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Most SBIG CCD cameras have a feature known as self-guiding.  Self-guiding is the easiest way to make guiding corrections 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 usually 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 Camera > Switch CCDs or press Ctrl-D to select the guide chip instead of the main chip.  Click Focus in the Toolbar and make sure the Update mode is set to Automatic.  Choose an appropriate exposure (usually 2-5 seconds).  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 suitably bright 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 Track > Calibrate from the main menu.  This will bring up the Calibrate Track window.

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

Exposure Time

This sets the exposure time and should be the same as that used to find the guide star (typically 2-5 seconds, possibly longer if using narrowband filters).

X/Y 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.

Recommended Calibration Times

Focal Length                Drive Speed

0.5x Sidereal

2x Sidereal

8x Sidereal

400mm

N/R

N/R

15 sec.

600mm

N/R

N/R

10 sec.

800mm

N/R

25 sec.

6 sec.

1200mm

35 sec.

18 sec.

5 sec.

1600mm

30 sec.

15 sec.

4 sec.

2000mm

20 sec.

10 sec.

3 sec.

2500mm

15 sec.

8 sec.

N/R

4000mm

4 sec.

N/R

N/R

Declination

The short answer is, don't use this setting.  The idea is that if you tell the camera what declination you calibrate at, it will know the calibration settings for any other declination in the sky.  However, you will often rotate the camera to find guide stars, and on a German equatorial mount you will change sides of the sky (from east to west) and these will always require recalibration.  Calibration only takes a couple minutes, so it is almost always best to ignore this setting and calibrate before each new object is imaged.

Steps

Normally this is set to 1.  If you prefer to break up each calibration movement into smaller steps, you can increase the number, but there isn't a really compelling reason to do this with most mounts.

Active CCD

This simply determines whether you want to calibrate using the main CCD or the guide chip.  You will use the guide chip for self-guiding purposes.

Log Results

If you are having any trouble with calibration routines, this command can log the resulting five calibration images for later review.  Normally, though, errors can be discerned from the calibration results (see below) without the need to save the images.

 

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 Calibration Track window in CCDOPS (Track > Calibrate).

Begin by choosing an exposure time.  Be sure that the exposure is long enough to detect a relatively bright star in the image.  Guide star exposures are typically 2-5 seconds unless you are using narrowband filters, such as Hydrogen-Alpha, which can require 10-20 second guide exposures.

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.

The Calibration Results window appears.  After each exposure, a set of numbers is displayed.  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 317, 214.  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 313, 149.  This shows almost no change in the Y-axis and a change of 65 pixels in the X-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 317, 214.  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 Y-axis.  Similar results should appear:  the star should move in the Y-axis 10 or more pixels and move very little in the X-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 CCDOPS is that it plots the movement of the star during the calibration procedure in the lower right corner of the Calibration Results window.

 

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 SlfG (Self Guide) from the Toolbar.

This will bring up the Selfguide window.

Select an Exposure time for the main CCD chip.  Track time selects the exposure time for the guide chip.  This should be set to the same exposure used for the calibration routine.  Set the option for Guide to Cursor.  This will allow you to select the guide star manually by clicking on it with the mouse, in case there is more than one star in the image.  Click OK.  A dark frame and single guide exposure are taken and the image is displayed.

Above:  Click on the guide star to select it for tracking.  The box is drawn automatically.

Click Resume in the Self Guide window.  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 around 2 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 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.  The program displays the average value of the errors as well.  As can be seen they are below the maximum recommended value of 2 pixels, meaning the telescope is guiding sufficiently to begin the guided exposure.

 

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.

Focal Length

Maximum Error (+/-)

500mm

0.5 pixels

1000mm

1.0 pixels

1500mm

1.5 pixels

2000mm

2.0 pixels

2500mm

2.5 pixels

 

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 CCDOPS you can adjust Aggressiveness on the fly which is an extremely handy feature.

 

Starting Exposure

Click Start in the Self Guide window to begin the main CCD exposure.

Return to Imaging with CCDOPS Page



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