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.

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:

• 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 target.
• 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.

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.

1. 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).
2. Start Maxim DL by clicking on it.
3. 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.
4. Select the Setup tab in the Camera Control window (as shown above).
5. Press Connect to connect with the camera.
6. Turn the Coolers controller On.
7. 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.
8. Go to the Guide tab.
9. Pop the Options menu (right side, halfway down), and select Camera Settings, which pops this window:
10. 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 guide cycle.]
11. Click on OK to commit to open shutter; the window closes.
12. The Options menu under Guide also lets you set the Track Box size; 64 x 64 is usually a good choice.
13. 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.
14. Go to the Expose Tab (not the expose radio button in the Guide tab!), which looks like this:
15. Set 1 second exposures for Expose, and select continuous exposures.
16. Once again, in the Options menu, bring up Camera Settings, toggle the shutter button so that it reads Close Shutter and click OK.
17. In the Options menu, be sure that No Calibration is checked.

[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 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.]

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. 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.

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, 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.]

9. In Maxim DL, select the Expose tab and hit the Start button. You should see the guide star (see below if you don't).
10. 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.
11. 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.
12. Stop exposing, then go to the Guide tab and select the Expose radiobutton.
13. 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.
14. 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.

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 you have rotated the instrument, be sure that the exact rotation angle is entered in the box in the guide star selector window.
4. If the JSkyCalc guide star selector tool is not open, open it using the Guide Stars button at the bottom of the JSkyCalc 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.
    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.