This section describes the Track & Accumulate feature available using CCDOPS.  Track & Accumulate allows you to easily combine multiple exposures to create the equivalent of a single longer exposure (without requiring a separate guiding CCD).

Track & Accumulate

A self-guiding CCD camera (ST-7, ST-8, etc.) has a built-in guiding CCD.  While the main CCD chip is collecting the image, the guiding CCD is taking short exposures (usually once every few seconds) and sending corrections, as necessary, to the telescope's drive motors to keep the instrument tracking perfectly.  However, a camera such as the ST-402ME has only a single CCD and so cannot make corrections while taking an image.  However, by taking many short exposures (say, 10 to 60 seconds each) and making corrections between each exposure, an effect similar to using a self-guiding camera can be obtained.

Calibration

In order for the camera to be able to make guiding corrections, it is necessary for the camera to know how much correction is required.  Teaching the camera how to correct is called calibration.  Calibrating the CCD camera is relatively straightforward, but there are some tricks to making it easier.

How Calibration Works

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 in any direction.

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.

Calibration Steps

1.  Select an Appropriate Calibration Star -- Try to use a fairly bright star when possible to avoid having the camera confuse the star with noise or the occasional cosmic ray strike, which will cause a bright pixel to appear for a single exposure.

2.  Select a Calibration Time (X Time and Y Time) -- Calibration time is how long the telescope moves in each direction before the star's position is recorded.  The amount of time should be sufficient to move the star at least 10-20 pixels on the CCD chip.  How long is required will depend on the focal length of your scope, the pixel size of the CCD, and the drive speed of the mount.  The pixel size is probably the least important factor, since most CCDs have similarly-sized pixels, so below are some recommendations based on some common telescope focal length and drive speeds (N/R indicates a combination of drive speed and focal length which is not recommended).

3.  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.

Calibration Procedure

Let's give 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 and centering your target.  Open the Calibrate window by selecting 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.  Select a Calibration Time (use the above chart as a guide).  Click OK to begin the calibration procedure.

Above:  A typical calibration setup.  The settings will depend on the focal length of the scope used and the guide rate.  5 second calibration times would be appropriate for a long focal length scope using a guide rate of about 0.25x sidereal or a short focal length scope using a higher guide rate of, say, 2x sidereal.

A line appears in the Calibration Results window after each adjustment is made.  These numbers describe the current position of the star on the chip.  Once the calibration is completed, results are shown at the bottom of the window showing how much correction is necessary for each direction.  The following window is from a successful calibration;  results are interpreted below.

Above:  Calibration Results window.

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 at least 10-20 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, 213, not exactly the starting point but certainly close enough.

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.

The results are output at the bottom of the window.  No error messages occur so a successful calibration was achieved.  Time to image!

Next, Taking Track & Accumulate Images....

Next Page