Until 2010, the standard acquisition/guiding cameras at MDM were based on three-stage image tubes. These were deployed in the mid-1980s, and served well, but had numerous shortcomings.

Early in 2010 we took delivery of three Andor Ikon DU 937-N cameras, to replace the image tube systems. These are mostly aimed at the laboratory microscopy market, which is much larger than astronomy in dollar terms, so they represent a lot of expensive research and development. They are also designed with laboratory scientists in mind -- scientists who have other things to worry about than tweaking their CCD systems -- so they're basically 'turnkey' systems.

Salient features of the cameras are as follows:

• Thinned, backside-illuminated CCD sensors, with over 60 percent QE from 4000 to 8500 A, and over 90 percent between 5000 and 7000 A.
• 512 pixel square active area; 13-micron square pixels, 6.6 millimeter square imaging area.
• Frame-transfer (i.e., data are shifted from the active region of the chip to a covered 512-square area for readout, eliminating the need for a shutter).
• Up to 2.5 MHz readout speed, resulting in 8 frames per second; something like 10 e- read noise at 2.5 MHz, down to 2.5 e- read noise at 50 kHz (which is too slow for most purposes).
• Thermoelectrically cooled (they can go much colder than we need).

As of this writing, one of the Andor cameras has replaced the guide camera on the 2.4 m, and the image tubes have not been used at this port since the beginning of the 2010B semester. Also, in the summer of 2010, Yorke Brown (a consultant with ties to Dartmouth and the Sloan telescope) designed and implemented a simple slit-viewing port for the Modspec and Mark III spectrographs. Another Andor has been dedicated to this port, so that an observer can use one camera to view the slit and another to handle guiding.

We have at present two different software options to run the cameras. Both run under Windows. They have complementary strengths, but unfortunately neither one "does it all". They are:

• Solis. This is Andor's own package to run the cameras. It is quick and convenient to use, and takes advantage of the very high speed these cameras are capable of. This is what you want for quick centering, focusing, and the like, and it is the software of choice for slit viewing with the Brown optics (or with CCDS). There are no astronomy-specific functions, and in particular there is no autoguiding option.
• Maxim DL. by the Canadian concern Diffraction Limited. This package is aimed at the amatuer astronomy market -- you can see their ads in Sky and Telescope. Maxim DL is not built to handle the Andors' high frame rate, but it does have an autoguiding function. This saw first use in 2010 June and has been in routine use ever since.

Note that you'll need the observer password to get onto the PC that controls the camera. This is not the same as the password to the Linux boxes, so -- be sure to get it from the staff!

Maxim DL is a huge, feature-rich program aimed at amateur astronomers. We use only a small subset of the features.

As noted earlier, the autoguiding capability of Maxim DL is its great advantage over the Solis software.

To start the Maxim DL software, double-click on its icon. Unlike the Solis software, it doesn't look for the camera right away, but simply pops up a large grey window with menu bars at the top.

Find the Camera Control icon -- it resembles a little power plug, but it's supposed to represent a camera with a cable coming out of it. It's about the sixth icon from the left. When you click it, the Camera Control window appears.

The Camera Control window has three tabs at the top -- Expose, Guide, and Setup.

Start with the Setup tab. It has panels for Camera 1 and Camera 2, which for us are one and the same.

• Just to be cautious, verify that the shutter is being held permanently open (or else it'll wear out pretty quickly). To do this, click on Setup Camera in either Camera 1 or Camera 2; in the resulting window, hit the Advanced button; there's a check box at the bottom of that window (for which we paid extra!) that holds the shutter open. See to it that it's selected. Note that this needs to be done before the camera is connected (next step).
• Click on Connect -- the program should take over control of the camera. If it doesn't, either another program (e.g. Solis) is running the camera, or the camera needs to be power-cycled (do this as a last resort).
• Turn the Coolers On with the button provided. The other Cooler buttons in the Camera 1 and Camera 2 areas let you set the temperature; -40 (that's minus 40) seems to be a good set point.

Now that the camera is connected, you can move on to the Expose tab. This is what you'll be using for acquiring objects. On the right side is a column of three buttons; you want Continuous, and be sure Autosave is off. Toward the bottom, you'll want to set X binning to 2, and Y binning to Same. Near the top, set the exposure time to something short, like 0.7 seconds.

If you hit the Start button, the camera will start taking pictures continuously. If it doesn't, it might have hung up -- try disconnecting and reconnecting the camera in the Setup tab.

You'll see the image appear in a window. 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.

Note that the image updates rather slowly -- a couple of seconds -- even if your exposure time is short. This is somewhat inconvenient for fast centering, but it's not too bad. (This is why you want Solis if you don't need the autoguider, e.g. for the dedicated slit-viewer).

Note - you can of course display the Guide tab (next step) at any time, but the guider can't do anything until you stop taking continuous exposures in the Expose tab.

Finally, we get down to business with the Guide Tab. Here we have 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:

• 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. If you rotate the instrument you manually change the number under Angle. It appears to work to make this simply equal to the rotator angle, but your mileage may vary. (If in doubt, you can simply recalibrate at the new angle, until you're sure of your procedure).
• 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. I have found that with 2x2 binning and the offset guider, the X and Y rates were both around minus 13, and the rotation was close to the rotator angle.

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.

Here's a strategy for guiding and acquisition.

First, set up as follows:

• Select an unmistakable target, such as a bright star or anything else you're sure you can identify uniquely.
• Use JSkyCalc24mGS to drive the guide probe into the predicted position of a nice bright guide star (like 10-12 mag).
• In the Expose tab, take continuous short exposures.
• Center the telescope "science instrument" accurately on the target, e.g. by taking test exposures and ensuring you're centered, or by simply centering it in the spectrograph slit.
• Look for the guide star in the Maxim DL image. If the center of your instrument is offset significantly from the center of rotation of the guide star XY stage, the guide star may appear close to the edge of the image, or it might be off the image altogether. If so, move the guide probe to approximately center the guide star. [Note: if there are other stars near the guide star, the pattern appears flipped vertically compared to the field map in JSkyCalc24mGS.]
• Once you're happy with the guide star, stop the continuous exposures.
• Select the guide tab.
• Select the Expose radiobutton
• Hit Start to record the position of the guide star.
• Write down the XY coordinates that the program found for the guide star; they should be near the field center.
• Compare the true guide probe position in xmis -- they're shown in white letters on a blue background -- to the nominal JSkyCalc24mGS guide star position. Figure out (and write down) the delta-x and delta-y that, when applied to the JSkyCalc24mGS coordinates, will give you the coordinates you have. [Be sure to get the true coordinates from the xmis window, since the coordinates returned to JSkyCalc24mGS when you hit "Read Guide Probe" are often incorrect.]
• Calibrate the guider and set up the aggressiveness as described earlier.

You're now set up -- for subsequent targets, all you need to do is

• Use JSkyCalc24mGS to select a guide star and move the probe to its expected location. Be sure to include any non-zero instrument rotator position, and if you're using the inner slit on OSMOS be sure to select the red slit in JSkyCalc24mGS.
• In xmis, apply the "dx" and "dy" that you figured out earlier, by entering the values into the appropriate boxes and executing the moves with a carriage return (with the mouse pointer in the box). (This accounts for any misalignment between the instrument center and the center of the MIS stage. The dx and dy corrections are independent of rotator angle.)
• Select the Expose tab and start taking continuous exposures.
• Identify the guide star. If the telescope pointing is off, you may need to move a little to find it. If you have a slit-viewer running, and can identify the field, put your object near the slit and that should bring your guide star into view.
• Move the telescope to put the guide star where it should be in the guider field of view; anywhere within the tracking box will do fine. (This is why you want a not-too-small track box).
• Stop the continuous exposures.
• Select the Guide tab.
• Be sure the Track radiobutton is selected. (There is no need to recalibrate the guider parameters.)
• Click on the Start button to being guiding.
• Once the guider settles, your instrument should be tracking within a couple of arcsec of the target coordinates.
• If you need better centering than this -- e.g. to drop your object into a spectrograph slit -- apply small dx and dy motions in the xmis window. (This is much quicker than re-setting the guider box, and offers finer control.) Note: for Mark III and Modspec: if the slit-viewer has the orientation correct (N near top and E near left), then if the target appears to the left of the slit, you apply positive dy to bring it into the slit If the target pokes out on the right, and vice versa. One arcsec is 12.8 guider steps. [The directions for guide probe corrections are independent of the instrument rotator, since the guide stage co-rotates with the rest of the instrument.]

  These procedures seem very complicated when written out explicitly like this, but in practice they go pretty quickly. The procedure leaves you exactly centered, and guiding, in one go.

  Viewing the Slit Using Solis

  Focus. The staff focuses the slit-viewing camera on the slit jaws as part of the instrument change, using using the focus ring on the lens attached to the camera. The focus ring has a locking screw on it, so the slit-viewing camera should remain in focus once they've set it up -- you shouldn't have to touch it. Once the slit-viewing camera is in focus, one can focus the telescope by acquiring a star, putting it near the slit (the jaws are tilted with respect to the focal plane, so the focus varies slightly across the field), and focusing the telescope until the star appears as sharp as possible. This obviously requires a star and can't be done until after sunset; there are instructions further down as to how to set up the program for focusing.

  To start the cameras, simply log in to the control PC as observer and start up the Andor Solis control/analysis software by double-clicking on its icon. This will look for the camera and will not start up unless the camera is found.

  If the software fails to start and immediately complains of a timeout, you may be able to solve the problem by power-cycling the camera. Unfortunately, it looks as if this requires unplugging the camera's power brick out at the telescope.

  Once the program comes up, turn on the cooling (unless it's horribly humid and you're just playing around). This is in the Hardware menu, under Temperature. Minus-40 is a good operating temperature. A box at the lower left of the screen reports the temperature; it's red and turns blue when the operating temperature is reached. There's no reason not to take images when the camera is warm, but there will be substantial dark current (and hence noise).

  Now open the Acquisition menu "Setup Acquisition" item. This brings up a window with lots of tabs, most of which you don't need. Important -- the tabs don't all fit at once, so you scroll left and right with little arrows at the upper right of the window.

  The leftmost tab is "Setup CCD", which is obviously important. On this one:

  • Check the box that says "Frame Transfer"
  • Acquisition mode can be "single" or "kinetic"
  • Triggering is "internal" (default)
  • Note the "Horizontal Pixel Shift". The chooser there lets you select "2.5 MHz at 16 bit", "1 MHz at 16 bit", or "50 kHz at 16 bit". 1 MHz seems to be a good all-round choice, but 50 kHz is useful for when you want to go as deep as possible, with some penalty in read time. In that case read a subarray.

  The next tab is "Binning". The slit viewer gives 0.24 arcsec per pixel unbinned. 2x2 binning is adequate for most slit-viewing purposes; for the initial focus on the slit jaws you'll want 1x1 binning. (Also, very bright objects don't saturate as quickly with 1x1 binning). Binning reduces read time and read noise at the expense of resolution; it only takes a few seconds to change it "on the fly". Note that the Solis program still displays absolute pixel coordinates (512 square for whole frame) even if the pixels are binned.

  You can also select a sub-image (measured in original pixels). If you do this you can move it around on the tiny display with the mouse. There's seldom a need for this.

  The "Image orientation" tab: For the Yorke Brown Modspec/MkIII slit viewer choose:

  • no rotation
  • Select both Flip Horizontally and Flip Vertically. The cartoon will show an "R" standing on its head. This choice aligns the Brown slit viewer with North near the top and East near the left when the instrument rotator is at zero. The field is rotated by 18 degrees, and the field is about 122 arcsec square at the 2.4m.

  Once you're set up, you can start looking at stuff. In the bar of icons just under the menu bar, there are little red and green circles -- these are the start/stop buttons. You want the one that says "RT" -- it starts the camera taking data continuously and reading it out. The red button stops this; you need to stop to reset the CCD parameters.

  The exposure time is set by a control box that comes up when you click on the "run-time" icon, which looks like a little tv remote held diagonally. The Run time box that pops up has a slider that says "Exposure (seconds)". It starts at the fastest possible exposure, then you slide up and when you get to the top it trips over to a new range, and so on up -- when you start over, it reverts to the shortest time, which is a little awkward but not hard to get used to. You can also simply type your desired exposure in to the entry box at the bottom.

  Note that you can make the picture bigger by dragging out the corners of the Solis window.

  Much of the power of the Solis software comes from the speed and convenience with which you can control the display to optimize your view. These controls are as follows:

  • Farther along the icon bar is one that has a red arrow pointing to the lower left and a blue one to the upper right -- this is the "Reset" (reset view) icon. It does two things: (1) restores the display to show the whole field being read (2) autoscales to min/max values. You'll use this a lot.
  • Two icons to the right is a little thing that looks like two bold blue arrowheads, up and down. This toggles autoscaling; if it's on, the greyscale is adjusted image-by-image (which makes it look pretty crazy but can be good if you're moving onto a bright star) If it's off, the greyscale stays stable.
  • There's a cursor on the image display. Clicking on the image moves the cursor to the spot you click on . X and Y cuts of the image through the cursor's location appear to the left and bottom of the image.
  • Here's something really cool -- if you click on the image and hold the mouse down, you can drag out a rectangle. When you release the mouse, the rectangle pops out to fill the whole display. This lets you instantly blow up any region -- like the slit, or a star you're using for focus. The "reset" button described above (diagonal blue and red arrows) restores the whole area.
  • At the top of the image is a slider bar for the greyscale. It only makes sense to use this with autoscaling off. Click-and-hold on the arrows on the left and right and you'll see how you can manipulate the greyscale (unfortunately, these move the greyscale range rather slowly). Also, if you click-and-drag the greyscale bar, it drag the interval up and down while keeping the separation of the black and white levels constant.
  • If you right-click the mouse, it brings up a menu that includes "Palettes" -- you can choose various color schemes for the display. I find two of them to be useful: "Grey.pal" and "false1.pal". It defaults to grey. The false color can be helpful for seeing faint features, if you choose your levels carefully.
  • The Display menu item at the top has a 'preferences' tag which you can use to set the color of the cursor to some contrasting color. Cyan works nicely on greyscale.

  The power of the Solis software is evident when you're trying to focus the telescope on a star. Here's a procedure for this:

  • Slide exposure to very short (like 0.3 sec) (you can do this "on the fly" but it waits til the current exposure is done to take effect).
  • Set the greyscale. This is best done by autoscaling, then turning off autoscaling (which gets you close), and then using the sliders to fine-tune. You don't want autoscaling on during focusing, because you want to see the peak level change rather than having it normalized away.
  • draw a small box around the star to blow it up.
  You get a rapidly dancing star, and the seeing is really dramatic. Note the starting telescope focus value, and focus the telescope on the dancing star. You don't have to worry about afterimages or gain like you would with the old TV.

  For acquisition, you'll want to see the whole field, and set the exposure to something reasonable. Note that if you have faint targets, but also have some brighter stars in the field, it's best to use short exposures to get near the target (because the feedback is quick), and then set the exposure long to see the actual target.

  Once your target is in or near the slit, you can use the drag-and-expand feature to get it dead-center, and so you can keep an eye on it without having to squint.

  Fast Photometry with the Andor

  In 2011 January I had cause to use one of the standard Solis features to do fast photometry - I'd discovered a system with a white dwarf eclipse, which basically made the star disappear in a couple of seconds, and reappear in a similarly short interval some 7 minutes later. I was delighted to find that the Andor could record this. Here are some notes on how to do this.

  Timebase. The windows control computer is supposedly on NTP, but it synchronizes very infrequently. If absolute timing is important, work with the staff to get it synched during the day. If you don't have a chance to do this, collapse the Solis window with the little "underscore-like" window control button, bring up the control panel, look at date and time, and figure out the offset by comparing the computer clock to something more reliable (e.g. the Linux system clocks, which are carefully synched via real NTP).

  In the instructions below, recall that the tabs in Acquistion are accessed using the left-right arrows toward the upper right.

  Setup. In Acquisition, under the Setup CCD tab,

  • Be sure that the Frame Transfer checkbox is selected
  • Set the exposure time to the desired value, like 0.5 sec
  • Leave the number of accumulations at 1
  • Set the Kinetic Series Length to the number of exposures you want; for example, 900 seconds at 1/2 second would be 1800.

  Now turn to the Binning tab. To avoid huge data set and keep the frame rate high, you may want to bin 2x2 and specify a subarray. To do this,

  • Select 2x2 binning using the radiobuttons
  • Click on Custom, and then fill in values for Left, Right, Bottom, and Top; these are in unbinned pixels.
  • There's an button labeled Draw >> that draws your custom subarray on the display. Use it to check that your boundaries are correct, and then accept it.

  Now select the Auto-save tab. In here,

  • Click to enable auto-save;
  • Select file type Fits (most likely)
  • Give a file stem
  • Give a directory for your data. I was using C:\Users\Observer\thorimages, which I created; feel free to write in it.
  • You can elaborate on this by automatically appending numbers, and so on; note that each time series will give only one file, so there's no need to have an elaborate numbering scheme.

  You should be ready to go now. To take signal, go to the Acquisition menu and simply Take Signal. The images will be displayed as they come in, and you can play with the stretch, etc.

  Once the operation finished, you'll have a 3-dimensional FITS file in the directory you specify. The file is time-stamped to the nearest second (only), and the time between frames is given in the header to five digits or so. To retrieve the FITS file(s) from the Windows machine, it's easiest (at this point) to use a USB memory stick. If you display one of these datacubes in DS9, it will let you step through the images and see what happens. For further reduction, you're on your own!