Autoguiding and Acquisition at MDM
John Thorstensen, Dartmouth College
Basic Concepts and Terminology
The MDM telescopes point pretty accurately, but not to
arcsecond accuracy. They track pretty well, but exposures
longer than a minute or so are likely to be trailed.
It's therefore important to be able to acquire objects
and guide the telescope. This document describes how to do
Because the telescopes are on equatorial mounts, the field
does not rotate on the focal plane as the telescope tracks
across the sky. This is a great simplification.
To keep the telescope from drifting off target as it tracks,
the usual strategy is to use an offset guide star.
This is a star in some part of the focal plane away from the
science field of view, which is reflected by a
"pickoff mirror", mounted on a movable guide probe),
through some optics into a guide camera.
An autoguiding program watches the guide star, and if
it appears to drift -- indicating that the telescope is
going off-target -- it sends a correcting signal to the
telescope. With proper tuning, this can be very accurate,
holding the telescope at a constant celestial position
to within a fraction of an arcsecond. Such accuracy
is necessary for good direct imaging,
and is also very desirable for spectroscopy.
The figure below is a sketch of the telescope
focal plane (the 2.4m and 1.3m are similar).
The pickoff mirror is mounted on the
guide probe, which moves on an XY stage as shown.
The pickoff mirror is above the science instrument (it's
in the Multiple Instrument System (MIS) guider unit),
so it can't collide with the science instrument; however,
it can block the science instrument field of view.
This can be a useful capability (e.g. for finding a bright
star when you're lost), but it's important to retract it
into a safe position when you're taking data.
A Fast, Accurate Way to Acquire Targets
The Hubble Space Telescope acquires targets by searching for
pre-selected guide stars at their expected locations in the
focal plane. We can do the same at MDM, and it makes
operations very efficient. Here's the idea:
That's the core concept -- using the guide probe and
autoguider as a powerful
aid to accurate centering as well as guiding. The enabling
technologies are the accurate guide stage, and the availability
of catalogs such as UCAC3.
- The guider pickoff mirror can be positioned accurately
and reproducibly, and can be moved in very fine steps.
- Over the last decade or so, deep, accurate star catalogs
have become available for the whole sky. In particular, the
UCAC3 goes to around 16th magnitude with roughly 0.1 arcsec accuracy.
- It is not difficult to compute where a guide star will land
in the focal plane when your target is centered, and to move
the guide probe into this position. When you set on the
target, the guide star should appear. You can then fine-tune
the pointing by moving the telescope so the guide star falls
in the right place in the guide camera. Assuming the guide
probe is set correctly, you should now be very close to the
- Wait, it gets better! Suppose you now want to make really
small adjustments to the telescope pointing, for example to center
a star in a spectrograph slit. The best way to do this is to
move the guide probe a bit -- the autoguider will cause the
telescope to follow, within a few guide cycles. Presto, you're
now perfectly centered, and -- as a bonus -- you've locked
the telescope into position with the autoguider.
The hardware and software at MDM make this strategy relatively
easy to implement, but of course you need to "know what buttons
to push". That's covered in the rest of this document.
Guiding with Maxim DL and the Finger Lakes cameras
As of mid-2011, the ancient image-tube guide cameras at both
telescopes have been replaced with modern, self-contained
CCD cameras. An extremely
fancy (and expensive) Andor unit was used at the 2.4m during 2010-2011,
but it has been replaced with a more modest Finger Lakes Instrumentation (FLI)
camera that offers some advantages for this task. The Finger
Lakes cameras are run using commercial
Maxim DL software, which provides autoguiding capability.
Camera Startup Procedure.
The camera is all set up. If you go into the Expose tab
and hit the Start button, it should fire off repeated
exposures and display them.
- The Maxim DL software runs
under Windows. If you're logged out of the machine, log in as
OBSERVER (the password is posted on the monitor frame).
- Start Maxim DL by clicking on it.
- In Maxim DL, click on the camera control
icon highlighted in the picture above. [It's supposed to resemble
a camera with a cable hanging off it.]
The Camera Control Window appears; you'll spend almost
all your time here.
- Select the Setup tab in the Camera Control window (as
- Press Connect to connect with the camera.
- Turn the Coolers controller On.
- You can check the temperature setpoint under Camera 1
(which should show FLI-New); the cooler button
opens a dialog box. A good setpoint is negative-35 C, which
the camera can maintain even when it's fairly hot out. In
the winter you might be able to go somewhat cooler, but -35
is cold enough for our purposes.
- Go to the Guide tab.
- Pop the Options menu (right side, halfway down),
and select Camera Settings,
which pops this window:
- In the dialog box, be sure that the third button down on the
right reads Close Shutter, as shown. [The button toggles
between Open Shutter and Close Shutter. When it reads
Close Shutter, it's waiting
for a command to close it, and hence keeps the shutter open. We want to
keep the shutter open to avoid wearing it out by running it with every
- Click on OK to commit to open shutter; the window closes.
- The Options menu under Guide also lets you set
the Track Box size; 64 x 64 is usually a good choice.
- Still in the Guide tab, select a 2-second
exposure, and set aggressiveness to 5 in both axes. You
can set this higher if you find it too sluggish, but beware
of overshoot; in bad seeing, you might want to dial it lower
to avoid pointlessly chasing a jumping image.
- Go to the Expose Tab (not the expose radio button
in the Guide tab!), which looks like this:
- Set 1 second exposures for Expose, and select continuous
- Once again, in the Options menu, bring up
Camera Settings, toggle the shutter button so
that it reads Close Shutter and
- In the Options menu, be sure that No Calibration
[Here's a sample 512-square frame from the Expose control.
The faint vertical stripes are probably due to the fact that
we're operating without a shutter, so that the bright star 'paints'
those pixels during the (very fast) read cycle.]
Manipulating the Image Display
Here are some things you can do to change the display:
- To change the magnification, put the mouse in the
image area and use the scroll wheel.
- To change greyscale stretch, you have
two options: (1) Right mouse button, "Screen Stretch"
item; useful choices are "low", "medium", "high", and
"range"; or (2) Hold down the Shift key and
the left mouse button simultaneously, and
move the mouse in the image area.
- With the right mouse button, you can draw a box, and
if you click on it just so you can drag it around. This will
What you're seeing:
In the standard orientation of the FLI camera, with the
instrument rotator at zero, the guide field
appears with North at the top and East to the left.
This is rotated by 90 degrees from the guider cartoon in
JSkyCalc. At the 2.4 m the scale is 0.223 arcsec per pixel
that the whole 512-square field subtends 114 arcsec.
However, the corners are vignetted -- especially the upper
left -- so the useful field is somewhat smaller.
[At the 1.3m, the field should be larger, but as of this
writing it has not been measured directly.]
Using the Autoguider
[This is organized a little awkwardly, having been pasted from
another document. Read this and the section on
Establishing offsets at the start of
your run and it should all become pretty clear.]
In the guide tab there are three important options in the buttons on the
right - Expose, Calibrate, and Track.
Some notes about the user interface and nomenclature:
- The "Expose" radiobutton inside the Guide tab
is entirely different from the "Expose" tab. I feel
that it was a design error on Maxim DL's part to give them
the same name.
- MaximDL uses the word "track" to mean "issue guiding
corrections to the telescope". The "track" switch on the
2.4m control panel means "drive the telescope in HA
to follow the stars."
- Note that the radiobuttons do not initiate the
action you want -- they only select what will
happen when you push Start.
If you select the Expose radiobutton under the Guide
and hit Start, then:
- A single full-frame picture is taken and displayed
in a separate window;
- The software automatically finds the brightest
star in the picture and enters its coordinates in the
Guide star X and Y
boxes. If you don't like the star it selected, you
can click on another star with the mouse, and its
coordinates will be entered into the boxes. (Note:
I believe the integer pixel you click is selected --
it doesn't centroid the image, unfortunately.)
- Note that the Expose action under Guide resets the
fiducial XY location of the guide star. This
will be necessary when you set up on your first night,
but you probably do not want to do this more
than once -- there is much to be gained by keeping the
guide star fixed.
In the Options menu (right side of the box)
there's a control for Track box size.
64 pixels seems to be a good option. The track
box needs to be contained entirely within the image, so
the guide star can't be close to the edge of the field.
Now you can move on to the Calibrate
button. This automatically establishes the scale, orientation,
and parity of the camera image, and the sensitivity
of the guide buttons, so that the program knows what
buttons to push to bring the star back when it drifts.
Note that you probably only need to do this at the
start of your run; you may even be able to skip this
if you know nothing changed since the last observer.
- First, go into Settings; under
X Axis and Y Axis, set the
Cal time settings both to 5.
- Hit Start and sit back -- After a while
a little red line appears by your guide star, the guide
star moves over, then it moves back, and then it does
the same thing in a perpendicular direction. The
program has mashed the guide buttons and watched the
star, then figured out the guide speeds and
directions! If you now look in Settings, in the
manual calibration section, you'll see
numbers for X speed, Y speed, and
angle -- the program has just set all these
for you. [With the 2.4m and the RA and dec guide rates
set to the default value of 2, the numbers should
be something like: X speed = -8, Y speed = +8,
angle = -90 degrees..]
- If you rotate the instrument (very
seldom done on the 1.3m) you should
not need to do the calibration rigamorole every
time you rotate -- simply set the Angle parameter
in Maxim DL to include the rotation. The formula is
angle = -90 - [rotator setting].
For example, with the rotator at +45, the guider
wants an angle of -135 degrees; with the rotator
at -30, the guider wants -60. [You can't choose
a more convenient quadrant; Maxim-DL unfortunately
insists on an angle between +180 and -180.]
- Once you have the calibrations, you'll want to
write them down. Unless you change the guide rates
on the TCS, or rotate the instrument, they should
be good for the rest of your run.
You're finally ready to guide!
- Select the Track radio button.
- For Aggressiveness, enter something like
5 in both axes -- the guider will then try to correct
50 percent of the image decentering.
- With the guide star somewhere near its fiducial position,
hit Start. A little postage stamp
around the guide star will be displayed in the
guider image window (you can blow this up and
stretch it to your heart's content); the guide
star should walk to the middle of the box, and
you're off and running.
Establishing Offsets at the Start of the Run.
Here's a procedure to get you going.
- Turn to one of the main Linux computers (not the
guider PC), and bring up JSkyCalc24mGS or
JSkyCalc13mGS (depending on your telescope).
There's a manual for JSkyCalc24mGS here.; if the link doesn't work, look
at the mountaintop web page.
- Point at a bright star, or some other unmistakable
target, and find it with your
science instrument. Focus the telescope on your
- Load the exact coordinates of the bright star (or target) into
JSkyCalc24mGS. The entry boxes are in the main window, with the
Dartmouth-green bar; to make this easier, hit "Stop Update"
and then "Resume Update" when you're done. Don't forget
the equinox. Hint: Change the "sleep for" field to 30 seconds
(instead of the default 15), and you'll be a lot happier.
[It's highly recommended that you load the coordinates from an
list, using the Object Lists facility of JSkyCalc.]
- Click on the Guide Stars button at the
bottom of the main JSkyCalc window. This brings up
the guide star selector, which looks like this:
- Verify that the coordinates in the upper right
corner of the guide cartoon are appropriate, and that the
Rotator angle is set correctly.
- Select a guide star by clicking on it -- a little
blue box appears around it, and its particulars are reported.
Guide stars in the range 10-14 work well.
[Sometimes the thread that updates the guider cartoon
stops. If this happens, you need to press "Read Guide Stars"
to force an update.]
- Press Move guide probe to move the guide
star to the star's expected location in the focal plane;
it won't move until you confirm it.
- Once the probe is in position it'll report its position
in the xmis window on the main computer, as shown below:
Ignore the little red circle in the guider cartoon --
it should indicate the guide probe position, but
it's often way off for reasons as yet undiagnosed.]
- In Maxim DL, select the Expose tab and
hit the Start button. You should see the guide star
(see below if you don't).
- Focus the guider using the little rocker switch
(for now, this may change when the MIS is overhauled).
If the guider focus reaches the end of its travel without
getting into focus, you're not dead - Maxim DL guides
amazingly well on out-of-focus images.
- The guide star
won't necessarily be exactly centered (that's ok).
If your guide star has little friends bright enough to
be in UCAC3, you may see them -- the pattern will be
rotated 90 degrees from the JSkyCalc diagram.
- Stop exposing, then go to the Guide tab
and select the Expose radiobutton.
- Hit Start -- a single image will be taken
and the coordinates of the brightest star in the
field -- hopefully your guide star -- will appear in
the Guide Star X and Y fields.
WRITE THESE DOWN.
- Grab the mouse and draw a nice box around the
star -- this will be useful later.
Now you're all set up.
A Protocol for Acquisition.
- Be sure JSkyCalc has your target's coordinates
in the RA and dec window. [This is easiest if you
read them from a list.]
- Set the telescope on your object's coordinates.
- If you have rotated the instrument, be sure that
the exact rotation angle is entered in the box in the
guide star selector window.
- If the JSkyCalc guide star selector tool is not
open, open it using the Guide Stars button
at the bottom of the JSkyCalc window.
- Look at the JSkyCalc guide star selector cartoon.
Select a suitable guide star, and move the guide
probe into position as described earlier.
- In Maxim DL, select the Expose tab, and
press the Start button. The image should
- Find the guide star in the image. You may
have to paddle around a little. If the
rotator is at zero, the image
has north at the top and east to the left.
[If you're trying to match the pattern in the
guide star cartoon, note the directions indicated
in the cartoon.]
- Using the hand paddle, move the telescope to
put the guide star in the box you marked earlier.
- If you're concerned that you may not have
the right guide star, you can select another
guide star and move the guide probe to it.
It should land right in the box.
- Once you're confident of your centering,
stop taking exposures with the Maxim-DL Stop
button, select the Guide tab, and
the Track radiobutton; then press Start
to start guiding.
- If there is fine adjustment to be done -- e.g.
for getting a star into a spectrograph slit -- then
it can be done using the dx: and dy:
entry boxes in xmis. Note that the OSMOS
has an elaborate centering sequence built around this.
Miscellaneous oddities etc.
Guide star coords are not with respect to full frame. I've seen
Maxim get into a state where it reads a subframe while guiding, and
then the guide star coordinates are not with respect to the full
frame. In principle it's faster to read a subframe, but in practice
the read is fast enough that it makes no difference. To cure this,
In the guide tab,
in the Settings pop-up window, lower right side,
in the Exposure Settings sub-box, hit the Reset
button, and then the OK button so that it 'takes'.
It may not look as if it has, if you've zoomed the guider
image display to see only the guide box; just de-zoom with the
mouse wheel to see the whole field.