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 this.

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.

1: The Cheat Sheet ...

1.1 : First night with a newly-changed instrument:

2.1.2 : Port for Autoguider Output

If for some reason Maxim DL gets reset to its default values, then you have to tell it which of many methods it should use to control the telescope. You do this on the Settings window (again), in the Autoguider Control box, using Control Via pull-down menu; you want:

ASCOM Direct for the 2.4m or

LPT378 for the 1.3m

2.2 : Manipulating the Image Display

Above: 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.]

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 be useful.

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 , so 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.]

2.3 : 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 tab, 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.. If you're using the guide probe as a tool to center your science instrument, this may not be what you want.

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.

The guider has to know how quickly the telescope will move when it hits the buttons, and in which direction. There's an autocalibration procedure for this which is not recommended -- it's not necessary. For completeness, it's described later in greyed-out text.

It's quickest to simply set the guide parameters by hand. In the Guider Settings window, at the lower left, under Manual Calibration, simply set:

For the 2.4m:

X Speed = -7.8, Y Speed = -7.3, Angle (deg) = 0 for the 2.4m

X Speed = -8, Y Speed = +8, Angle (deg) = -90 for the 1.3m

At the 2.4m these numbers work if the guide rate is set to 2.0, with "Cos Dec" on.

At the 1.3m similar speeds are obained if the paddle gain is 0.02; you set this by going to the box on the telescope control GUI provided for typed input, and typing

tx paddle gain=0.02

followed by "enter". Be sure to do this every day, since the paddle gain is often set higher by people needing to move the telescope faster. Also, it may change to some other value if the TCS is restarted.

Note that the 1.3m TCS does not have a "Cos Dec" option that automatically increases the EW speed far from the equator. To account for this use the "Scope Dec" parameter in the Guide tab. Jules Halpern of Columbia contributed some draft notes (reproduced below) on guiding the 1.3m, which describes in detail the procedures that are particular to the 1.3m, and gives more precise values for the speeds.

If you rotate the instrument (very seldom done on the 1.3m), change the Angle parameter to adjust for the rotation. The formula is

NOTE: the calibration procedure described in the greyed-out text below is deprecated at the 2.4m -- it is not working correctly as of 2014 January, apparently because a hot pixel is causing it to get confused. It is unnecessary in any case.

The 1.3m camera may not have this problem; Jules Halpern's notes discuss some considerations for calibrating the 1.3m guider, in the event it's needed.

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. Note especially that you don't have to recalibrate the guider at every new target; doing so is a waste of time.

• 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..]
• 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.

2.4 : Establishing Offsets at the Start of the Run.

Here's a procedure to get you going.

1. 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.
2. Point at a bright star, or some other unmistakable target, and find it with your science instrument. Focus the telescope on your science instrument.
3. 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 MDM-style pointing list, using the Object Lists facility of JSkyCalc.]
   

4. 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:
   

5. Verify that the coordinates in the upper right corner of the guide cartoon are appropriate, and that the Rotator angle is set correctly.
6. 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.]
7. 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.
8. Once the probe is in position it'll report its position in the xmis window on the main computer; they show in green when the probe is at its destination. [The numbers don't agree with the earlier figure - this is a later screenshot.]
9. The little red circle in the guider cartoon is supposed to indicate the guide probe position, but it usually doesn't work. Ignore it.

10. In Maxim DL, select the Expose tab and hit the Start button. You should see the guide star (see below if you don't).
11. Focus the guider using the program focus.exe, found on the desktop.  Click STOP when done focusing to quit the program from running.  Click the white arrow to restart the program and focus further as needed.  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.

12. 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.
13. Stop exposing, then go to the Guide tab and select the Expose radiobutton.
14. 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.
15. Grab the mouse and draw a nice box around the star -- this will be useful later.

Now you're all set up.

2.5 : Acquiring targets by offset.

This is especially appropriate if your instrument reads slowly, so that it's awkward to center your target by simply looking at it. The basic idea is to put the guide probe where you know the guide star will be, and then center using the guide camera:

1. Be sure JSkyCalc has your target's coordinates in the RA and dec window. [This is easiest if you read them from a list.]
2. Set the telescope on your object's coordinates.
3. If the JSkyCalc guide star selector tool is not open, open it using the Guide Stars button at the bottom of the JSkyCalc window.
4. If you have rotated the instrument, be sure that the exact rotation angle is entered in the box in the guide star selector window.
5. Look at the JSkyCalc guide star selector cartoon. Select a suitable guide star, and move the guide probe into position as described earlier.
6. In Maxim DL, select the Expose tab, and press the Start button. The image should start updating.
7. 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.]
8. Using the hand paddle, move the telescope to put the guide star in the box you marked earlier.
9. 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.
10. 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.
11. 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.

    2.6 : Building in a target offset.

    As noted earlier, if the center of your instrument's field is offset from the nominal center of the guide field, you'll have to adjust for this, because JSkyCalc's nominal coordinates won't be quite right.. One way to do this is to offset the guide probe away from the JSkyCalc coordinates every time you set on a new object, by manually entering dx and dy into xmis. This is tedious and error prone.

    It's easier in the long run to build this into the calculation, using the provision in JSkyCalc. Suppose you have already determined the dx and dy values for your instrument. Install them as follows:

    1. Compute these values: Target X offset = dy / 12.8 Target Y offset = dx / 12.8

       That's not a misprint - the X offset depends on dy, and the Y offset depends on dx.

    2. In the Guide Star Selector, click the Other radiobutton (to indicate you're not using either OSMOS slit).
    3. Enter the number in the fields of the same name in the guide star selector.

    Now that you have non-zero numbers in those fields, when you read guide stars, you'll two sets of coordinates in the selector -- the yellow numbers are for the target, and the green the nominal center. A yellow box marks the target's off-center location in the diagram.

    2.7 : 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.

    Appendix : Jules Halpern's notes on guiding the 1.3m.
    In 2013 December, Jules Halpern had a run at the 1.3m in which he paid careful attention to the guider. Here are his notes, which should be very useful.

    
                                                       February 9, 2014
    
     ===================================================================
     Supplemental notes on autoguiding at the 1.3m, with help from the
     Maxim DL User Manual; some of this would apply to the 2.4m as well.
     ===================================================================
    
    This writeup assumes that you are already familiar with John Thorstensen's
    manual on Autoguiding at MDM, therefore, with the basic operation of the
    Maxim DL guiding system.
    
    If you or the autoguider (figuratively) depress a guide button on the
    hand paddle. the telescope will move a certain number of arcseconds
    per second (the guide rate).  This is also what happens during the
    Calibrate function of the guider.  If you use the Calibrate function
    at the 1.3m, as described below, you should do it with a star on the
    celestial equator, and enter 0 in the "Scope Dec" field at the top center
    of the Guide window.  This is necessary because the 1.3m TCS does not
    adjust the HA guide rate for declination; the EW guide motions are slower
    across the sky at high absolute values of the declination.  By the same
    token, the guider doesn't know at what declination you are pointed unless
    you tell it.  So you have to enter manually the declination in the
    "Scope Dec" field of the Guide window each time you move to a new target.
    The guider will use the declination to adjust how long it presses the buttons.
    
    Moral: at the 1.3m, YOU have to tell the autoguider what the declination is.
    (At the 2.4m, ignore the "Scope Dec" feature of guider because the TCS takes
    care of it. The 2.4m guider always sees the same speed in pixel units.)
    
    Here. we refer to the telescope guide "rate" in arcseconds per second, while
    a guide "speed" is in pixels per second, the pixel of the autoguider CCD.
    
    Automatic calibration is optional
    ---------------------------------
    
    In order to get an accurate calibration, the telescope should move at
    least 20 pixels in each of X and Y.  The length of the move will be
    proportional to the product of the "Cal time", which you enter in the
    Settings pop-up of the Guide window, and the paddle gain, which you set
    in the Command line of the TCS Control Panel (running on mcgraw). 
    The "Cal time" is how long the guider will press the guide buttons to
    determine the guide speeds.  The default Cal times are 5 seconds in both
    X and Y, and the recommended paddle gain is 0.02.  To see or change
    the paddle gain, type in the Command line of the TCS:
    
         tx paddle gain       (to see what it is)
    
         tx paddle gain=0.02  (to set it)
    
    Moral: The guide speeds are only valid in conjunction with a particular
    paddle gain.
    
    Check the paddle gain at the beginning of each night, as the staff
    may have increased it to slew rate if they were working on the
    telescope during the day.  Also, any time you restart the TCS, the
    paddle gain may reset to a default value.  The last time I restarted
    the TCS it came up with paddle gain=0.01, which is not a bad value to
    use.  But you would have to calibrate the guider for that gain if that
    is what you are going to use.
    
    For paddle gain=0.02 and Cal time of 5 seconds, the calibration should
    give the following speeds AT THE CELESTIAL EQUATOR.  The values are only
    repeatable to +/-0.5 and are rounded off here to that level of precision:
    
         X Speed  -8.5
         Y Speed  +6.5
           Angle  -90
    
    Remember that these speeds are in pixels per second, so it implies that the
    telescope moved about 30-40 pixels during the calibration, which is good
    enough.
    
    Sometimes the calibration will fail because of backlash in declination;
    the telescope may not move in declination (X) even if the guide signal
    is activated for 5 seconds.  If this occurs, a little error message
    will pop up to the effect that the star moved less than 5 pixels in X,
    and you will see that the speeds were not updated. If this persists
    after several tries, you could try increasing the Cal time to 10 seconds
    to get it to move.  It's okay to do this as long as it doesn't cause the
    star to move completely off the field.  However, you may never take out
    the backlash (more on this below), so you may not get a reliable result.
    As a last resort, just manually enter the above values, and they should
    work well enough, providing that you also set the paddle gain to 0.02.
    
    Manual calibration is preferred
    -------------------------------
    
    Now that you understand how the fancy auto-calibration works, and
    why it can fail at the 1.3m, you may realize that the guide speeds
    can be determined during the day by using the physical hand paddle,
    the clock, and the coordinates on the TCS GUI.  In fact, the values
    that I tabulated above were determined in this way, and are consistent
    with what the guider measured by itself on the sky.  You could just
    adopt these parameters (including paddle gain=0.02), and stop reading
    here.  I only really wanted the calibrate function of the guider
    to determine the signs of the Speeds and the Angle, which I would
    otherwise have to guess.  To calculate the calibration, I just had 
    to know the pixel scale of the guider CCD, which is 0.67", to convert
    the speeds that I measured into pixels per second.  I have only
    determined the pixel scale to within about 10%.  It should be possible
    to be more accurate, but I haven't done that yet.
    
    Aggressiveness is not a virtue
    ------------------------------
    
    An exposure time of 2 seconds for guiding works pretty well.
    The recommended value of "Aggressiveness" is 4-5. Aggressiveness is the
    fraction of the calculated offset that it actually corrects, with 10
    being the full move.  So 4-5 is a conservative amount that prevents
    overshoot.  Overshoot causes an oscillation at a small multiple of the
    period of the guide exposure, with an amplitude of +/- a couple of pixels.
    (In normal guiding, with good seeing and no wind, the typical errors
    can be 0.1-0.2 pixels.)  If the guider appears to be oscillating,
    first check that the paddle gain is correct for the guide speeds that
    you are using.  As a second resort, reduce the Aggressiveness. 
    Make these changes to avoid chasing after momentarily jumping images
    when the seeing is bad or when wind buffets the telescope.
    
    "Nodding" is not bad guiding 
    ----------------------------
    
    An oscillation caused by the guider is NOT the same as the intermittent
    "nodding" behavior of the 1.3m, which is a feature of the TCS alone.
    It's easy to tell the difference.  A guider-induced oscillation will stop
    as soon as you stop guiding.  On the other hand, if the TCS is nodding,
    it is a more pernicious effect that occurs whether or not you are guiding.
    The TCS nodding can manifest itself as repeated jumps of 10"-60" that
    quickly return to the original position.  Or it can be a jump followed
    by a return in several discrete steps.  Or an oscillation of smaller
    amplitude that is more-or-less continuous.  These motions occur only in
    declination.  The guider is not capable of compensating for the nodding.
    The user can't prevent such nodding either, except perhaps by wishing,
    but more often by moving to another part of the sky.
    
    Backlash is unpredictable
    -------------------------
    
    Sometimes backlash in declination will prevent the guider from correcting
    for many cycles, up to a few minutes.  The telescope will drift away by
    a couple of pixels in declination before eventually coming back.  That's
    when your lack of aggressiveness becomes a handicap,  because it takes
    a longer time to take up the backlash.  There is a Backlash parameter
    in the Guider Settings next to the Speeds.  It units are seconds, and it
    is supposed to account for the time delay due to backlash in exactly the
    way that you might imagine.  It presses the button for that much extra time
    that it takes to start the telescope moving.  Backlash is usually a problem
    in declination but not in RA.  However, you should probably set both backlash
    times to zero unless you are exactly sure about the behavior of the telescope,
    which I'll bet you aren't.  The 1.3m backlash is intermittent, and sensitive
    to the mechanical adjustment of the telescope as well.
    
    Conflicting coordinate conventions are confusing
    ------------------------------------------------
    
    The guider image is oriented such that North is up, and East is to the left.
    The X coordinate increases to the West (right), and Y increases to the South
    (down) as you pan the cursor across the image.  This is for the default
    telescope rotator position of 0 degrees, and applies at both telescopes.
    The Guide Star X,Y coordinates and the scrolling X Err and Y Err
    in the Guider Information window are consistent with this convention.
    The sign of the error is the direction that the star moved.  However,
    the points plotted in the Tracking Error Graph have X and Y switched
    from the image coordinates, and one of their signs is reversed, too.
    X Err is graphed as -(Y Error) and Y Err is graphed as X Error.  So you
    have to look at the X Error graph to see errors in declination.  If you're
    confused now, join the club.  These conflicting coordinate conventions
    appear to be an unfortunate design feature.
    
    This raises the question of which coordinate system applies to the
    Speeds and the Aggressiveness, which are the properties you can evaluate
    and modify.  The X Speed is for declination because that's the one that
    usually fails  in the self-calibration.  And the guide rates determined
    "by hand" for RA and declination are different; their magnitudes are
    consistent with X Speed being for declination.  So X Aggressiveness
    is probably also for declination.   In summary, the graph's coordinates
    are correct and the image's are not, if that helps you think about this.