IntroductionThe Santa Barbara Instruments ST4 autoguider is by far the most
popular autoguider in the world, and for good reason: It is arguably the best small
autoguider ever made. It's simple to use, very adaptable to almost any kind of mount, and
guides very accurately. It's so good that this inexpensive device is used to guide many
large observatory telescopes, including the world's largest wide-field telescope, the
48-inch Oschin Schmidt at Mount Palomar.
The ST4 is simple to use, but it can be a challenge to find the settings that work for
your scope and mount, and therefore at first it can be rather frustrating. The following
is a set of instructions for getting started with the ST4, including how to set it up and
how to acquire guide stars with it.
Getting started
OK, first things first: On any new mount & guider combination, practice guiding with a
short focal length before you try guiding with a long one. If this is your first time
using this guider and mount, start with a very short scope. A short (400m - 600mm FL)
refractor is ideal. Forget about a long-focus scope for now. If at all possible, set up
the mount with just a short scope on it, removing the larger scope altogether.
Next, on the initial session forget about trying to take a real picture. If you're trying
to take a photo during your learning session you'll be very frustrated with each little
thing that goes wrong and the whole learning experience will be infuriating instead of
enlightening and fun. Don't put that pressure on yourself. Make it a pure learning session
of exploring what the guider can do, and don't even bring the camera with you.
Also, before you start playing with the guiding, make sure you're properly polar aligned.
Do a good drift alignment, don't just use the polar scope. The polar scope gets you close,
but not close enough to avoid polar rotation, which by itself can make guiding difficult.
The drift alignment procedure is described in most how-to books on astrophotography. It's
a bit tedious, but not very difficult and certainly more effective than anything else.
Now that you're using a short scope, you've put the camera at least 50 feet from the
scope~, and are accurately polar aligned, you're ready to begin.
Start by making a preliminary check of the seeing. Using a high power eyepiece (150x or
more), take a look at a bright star in the direction where you want to guide. Throw it
slightly out of focus and examine the concentric-rings outline of the star image. If it's
bouncing or being distorted a lot, you're going to have poor guiding. Try pointing in
another direction, or higher in the sky. If neither of those helps then it might be a good
night to get the binoculars, enjoy the wide views, and try the guiding another night.
Trying to achieve fine guiding on a night with poor seeing is like banging your head
against the wall: Persistence will only get you a headache.
Another thing to check: If when you throw the bright star slightly out of focus the
concentric rings aren't exactly concentric, your scope is out of collimation. Collimate it
properly before proceeding.
Assuming the seeing is decent and the scope is collimated, power up the ST4, leave the cap
on the guider head, and let it sit for a few minutes until the readings stop changing.
When they stop changing that indicates that the guider head is cooled to operating
temperature.
Without removing the cap from the guider head, take a dark frame, then go back to
find-and-focus mode. You should be getting nothing but zeroes in the X and Y numbers. If
you don't then either the guider cap is not light-tight (easy to check), the head wasn't
fully cooled down, or something is wrong with the ST4. The latter is very unlikely, but
it's something to check the first time out or anytime you suspect a problem.
Basic settings for the ST4
If everything checks out OK, then enter the following settings in the ST4:
b = 1
Fl = S
SA = 4 (this assumes you are using a short scope)
bA = A (don't use faint mode for now)
C1 = 8
C2 = 8
H1 = 0
H2 = 0
AA = 0
AL = 0
(If you don't know how to enter these settings into the ST4, read the manual. This
article is not intended as a substitute for reading the manual.)
Setting up the mount
Power up the mount, if you haven't already, and set the guiding speed to 1x. If you're
using a Losmandy mount, set it to 2X, which is the equivalent of 1x on other mounts. (No,
I don't know why; that's just the way Losmandy did it.)
If your mount has a declination backlash adjustment, set it to eliminate all the backlash,
but be sure you don't go too far and cause it to overshoot on each correction. If you have
a Losmandy mount and haven't calibrated the TVC (anti-backlash) setting, set the TVC
setting to about the mid-point of its range, i.e., with the LED in about the middle. You
can fine-tune it later, but that's a good start. If you have an AP QMD mount, you'll have
to fiddle with the DIP switches. (Don't blame me; it wasn't my idea to make the backlash
adjustment such a pain to set.)
If your mount has PEC (periodic error correction), turn it off. (Yes, off.). The
only exception to that is the Meade LX200 mounts, which often have gears with such large
errors that they cannot be effectively autoguided without the PEC. Make sure the LX200 PEC
has been very carefully programmed, else it's worse than nothing.
Focusing
Now point the scope at a very bright star and center it in the field of view. Use a
crosshair guiding eyepiece to make sure it's really centered in the field of view. Once
it's centered, tighten (but don't over-tighten!) the clutches so the scope will not be
easily bumped from its position.
Remove the eyepiece and diagonal, and tape a piece of frosted package tape over the
focuser opening. (If you don't have frosted tape use some kind of material that is thin
and translucent such as a piece of ground glass or translucent plastic.) Once it's in
place, move the focuser (probably outward) until you have the image of the bright star
sharply focused on the tape or ground glass. Make sure it is as sharp as you can get it.
With the star sharply focused, draw a line on the focuser drawtube marking the position of
the focuser. (A pencil works well if the tube is not chrome-plated, else try a thin
marker.) This will be the starting position for focusing the ST4. It won't be exact, but
it will be close enough to get started.
But you can't guide on such a bright star, so now remove the tape, re-insert the diagonal
and eyepiece, and re-aim the scope at dimmer star, something of 4th magnitude or so. After
you've carefully centered the new star with the high-powered eyepiece, remove the
diagonal, return the focuser to the mark you made before, and insert the ST4.
Acquiring a guide star
You should now be getting some solid readings on the ST4's display. Using the hand guider
of the mount at the 1x speed, slew the mount until the X and Y numbers are both in the
range of 30 to 60, which means the guide star is somewhere near the center of the chip.
With the star now in good position, adjust the focus a *tiny* bit at a time to maximize
the brightness readout. Don't rely on just one reading; after each adjustment wait for two
readout cycles for the mount and guider readout to stabilize, then observe the next 3 or 4
readings to see if on average they are higher or lower than before. But remember to make
only tiny adjustments; you will likely have to move the focuser 1/4 inch or less in one
direction or the other to reach perfect focus. If you move more than that you are probably
heading in the wrong direction; go back to the marked position and try going the other
way.
If you find that the star brightness reaches 99, the star is too bright and you'll need to
find a fainter star. Before you insert the eyepiece, however, erase the old marker line
and mark the new one, which is now closer to exact focus than your original line was.
After you center the new guide star, go back to the marked position (the last one, not the
first one) and continue optimizing focus. Once you reach optimum focus, lock the focuser,
mark that position and erase all previous marks; that will be your new starting position
for next time.
With good focus now achieved and the brighness readout something reasonable (between 20
and 40 is about ideal, but anything from about 8 to 80 will work), stop the find-and-focus
mode, cap the scope, and take another dark frame. Remove the cap and go back to
find-and-focus mode; the readings should be similar to what they were before, but you now
have a more accurate dark frame. (The ST4's cooling is not regulated, so it tends to drift
as ambient temperature changes. Always take a dark frame before starting to guide.)
If the ST4 doesn't see the guide star...
If you are centered on a star in the guide scope but the ST4 doesn't see it, here are
some things to check:
- Check to make sure you're properly focused. Check again.
- Double-check to make sure it's really centered. Use a crosshair eyepiece; it's hard to
judge center on a conventional eyepiece.
- Increase the "b" parameter in the ST4 to 2, 3, or 4. Take a dark frame and see
if it sees the star now.
- If at a setting of 4 it still doesn't see it, switch to b=3 and set bA=F, take another
dark frame, and now check it. (Do NOT use b=4 with Faint mode; it can cause erratic
readings that will throw off your guiding.
- If it still doesn't see the guide star at all, re-check aiming and focus. Try making
tiny focus adjustments to see if the brightness increases. If after optimizing focus the
brightness is still too low, find a brighter guide star.
Calibrating the mount
Before calibrating, make sure of the following:
- The guiding speed is set to 1x (2x on Losmandy mounts)
- The ST4 guider cable is plugged into the mount controller
- The ST4 is oriented in the focuser so its rear label is eitherperpendicular or parallel
to the Dec axis, i.e., not at an oblique angle. On a Newtonian, rotate the tube so the
focuser centerline is either straight up or perpendicular to the Dec axis.
- The scope cap is removed (I can't tell you how many times I've forgotten this)
To calibrate, stop the find-and-focus mode and press the Calibrate button. Watch the
readings carefully as it's calibrating:
- On each calibration slew, see if either the X or Y number on the readout moves more than
3 units. If it doesn't you'll get an error. If you get an E1 or E2 error; increase the C1
parameter. If you get an E3 or E4 error, increase the C2 parameter. Try to set the C1 and
C2 values so you get at least 10 units of movement in each direction. That is not a
hard-set number; anything from 6 to about 20 will usually work fine.
Note: If during calibration either the X or Y number goes past zero or past 99, you'll
lose the guide star. Let it finish the calibration (which will usually bring the guide
star back onto the chip), then either decrease C1 or C2 to reduce the slew distance, or
re-center the guide star on the chip.
- If the X and Y values both move together, i.e. there is significant motion in both axes
at the same time on every slew, the ST4 is not properly oriented to the mount. It can work
that way, but it's easier to interpret readings if it's aligned to the mount. Rotate the
ST4 in the focuser so that it its label is aligned as cited earlier.
- If during the Dec axis calibration the movement is significantly more in one direction
than the other, try adjusting the TVC setting on the mount controller. If the second Dec
slew is shorter than the first, increase the backlash setting; if the second one is longer
than the first, decrease the backlash setting.
If you don't know which is the Dec axis calibration, listen to the drive motors as the
calibration is happening. On most setups the RA axis is the first one calibrated, and
you'll hear that the RA motor speeds up or stops as the RA is being calibrated. During the
Dec calibration you'll hear the Dec motor moving very slowly first in one direction, then
the other. You have to put your ear close to the motors to hear them running and to be
able to tell which one is running.
Guiding and checking the numbers
If the calibration is successful, you'll get a perky "HELLO" from the ST4.
Without further ado, press the Guide button and see what happens.
For the first minute or so that the ST4 is guiding you are likely to get some erratic
readings as the mount and guider settle down and the ST4 optimizes its internal
parameters. After a minute or two you should see that the guiding readings become fairly
steady; that's the time to analyze how it's doing:
- If you are getting small numbers on both axes, things are probably well. On my G11
guiding at a half-meter focal length under good seeing I typically get long strings of X
and Y readings of zeroes and an occasional 1, sometimes a 2 when there are burbles in the
seeing. At a 1-meter focal length I get more 1's and 2's because seeing effects are more
magnified, but if the seeing is very good I get mostly zeroes and ones. Anything up to a 3
or 4 is OK for most purposes. You should not be getting strings of 5 or higher unless you
are using a long focal length and/or seeing is poor. A long focal length will magnify
seeing effects, so a long focal length will always generate higher error numbers unless
the seeing is truly excellent.
- If you are seeing large error numbers, see which axis is doing it:
- If the RA errors are much larger than the Dec errors, that indicates poor drive
accuracy.
- If the Dec is larger than the RA, it indicates poor polar alignment. This is
particularly true if the Dec errors are mostly in one direction.
- If both the Dec and RA are large, it typically indicates either poor seeing or something
is vibrating the scope. The latter could be wind, could be an unbalanced fan (scope
cooling fan or CCD camera fan), or could be that the scope is too large for the mount and
therefore small vibrations are not dampened. (See below for some ways to check if the
scope is too large for the mount.)
- If the problem is poor seeing causing large readings, try setting the AA value to 3.
That will average three readings before making a correction, which will tend to average
out seeing effects. That can work wonders sometimes.
- If the readings start out low but become larger and larger, eventually becoming E's, try
setting the SA value to 9. That will effectively turn off the self-optimizing feature of
the ST4, which can sometimes be fooled by poor seeing. When guiding with long focal
lengths you should set SA to 9 as a matter of course.
Troubleshooting
Once guiding is underway the ST4 will happily guide for hour after hour until you stop
it or it runs into a problem. Here are some of the possible problems and how to deal with
them:
- Everything is going perfect, but suddenly you get the plaintive "lost guide
star" wail from the ST4. Things to check:
- A cloud, perhaps a very thin nearly-transparent cloud, moved in front of the guide star.
The ST4 will set off the alarm if the guide star brightness drops to half of its initial
value, and it doesn't take much of a cloud to do that. You probably would not be able to
see it, but it would be enough to trip the alarm. If guiding with a guide scope, put
something (a hat?) in front of the main objective to block the camera while you wait to
see if the star will come back; usually it will. If guiding off-axis cross your fingers
and pray. If it doesn't come back within 30 seconds or so, remind yourself that you are
having fun, then close the shutter and hope you got enough to make it worthwhile.
- The mount has hit something and cannot track. Could be that you allowed a cable to get
wrapped up in the mount, could be that you allowed the scope to bump into the mount base
or tripod, could be you nodded off and your nose is now resting on the side of the tube.
Kiss off that exposure and yell at yourself. Do not pass GO, do not collect $200.
- The mount was so unbalanced that as it shifted in orientation it reached a position
where it could no longer pull the unbalanced load. This can happen, for instance, if you
have too much counterweight and start guiding with the scope pointing low in the east.
It'll be fine at first, but as it tracks higher and higher the weights will exert more and
more of perpendicular pull and the mount could reach a point where it can no longer pull
all that extra weight.
- The clutches could have been so loose that at some point they slipped and the scope
shifted. Even a small shift will lose the guide star. You should have the clutches so that
they will not move with a light touch, but still move with a mild but deliberate push. You
do *not* watch to crank down the clutches hard; there is no need for that and such
tightness can result in gear damage if the mount suffers some kind of impact because the
clutches will not give, so will transfer the shock to the gear faces.
- The mount tracked across the meridian and shifted its weight balance across the top,
flopping from one side of the gear teeth to the other. The best solution is to plan your
exposure so this doesn't happen.
- You have a loose element in the optical train. The primary, secondary, or spider could
be loose; the focuser could be unlocked; the guider could be loose in the focuser. It's
even possible you forgot to tighten the scope to the mount. If the result of such a
bonehead error is the loss of a once-in-a-lifetime image, suicide is the only option. A
knife works, but a bullet is quicker. Try to avoid damage to the equipment so it can be
resold on Astromart.
- Your battery is low and either the ST4 or the mount has stopped working properly. A
quick check on most mounts is to see if it will do a high-speed slew on both axes. Most
mounts will lose the Dec slew capability first. Most will continue to drive at a sidereal
rate long after they are incapable of slewing, which may make it see it's working fine,
but it cannot respond to guiding inputs, particularly in Dec. The ST4 has no specific
indication of low voltage. It will just start operating erratically or lose the guide
star, but the the lights still be on, which can be very deceptive.
Next steps
I hope the foregoing gets you guiding in your first ST4 session. Practice with it a
bit, learn what parameter settings work for your particular equipment, write them down if
need be.
As an accessory to your ST4, you might also consider getting an eyepiece that can be made
parfocal with your ST4; it will make finding guide stars and focusing much easier. Both
Meade and Celestron have made eyepieces like this; check to see which ones are available.
Enjoy, and remember to keep chanting "serenity now" as you go climb the
learning curve. :-)
--Wil Milan
Appendix: Is your mount steady enough?
One of the common reasons why you may have problems getting good guiding is that the
mount is not sufficiently solid, which usually means the scope is too large for the mount.
Most scopes are sold with mounts that are suitable for visual use, but not for
photographic use. For photo use you'll want a mount that would be overkill for visual use.
A simple check of mount steadiness is to try focusing on a planet at high power: If
operating the focuser causes vibration blur so that the planetary details cannot be seen,
that mount is not up to photography with that scope. You should be able to operate the
focuser at high power without significant vibration.
A more precise measure of mount steadiness is to time the dampening time with a stopwatch.
Focus on a bright star at high power, then give the scope a hard "pluck" at the
focuser or objective end, simultaneously starting the stopwatch. The vibration should be
completely damped within 2 to 3 seconds, even from a fairly hard pluck.
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