Most SBIG CCD cameras have a feature known as self-guiding.  Self-guiding is the easiest way to make guiding correction during a long exposure.  Guiding is necessary because no telescope mount can track perfectly.  During a long exposure, corrections must be made to keep the telescope tracking on a guide star.  This is often done with a second CCD camera called an autoguider.  In a self-guiding CCD camera, the autoguider is built into the camera as a second, smaller CCD chip mounted just above the main CCD.  This is a much easier system than having to use a second separate CCD with a guidescope or off-axis guider.  The following section will walk you through calibrating and using the autoguider in a self-guiding CCD camera.

Finding a Guide Star

The trickiest part of using a self-guiding CCD is acquiring a guide star, but there are some tricks to make is easier and once you have the hang of it the process becomes much easier.

Begin by selecting the autoguider CCD if the main chip is currently being used.  Select the Focus tab in the MaxIm CCD control window, then click the button next to Guider to select the guide chip.  Next, select an appropriate exposure time (usually 2-5) seconds.  Make sure Continuous is checked, then click Start Focus.  This will show you what the guide chip is seeing and will allow you to hunt down a guide star.

Locating a Guide Star

Far and away the easiest way to find a guide star is to use software with a field of view indicator function, such as Starry Night Pro or The Sky.  Using the field of view indictor function in these programs, you can see exactly the field of view for both the main CCD chip and the guide chip in combination with a given telescope.  This can show you nearby guide stars that can be found by either moving the scope slightly or by rotating the CCD camera.

Above:  Field of view indicator shows no bright guide stars visible on the guide chip (right side).

Above:  By rotating the camera 180 degrees, a brighter guide star is placed on the guide chip (left side).

Calibrating the Autoguider

Calibration teaches the autoguider how to correct for tracking errors.  The calibration procedure essentially pushes the telescope slightly in each direction to determine how much movement is necessary to correct for a given error.

Tip:  If you used a higher guide rate (say, 16x) for centering your target and acquiring a guide star, be sure to switch back to an appropriate rate for guiding (usually around 0.5x or 1x for long-focal-length scopes and around 4x or 8x for shorter scopes).  Some telescopes keep their guide speed setting separate from the "button rate" that is used to move the scope for centering, but not all of them do.

To calibrate the CCD, an exposure is taken and the position of the brightest star is recorded.  The CCD then moves the telescope slightly in one axis to shift the star and the new position is recorded.  By knowing how far the telescope moved during a given amount of time, and in which direction, the CCD knows how to correct the telescope if the star drifts in that direction during an exposure.  Next, the telescope is moved the opposite direction in the same axis, and then each direction in the other axis.  Now the CCD knows exactly how to correct the telescope for any tracking errors.

Note:  Usually calibration is done first in the X-axis, then in the Y-axis.  It is possible to determine which telescope axis (RA or Dec) corresponds to which CCD axis if your CCD camera is oriented in the standard position (cables hanging down toward the bottom of the telescope tube, i.e., away from the finderscope).  On a fork-equatorial mount the X-axis will be right ascension and the Y-axis will be declination.  On a German-equatorial mount the opposite is true, the X-axis is declination and the Y-axis is right ascension.  Whether plus-X equals east and minus-X equals west, etc., is a function of how the camera is attached to the telescope (straight-through, diagonal, HyperStar, etc.) and is also less important, so we won't worry about it.

Autoguider Settings

Select the Guide tab in the MaxIm CCD window.  Click the Options tab and select Guider Settings button to bring up the Guider Settings window.

Below are descriptions of the various settings along with recommended values.

Cal. Time

This tells the camera how long to "push" the telescope in each axis.  Ideally the guide star will move about 10-20 pixels during each calibration movement.  See the table below for recommendations using various focal lengths and drive speeds.

Recommended Calibration Times

Backlash

If there is backlash in your telescope mount, this setting will help compensate for it.  Ideally, a mount for CCD imaging will have minimal backlash to begin with, and if there is any it is better to compensate for this using the telescope's built-in backlash compensation (if available).  If the telescope does not have anti-backlash control, set this value to compensate for the backlash.  Whatever time you set will be added to the calibration time when the motors reverse direction.  Backlash control is typically only necessary in declination, since the RA motor is almost always running in one direction.

Guider Enables

Sets whether the X axis or Y axis is calibrated.  Unless you do not have control over one of the telescope's axes, leave these checked.  You can also select individual + and - controls on each axis.  Normally there is no need to deactivate any of these controls.

Control Via

When using a self-guiding CCD set this to Camera Relays.

Exposure Settings

Binning can be left at 1 for more accuracy.  If you have a very faint guide star, binning can be used to increase sensitivity.  There are subframe controls, but these are not necessary for the calibration routine.

After choosing the proper settings, click OK to confirm.

Calibration Procedure

The following is an example of calibrating the CCD.  Usually you will calibrate the camera while pointed at the object you wish to image, so begin by focusing the telescope and then finding a guide star with the autoguider CCD.  Open the Guide tab in the MaxIm CCD window.

Begin by choosing an exposure time.  Be sure that the exposure is long enough to detect a relatively bright star in the image.

Click on the Options tab and select Guider Settings.  Select a Calibration Time (use the above chart as a guide).  The default values for the other parameters will usually suffice for now.  Click OK.

Make sure Expose is set in the Guide window and click Start. 

An image is taken and displayed.  Select the guide star in the image by simply clicking on it with the cursor (do not draw a box around it as you would for a subframe).

Now select Calibrate in the Guide window and click Start again.  The calibration procedure now begins.

A description appears in the Guide window after each adjustment is made.  These numbers describe the current position of the star on the chip.  The position should change after each calibration adjustment.  The following window is from a successful calibration;  results are interpreted below.

Five exposures are taken during a calibration routine.  The first determines the starting position of the star, in this case 108, 93.  The telescope is then moved for the duration of the Calibration Time (5 seconds in this example).  Another exposure is taken and the new position of the star is shown.  Here the star moved to 106, 61.  This shows almost no change in the X-axis and a change of 32 pixels in the Y-axis.

This is the expected result.  If the camera is properly oriented to the RA and Dec axes of the telescope, there should be little or no change in one axis.  If the calibration time is set properly we should see a change of at least 10 pixels in the other axis.  So far, so good.

The next exposure is taken after the star has been moved in the opposite direction for the same amount of time as the first adjustment.  In theory this should put the star right back where it started.  In practice there will be some difference, but ideally it will not be too great.  Above, the star returned to 108, 93.  The star may not return to exactly the same spot (due to atmospheric turbulence or backlash in the mount) but it should be quite close.

The star is then moved in each direction in the X-axis.  Similar results should appear:  the star should move in the X-axis 10 or more pixels and move very little in the Y-axis.  The star should then return again to about the starting point.  Above it can be seen that this is the case.

A nice feature of MaxIm DL is that it plots the movement of the star during the calibration procedure in the image.

Above:  Guide star image after calibration.  The red lines show the movement of the star during the calibration procedure.  The star moved up and then back to its starting position, then moved left and returned again to the starting position.  A proper calibration routine should show a nearly perpendicular set of lines such as this.  The longer line in one axis indicates that the calibration time could have been shorter in that axis (or longer in the other axis), but this is a perfectly suitable result.

Interpreting the Results

If something really goes wrong, you'll know it because an error message will appear.  But, determining what went wrong is important, as is being able to tell if a calibration was ideal or just acceptable.  Ideally, the star should move at least 10 pixels in each axis and should come back to about the same position after each pair of movements (left then right, up then down). 

If the star moves far enough in one direction but does not return in the other direction (or, equivalently, if the star does not move in the first direction and then does in the second), this usually indicates a backlash problem.  This is most common in the declination axis, but can occur in right ascension as well.  Sometimes increasing the calibration time is helpful, but more often the backlash settings need to be adjusted (also, double check the telescope's balance).

Tip:  Increasing the right ascension calibration time is necessary the closer the telescope is aimed to the celestial poles.  Near the equator, a telescope may only require 5 seconds for an RA calibration time, but might need 15 or 20 seconds when pointing to an object above 60-degrees declination.  This is because the telescope must turn father in right ascension to cover an equivalent angular amount of sky at higher declinations.

Self-Guiding

Once the CCD is calibrated, the autoguiding procedure must be started.  Then the image may be taken.

Select Track in the Guide window and click Start.

The autoguider will begin tracking the guide star.  It is best to let the guider run for a minute or so to be sure everything is working correctly and to let the guider settle in.

Above:  Each time an image is taken and an adjustment is made to the mount, X and Y errors are displayed.

Guiding Results

It is important to know if the X and Y errors displayed in the Guide window are suitably small.  In the example above, the largest error is -1.21 pixels in the Y-axis.  The longer the focal length of the telescope, the greater will be the magnitude of the errors.  For the example above, a 2000mm focal length telescope was used.  A value of 1.2 is acceptable at this focal length.  At a short focal length of, say, 500mm, much smaller numbers are preferable.  Use the chart below as a rough guide, but be sure to examine the results from your own setup to see if the tracking is suitable.

Recommended Maximum Guiding Errors

There is handy rule of thumb for guiding errors:  take the focal length and divide by 1000 to find the maximum +/- error in pixels.  This assumes a maximum guide error of 3 arcseconds (comparable to long-term seeing conditions at most sites) and the use of a TC-237 guide chip.

Aggressiveness

Aggressiveness, as the name implies, controls how aggressive the guider is in correcting errors.  If you have constant large errors in one direction, try increasing the aggressiveness.  If you have large errors swinging from + to - on each exposure, try decreasing the aggressiveness.  In newer versions of MaxIm you can adjust Aggressiveness on the fly and independently in each axis.  This is an extremely handy feature.

Starting Exposure

Select the Expose tab in the MaxIm CCD window and begin the exposure as you normally would.  The autoguider will continue to track until you go back to the Guide tab and click Stop.

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