The user interface of the Palomar Autoguider.

An overview of the user interface

Cursor usage in the image-display window

Step-by-step instructions are displayed in a single line of text just below the image-display window. This should be sufficient for many users, but more detailed directions can be found later in this document.

There are three classes of buttons

Other buttons turn an option on when the button is depressed, and off when it is raised. Clicking on one of these buttons toggles its state between on and off. The ENABLE buttons of the AUTOGUIDER and FRAME GRABBER control areas are examples of on/off buttons.

The remaining class of buttons remains down only for as long as you hold them down. These are used to initiate transitory operations. For example, pressing the DELETE button in the TARGET control area, deletes the target area marker.

If the labels on the buttons don't make it obvious which buttons behave in which manner, either look up the confusing button in this document, or simply try pressing the button to see what happens.

Using sliders

The above picture shows an example of a slider control. Sliders are used to interactively change the value of a numeric parameter. The precise value of the parameter is displayed to the left of the slider, and this is reflected in the distance of the black rectangle from the left edge of the slider. To change the value, place the cursor over the slider, near where you want the black rectangle to go, then hold down the left mouse button and drag the cursor along the slider until the displayed value is satisfactory. Alternatively, to make fine adjustments move the cursor to one side of the black rectangle and click the right mouse button one or more times. Each click will cause the rectangle to move a small amount towards the cursor and have a corresponding effect on slider's value.

The main control area

The target control panel

This panel provides controls for manipulating the size and position of the target area. When guiding is enabled, the autoguider will attempt to keep the guide-star positioned at the center of the target area. The target area is denoted by a nested pair of green boxes. The margin between the boxes is used to estimate the background surface brightness, so the guide-star should not extend into this region.

To reposition the target:

1. If the guider is currently enabled, disable it (see below).
2. Press the POSITION button.
3. Move the cursor into the camera window and click the left mouse-button near where you want the target to appear, then let go.
4. The target will now move wherever you move the cursor.
5. Once the target is where you want it, click the left mouse button again, and the target will anchor itself at the chosen position.
6. The above operations can be aborted at any time by pressing the right mouse button. The target will then return to its original position.

To resize the target:

1. If the guider is currently enabled, disable it (see below).
2. Press the RESIZE button.
3. Move the cursor into the camera window and click the left mouse-button close to a corner of the target area, then let go.
4. The target area will now expand as you move the cursor away from the center of the target, and shrink as you move it towards the target center.
5. Once the target has the desired size, click the left mouse button again, and the target dimensions will freeze at the chosen size.
6. The above operations can be aborted at any time by pressing the right mouse button. The target will then revert to its original size.

If you don't intend to use the autoguider, but you do want its display, the target marker can be deleted from the display by pressing the DELETE button. This has the side effect of stopping guiding if the autoguider is enabled. The target can subsequently be resurrected by following the above instructions for positioning the target.

The Autoguider control panel

This panel controls the state of the autoguider. The ENABLE button is used to enable or disable the autoguider. The buttons to its right are used to select one of two modes of guiding.

Enabling and disabling the guider

The latest estimate of the guide star position will be indicated by a cross hair within the target area. When the signal to noise ratio of the guide star is sufficient for guiding, four lines will also be seen to extend outwards from the middle of the four sides of the target area. At other times, when the guide star is obscured by a cloud, or poor seeing diminishes the visibility of the star, guiding will automatically cease until the guide-star reappears.

Selecting a mode of guiding

1. GUIDE: This is the normal guiding mode. In this mode the telescope is moved to keep the guide star within the target area.
2. DRIFT: In this mode, instead of moving the telescope, the target marker is moved to follow the guide-star. This mode is used by the automatic camera-geometry calibration facility. It could also be used from the Tcl Autoguider interface to obtain a record of the trajectory of an object as it drifts through the camera field.

The Gray-scale control panel

This panel controls the contrast and brightness of the displayed camera image. The default contrast and brightness maps all available brightness levels in the image to colors spanning from black to white (or vice versa when the INVERT button is pressed). This default can be restored at any time by pressing the RESET button.

How to adjust the contrast and brightness of the image

1. Press the ADJUST button.
2. Move the cursor into the camera window and click the left mouse button somewhere near the center of the window, then let go.
3. The contrast and brightness will now change as the cursor is moved within the camera window. As the cursor is moved from the left edge of the window to the right edge, the brightness will increase from its minimum to its maximum (or the reverse if the INVERT button is pressed). As the cursor is moved from the bottom of the window to the top the contrast of the image will increase from its minimum to its maximum.
4. Once the desired appearance has been achieved, press the left mouse button a second time to adopt it. Note that as soon as you do this, not only will the display show the new selection, but the autoguider will start to use it. So before committing to a new appearance, be sure that the guide-star retains sufficient contrast against the background for the autoguider to continue functioning.
5. The above operations can be aborted at any point by pressing the right mouse button. If you just want to experiment with changing the appearance, without effecting the autoguider, follow the above directions, but when you have finished, instead of pressing the left mouse button to adopt the new appearance, press the right mouse button to revert to the old one.

Toggling reverse-video

The Camera-Gain control panel

The Xybion camera that the autoguider was designed for, contains both an image-intensifier and a gain-controlled CCD. There are three hard-wired CCD gains, 1, 2 and 4. The x10 gain is actually wired to the x4 because although the Xybion controller unit provides a x10 input, Xybion cameras only support up to x4.

Adjusting the CCD gain

Adjusting the image-intensifier gain

The Frame Grabber control panel

This panel controls how the autoguider collects camera frames for display and guiding. The ENABLE button should normally be pushed down. This enables the camera, and the capturing of camera frames. At the end of the night I suggest that you pop up this button. Not only does this halt frame capturing, but it also sets and holds the image-intensifier gain at zero to prevent damage.

The two mutually-exclusive mode buttons FRAME and FIELD select between capturing integrated frames and capturing individual fields. Camera frames are actually formed from two interlaced fields, so by reading out fields instead of frames, the autoguider sees twice as many images per second, sampled with half as many lines. Field mode was written for possible future use in fast guiding. The only possible use to normal guiding is to double the potential speed of drift mode guiding. Note that the integration and averaging sliders are ignored in field mode.

The FRAMES PER INTEGRATION slider specifies how many frame intervals that the frame-grabber waits before reading out an integrated image from the camera's CCD. Similarly, the INTEGRATIONS TO AVERAGE slider specifies how many of these integrated frames to average before displaying the result. The delay between displayed frames is equal to the product of these selections, multiplied by the basic 30th second sampling interval.

Note that the available frame buffers are only 8-bits deep, so averaging multiple 8-bit frames in-place results in some loss of information. The actual amount of averaging achieved depends on the brightnesses of the pixels being averaged. To be precise, only the first N frames of a mean contribute pixels with brightness values of 2^(N-1). Thus it is pointless to average too many frames if the features that you want to enhance are inherently dim. For more details see the architecture documentation on the Frame Grabber task.

The Fiducial Marker control panel

This panel provides a way to mark locations of interest in the camera window. Any number of markers can be placed in the camera window. These can then be moved, deleted, or act as the targets of cursor-directed telescope moves. You can actually define two pages of markers, and alternate between them by pushing the PAGE2 toggle button. To clear the displayed page of markers press the CLEAR button.

Adding a new marker

1. Press the ADD button.
2. Move the cursor into the camera window.
3. Press the left mouse button and let go.
4. A new marker will now appear, and it will move wherever you move the cursor.
5. Once you have moved the marker to the desired position, press the left mouse button a second time to anchor it there.
6. The above operations can be aborted at any point by pressing the right mouse button. The new marker will then be discarded.

Moving an existing marker

1. Press the MOVE button.
2. Move the cursor close to the marker that you wish to move.
3. Press the left mouse button and let go.
4. The marker will now follow the motion of the cursor.
5. Once you have moved the marker to the desired position, press the left mouse button a second time to anchor it there.
6. The above operations can be aborted at any point by pressing the right mouse button. The marker will then return to its original location.

Erasing a displayed marker

1. Press the DELETE button.
2. Move the cursor close to the marker that you wish to delete.
3. Press the left mouse button and let go.
4. The marker will disappear.

How to command a telescope move with the cursor

To move the telescope:

1. Press the GOTO button.
2. Move the cursor into the camera window.
3. Select the destination position of the move by clicking the left mouse button over the desired point. Then let go. Note that if you click within the rectangular bounds of a marker, then the destination arrow will snap to the center of the marker.
4. An arrow will now be drawn, with its head at the selected destination and its tail following the motion of the cursor.
5. Move the cursor until the tail of the arrow is over the astronomical object to be moved, then press the left mouse button a second time. The telescope will now move the object at the tail of the arrow to the position of the head.
6. The above operations can be aborted by pressing the right mouse button.

The Sky Coordinate control panel

This panel allows you to draw a straight line with the cursor and measure its direction and length on the sky in a variety of formats. To make such a measurement:

1. Press the MEASURE button.
2. Move the cursor into the camera window.
3. Click the left mouse button at the desired origin of the measuring line, then let go.
4. A line will now appear, anchored at the selected origin. Its other end, augmented with an arrow head to denote the direction of the measurement vector, will follow the motion of the cursor. The dimensions of the vector will be displayed at the top of the window.
5. To change the displayed format of the dimensions, move back to the control area and toggle the POLAR and/or PIXEL buttons. The former selects between polar and rectangular display formats, while the latter selects between arc-second and camera-pixel units. Once you have selected the desired format, move the cursor back into the camera window and measurement will resume with the new display format.
6. When you have finished making the measurement, press either the left or the right mouse buttons.

The Frame Memory control panel

A spare frame buffer is available for recording an image for later inspection. To record a copy of the currently displayed image press the STORE button. To toggle between displaying the current camera-image and a stored image, press the DISPLAY button (remember that you will need to press it again to return to the real-time display). To display the difference between subsequently received camera frames, and a previously stored frame, press the SUBTRACT button. Press it again to return to the un-differenced display.

The subtraction facility may be useful in removing hot spots. To experiment with this, first find a blank area of sky and set the integration time to the same value that you plan to observe with. Record this image by pressing the STORE button. Then press the SUBTRACT button so that subsequent images will have the hot spots removed.

Note that the stored image will be discarded if you press the the FRAME or FIELD buttons on the frame-grabber panel. Also note that if the integration time of the stored image is higher than that of the observed image then you may end up subtracting a inappropriately large background offset from the observed image.

The Compass control panel

This panel allows you to position, resize or erase the compass rose.

To position the compass:

1. Press the POSITION button.
2. Move the cursor into the camera window and click the left mouse-button near where you want the compass to appear, then let go.
3. The compass will now move wherever you move the cursor.
4. Once the compass is where you want it, click the left mouse button again, and the compass will anchor itself at the chosen position.
5. The above operations can be aborted at any time by pressing the right mouse button. The compass will then return to its original position.

To resize the compass:

1. Press the RESIZE button.
2. Move the cursor into the camera window and click the left mouse-button close to the arrow-head of the compass then let go.
3. The compass will now expand as you move the cursor away from the center of the compass, and shrink as you move it towards the compass.
4. Once the compass has the desired size, click the left mouse button again, and the compass dimensions will freeze at the chosen size.
5. The above operations can be aborted at any time by pressing the right mouse button. The compass will then revert to its original size.

The Pixel Calibration control panel

This panel is the interface to a facility that measures the size and reflection parity of the pixels of the current guide camera. The currently adopted pixel sizes are also displayed. Without a good calibration, the autoguider will not guide correctly, mouse directed moves will miss their targets, and vector measurements will be incorrect.

To re-calibrate the geometry of the camera wrt the sky.

1. Disable the guider.
2. Move the telescope until a bright guide-star is positioned near the center of the camera window.
3. Roughly center the target area on the guide star.
4. Press the CALIB button.
5. The autoguider will automatically now perform the following steps:
   1. Change to DRIFT guider mode.
   2. Enable the guider.
   3. Center the target area on the guide star.
   4. Move the telescope north while keeping the target area centered on the star.
   5. Stop and take a measurement of the target displacement from where the move started.
   6. Move the telescope back to its starting position while keeping the target area centered on the star.
   7. Move the telescope east while keeping the target area centered on the star.
   8. Stop and take a measurement of the target displacement from where the move started.
   9. Move the telescope back to its starting position while keeping the target area centered on the star.
   10. By examining the pixel displacements that resulted from the Northward and Eastward moves, the autoguider deduces the rotation angle of the image and whether it is reflected.
   11. Move the telescope diagonally such that the star moves at approximately 45 degrees over the displayed image.
   12. Stop and take a measurement of the target displacement from where the move started.
   13. Move the telescope back to its starting position while keeping the target area centered on the star.
   14. Compare the measured target area displacements to the commanded telescope moves to determine the dimensions of the pixels.

The configuration of the autoguider

Adjustment of the tracking algorithm

Two algorithms are provided for locating the position of a guide star within the target area. Both algorithms operate on row and column sums of pixel intensities within the target area.

1. Median
   Horizontally, the median location of the guide star is at a vertical-line in the target area that separates equal amounts of flux to its left and right within the target area.

2. Centroid
   Horizontally, the gray-scale centroid of the guide star is the intensity weighted mean column-index over the width of the target area.

Adjusting the background-subtraction threshold

The value of the background that is subtracted is the sum of two components:

1. An constant offset, equal to the mean brightness of the margin that surrounds the target area.

2. The standard deviation of the brightnesses of pixels in the margin of the target area, multiplied by the value of the THRESHOLD SNR slider.

Guiding constraints

The SEEING TOLERANCE slider can be used to guard against guiding on small offsets that are solely due to seeing variations. It sets the width and height of an error box surrounding the center of the target area. Only when the guide-star is seen to exit this box will a correction be made to move it back to the center.

The TELESCOPE MOVE RATE slider sets the rate at which the telescope is moved when re-centering a star. If this is set too low compared to the correction interval, then a given correction may not have time to finish before the next correction is measured, and the result will be oscillation. An optimal value that seems to avoid both this problem and the problems of overshoot at high move rates, is 10 arc-seconds/second.

The CORRECTION MULTIPLIER slider determines whether the guide-star is moved all of the way back to the target center in a single move. This is usually the desired behavior, so the value of the slider should normally be set to unity. However when guiding on an object that is completely obscured by a slit, and the object starts to exit one side of the slit, one doesn't want the emerging edge of the star to be moved all the way back to the center of the slit, because the other edge of the star would then probably exit the other side of the slit! To counter this, decrease the correction multiplier. The autoguider will then only move the edge of the star the specified fraction of the distance between it and the target center (which I am assuming is positioned at the center of the slit). The appropriate scale factor to choose depends on the visible dimensions of the star, compared to the width of the slit. If the star and the slit are roughly the same width, then a small factor will be needed. In such cases it is probably better to turn up the intensifier gain until light from the star spills over both edges of the slit, then use the centroid algorithm, but don't do this if there are any other stars in the field that are bright enough to burn out the intensifier at such high gains.

The MINIMUM MOVE INTERVAL slider sets the maximum frequency at which the autoguider will move the telescope to correct for the guide-star drifting. The TCS can't cope with updates of less than a second, and in practice the frequency will often be limited by the guide camera image update rate. Given that seeing effects generally swamp telescope tracking errors on intervals of a few seconds, increasing the update interval is one way to avoid tracking seeing-generated offsets.