle. Do this
perhaps ten times, that should be plenty. In fact, it’s not a bad idea to do this every six months to a
year, or anytime you notice that focus shift has become excessive (image jumps too much when you
reverse focus direction). This alone is one of the simplest, yet most noticeable things you can do to
improve your viewing experience; having objects jump out of the field while you’re twiddling the
coarse focus at high power is annoying. Note that this cycling of the focus travel is sufficient to do as
much as you reasonably can to eliminate focus shift; you can never get rid of all of it, as that’s the
nature of the beast. Nor should you consider opening the telescope to regrease it; at some point, it’s
best to leave well enough alone.
6.
Now you can remove the
focuser assembly from the
telescope. Just slide the focus
screw radially outward, off the
focus arm.
7.
If you are going to complete any planned modification in a single session, it’s not necessary to lock the
primary mirror (with the locking knob or transport screw). The only reason to do this is to keep the
primary mirror from slamming back and forth if you should move the telescope without the focuser
assembly installed; there would be nothing to hold it. So use your own judgement whether you need /
want to lock it or not.
8.
To prevent insects, dust or debris from
entering the telescope while the focuser is
removed, cover the hole with tape or a
suitable plug. (It doesn’t take long for a
tiny spider to crawl in, and start weaving
cobwebs in the optical path).
9.
Now that you have the focuser assembly removed, you can work on it on a convenient workbench or
table. It’s a good idea to put down some clean paper so that if you should drop any small parts they
will be easier to find.
10. This is a good time to familiarize yourself with how the mechanism operates.
a)
Hold the black-anodized bushing in one hand. Try to push or pull the focus knob axially (i.e. in
and out); does it move? That’s the primary source of backlash. When you reverse focusing
direction, you have to take up that excessive play before you start to move the primary mirror.
b) Place something like your allen wrench in the hole at the end of the focus screw and rest it against
the table or bench surface to keep the screw from turning, or clamp the allen wrench in a small
vise to act as a third hand. Hold the black-anodized bushing in one hand and turn the focus knob.
Notice how the focuser screw moves in or out. Turning the knob counterclockwise moves the
screw out; the brass focus shaft is effectively a trapped nut that engages the screw, and the focus
knob merely turns this trapped nut. So, turning the focus knob counterclockwise moves the screw
out, and pushes the primary mirror forward (toward the secondary mirror). (Counterintuitively,
this moves the focus outward – away from the back of the telescope. That’s just how Cassegrains
work).
c) This is why, when observing, it’s best to make your final coarse focus adjustment
counterclockwise. There will always be some mechanical backlash, but this pushes the mirror
forward instead of pulling it back, and tends to be a more positive motion (you’re pushing forward
against gravity, which is pulling the mirror back).
11. Now it’s time to disassemble the
focuser assembly. Loosen the two
setscrews on the focus knob. You
don’t need to remove them
completely, but they do need to be
backed out far enough to clear the
large thread on the brass focuser shaft.
12. While gripping the flange of the brass focus shaft with the fingers of one hand, unscrew the aluminum
focus knob with the other. Do not use pliers to grip the brass shaft or flange. Some people might tell
you that it won’t cause any damage if the marks are on the outside of the flange. It’s bad practice, yes
you can damage the brass enough to affect the action (especially with the needle bearing installation,
to come), and it’s not necessary. Wipe any stray grease off the brass and the grip of your fingers should
be enough; if you need extra grip, try grabbing the brass with some rubber, for example a piece of old
innertube, rubber gloves, or slip a rubber washer over the focus screw and press the black focuser
bushing down (by pressing two fingers on the focuser bushing’s wide flange) against a flat surface
using the rubber for extra grip. Once broken loose, the focus knob should unscrew easily (make sure
the setscrews are backed out far enough).
13. When you have unscrewed the aluminum focus knob, note that there may be some white plastic shims
stuck in a recess in the knob. Others may still be on the brass focus shaft. Note and record how many
there are on this side of the bushing. Remove these and set them aside.
14. Now you can slip the brass focus shaft and the stainless steel focus screw out of the black focuser
bushing. Again, note that there will be white plastic shims between the wide flange of the brass shaft
and the bottom end of the black focuser bushing. Again, count them, record the number and set them
aside.
15. This completes removal and disassembly of the focuser assembly. You won’t be needing the focus
knob, black bushing, or white plastic shims anymore for installation of the Digi*Focus. It would be a
good idea to save these in case you wish to revert to the factory original focuser configuration at a later
time.
3. Reassembly and Installation
The Digi*Focus kit includes the following items:

Focus counter
Focus knob with three setscrews
Adapter plate
Shortened, larger diameter focuser bushing
(2) needle roller thrust bearings
(4) hardened thrust washers
5/64” and 2.5 mm hex keys
Syringe of low-viscosity, all-temperature red grease
1.
Begin by cleaning the existing grease from all moving parts.
a)
It is not necessary nor desirable to disassemble the stainless steel focus screw from the brass shaft;
to do this, you would have to break the Loc-tite bond on the long socket head screw (possibly by
heating it – another invitation for damage), then re-cement it when you’re done. This invites
problems, is not necessary, and would require running out to buy a tube of Loc-tite. As long as the
capscrew is still firmly Loc-tited to the focus screw, leave it alone.
b) If you wish, you can use solvent such as turpentine, mineral spirits, or paint (not lacquer) thinner
to help clean the grease from the focuser parts, but it’s not absolutely necessary. A good wiping
with clean paper towels is more than sufficient.
2.
The Digi*Focus kit includes a syringe of red synthetic grease, which has been formulated to work well
even when used on machinery operating at extreme Arctic temperatures. It is not quite as “thick” as
Mobil 1. (Photographs that follow show a white Teflon grease used on bicycles; the procedure is the
same for the red grease packed with the kit).
3.
Begin reassembly of your cleaned
parts by smearing a dab of grease
under the flange of the brass focus
shaft. Most of this will be squeezed
out during assembly, but try to get a
good film all the way around the
flange.
4.
Slide one of the four hardened thrust
washers onto the focus shaft, against
the greased flange.
5.
Grease one of the needle roller thrust bearing
assemblies. Pack grease into both sides, using your
fingers. Don’t worry about using too much; excess
grease will be squeezed out during assembly. Too
much is better than too little as it’s “selfregulating” (excess is squeezed out during
assembly).
6.
Slide the needle thrust bearing onto
the brass focus shaft, against the thrust
washer.
7.
Drop a second hardened thrust washer
onto the needle bearing. Grease its
exposed face.
8.
Grease the focuser shaft and slide the black focuser
bushing onto the shaft. Note that the Digi*Focus kit
uses a shorter, precision machined and anodized
focuser bushing to allow enough space for the bearings
and hardened washer package.
9.
Grease one face of the third hardened
thrust washer and drop the greased
face over the focus shaft and against
the flange of the black focuser
bushing. Note that shorter bushing
exposes a longer “pilot” section of the
brass focuser shaft, to better locate the
bearings and washers.
10. Pack grease into the second roller thrust bearing. (See Step 5, above).
11. Slide the second bearing onto the
shaft, against the thrust washer and
focuser bushing.
12. Drop the last hardened thrust washer
over the shaft. Grease the exposed
face.
13. Grease the large thread on the focus shaft.
14. While holding the brass flange of the focus shaft in one hand, thread the focus knob onto the focus
shaft. Grasp the brass flange firmly, and tighten the aluminum. Do not use pliers! Hand tight is plenty
tight enough, and may even be too tight; we’ll check this later. Again, this would be a good time to
wipe away excess grease.
15. The assembly should looks like this.
16. Gently tighten the three setscrews. Tighten
them alternately, in stages, to ensure that the
focus knob is not tilted out of axial
alignment. Final tightening should be firm,
but don’t overdo it. The screws are
bottoming against soft brass and excessive
force isn’t needed to seat them.
17. Check the focuser action for smoothness and ease of movement. “Exercise” it a bit by holding the
black focuser bushing and twisting the knob back and forth. (I like to put the knob in a lathe, at low
speed, and hold the bushing in my fingers). If it feels “lumpy,” chances are it’s not dirt or surface
irregularities; you probably have too much load on the bearings. Loosen the setscrews, back off the
focus knob slightly, reseat it but with a little less force this time, and again tighten the setscrews
sequentially. If the focuser is too loose (to the point where it has axial play – you can wiggle the knob
in and out relative to the black focus bushing) you didn’t tighten it enough. Loosen the setscrews, get a
firm grip on the brass flange, and tighten it slightly. Retighten the setscrews, sequentially. If you need
more grip, do not use pliers. Using a piece of rubber, a rubber washer, or rubber gloves to provide
added grip. (See Section II, Disassembly, Step 12).
18. Put your allen wrench or any other convenient pin through the hole in the end of the focus screw, and
spin the screw out all the way (or just use your fingers on the cross-hole end). Lightly grease the
threads; most of this will be forced out anyway. Run the screw back into the shaft as far as it will go to
get grease into the threads. Wipe away the excess grease. Run the screw all the way out again.
19. Temporarily tape the adapter plate to the
back of the telescope, with the three
mounting holes aligned and the recess for
the focuser bushing facing outward. You
may find it useful to use a hex key or
other convenient object as a handy “pin”
on which to hang the plate while taping.
Remove the pin after the plate is taped
down.
20. Make sure the focus lock, if equipped, is
fully “off.” Insert the focuser assembly
through the hole in the adapter plate and
telescope backplate. Hook the crosshole in
the focuser screw over the focus rod
inside the telescope.
21. Turn the focus knob about five turns clockwise. This is to avoid any possible interference with
telescope internal parts that may occur if the screw is fully extended when the assembly is tightened
into place. Push the focuser assembly into position, seating the flange of the focuser bushing in the
adapter plate recess.
22. Re-install the three button head cap
screws through the focuser bushing and
the adapter plate. This clamps the adapter
plate into position on the back of the
telescope. Remove the tape used to hold
the adapter plate to the telescope.
23. Install the adhesive-backed foam gasket
against the underside of the counter. Slip
the counter over the focus knob and insert
its locating pin in the corresponding hole
on the adapter plate. Do not force the
counter down against the adapter plate;
just having the gasket touch, or even have
a little bit of clearance, is best. Otherwise,
the counter may be tilted, increasing load
on its internal bearings. Do not tighten the
setscrew on the counter yet.
24. Turn the focus knob fully clockwise, to
the limit of its travel. Don’t force it.
Generally, this is a good choice for your
focus counter’s “zero” position: it
represents the farthest “inward” focus
position, toward the front of the telescope.
(Turning the focus knob counterclockwise
from this limit will push the mirror
forward, thereby moving the focus
backward, farther “outward” from the
telescope). With the focus at this limit,
turn the black steel collar of the counter to
set the numbers to zero. Using a 2.5mm
hex key, tighten the counter setscrew
against the focus knob. (Alternatively, you may choose to have some often-used focus position as your
zero. The disadvantage is that when inward of this focus position, the counter will not read negative
numbers, but rather numbers like 99x.xx Loosen the setscrew, set the desired focus position, set the
counter to zero, retighten the setscrew. It is not recommended that this procedure be done routinely as
it will eventually cause many setscrew marks on the focus shaft).
25. Turn the focus knob counterclockwise, away from its limit position, and check for binding (knob turns
easily for part of each turn, stiffer for other parts). If you think there’s any binding, confirm that it is
caused by the counter: temporarily loosen the counter setscrew and pull the counter away from the
adapter plate to disengage the locating pin, to see if the localized (over only part of each rotation)
binding goes away. If so, the counter had been forced against the adapter too tightly (see step #23,
above) or the pin is too tight in the locating hole, and there is a side load on the focus knob for part of
each rotation. (Because of the additional gears and parts inside the counter, focus action will inevitably
be somewhat stiffer than the plain, original focuser, but we want to make sure that tolerances aren’t
adding up to make the focus knob “run out” too much, stiffening it excessively anywhere in its
rotation). There are three remedies: a) don’t force the counter against the adapter plate; b) there is some
play (very little; a few thousandths) between the three mounting screws, focuser bushing, and adapter
plate; remove the counter, barely loosen the three button head capscrews, wiggle the adapter to a new
(random) position, and retighten. Reinstall the counter and try again. This is a trail and error process. c)
If unsuccessful after a few attempts, remove the counter, and carefully sand or file a little bit (only a
thousandth or two) of material from the top and bottom of the alignment pin, to let it “float” vertically
in the adapter plate locating hole. This is a noncritical fit, as long as you don’t overdo it; even a few
thousandths of side play won't affect focus counter readings enough to be readable, and vertical play
won’t affect it at all.
26. Done!
4. Assembly drawing
5. Hints for using the Albrecht Digi*Focus
Technical note: the Albrecht Digi*Focus counts turns of the focus screw, to a numbered resolution of
1/100th of a turn, subdivided by lines to 1/1000th of a turn. A yellow vertical stripe between the third and
fourth (of five) digits indicates the decimal point. The focus screw has 28 threads per inch, so one turn is
1/28th of an inch, and 1/100th turn is 1/2800th of an inch or 0.000357”. For a typical, nominally f/10 SCT
system with an f/2 primary and 5x secondary amplification factor, this movement becomes magnified not
5x, but by a factor of 52 = 25. In other words, 1/100th turn of the focus knob will move the focus by about
0.009”. This is why moving your primary mirror only an inch or so can change focus position by about two
feet.
1.
When you focus your telescope with a given viewing or imaging configuration, note the Digi*Focus
readout and exactly what hardware you have in the optical path – microfocuser (or none), adapters,
spacers, flip mirror boxes, filter wheel or individual filter, camera, eyepiece, etc. Also note the ambient
temperature; because of that 52 = 25 factor, small changes in mirror separation due to thermal
expansion or contraction can have a visible effect on focus position. Over time, you will build your
own “lookup table” of commonly used coarse focus positions for all of your viewing or imaging
situations.
2.
If you use an on-axis microfocuser, set this to the center of its range before coarse focusing; that way,
you can use the Digi*Focus to quickly set coarse focus “in the ballpark,” for fine tuning with the
microfocuser. You’ll find that Digi*Focus is repeatable and accurate, saving valuable time when
changing setups.
3.
When imaging without the benefit of a microfocuser, and dealing with the extremely sensitive coarse
focus, you can use Digi*Focus to obtain a “V curve” of a focus star’s FWHM size (using your
camera’s software if so equipped) vs. focus position. Because of backlash in the mechanism inside the
telescope, remember to move through focus in small steps (perhaps as small as 1 count on the dial), in
a consistent direction. Once you have determined which number provided best focus, go back and
approach your focus position from the same direction. Usually, this will be by turning the knob
counterclockwise to push the mirror away from you.
We hope you enjoy your Digi*Focus. Clear skies!
Pete Albrecht
2339 Notre Dame Rd.
Costa Mesa, CA 92626
E-mail: [email protected]
Digi*Focus web site: http://www.petealbrecht.com/digifocus