Once the CCD is calibrated it is ready to take Track & Accumulate exposures.  The procedure is quite simple:  an exposure time is selected, along with the number of exposures to be taken.  Certain other parameters can be set, such as whether you want a color image, etc.  Once the first exposure is taken, it is displayed and you are asked to select a guide star.  The position of this star is recorded and any drift is compensated for between each exposure by the CCD camera.

Start by opening the Track & Accumulate window by clicking the T&A button in the toolbar, or by selecting Track > Track & Accumulate from the main menu.

Above:  The CCDOPS Toolbar.

Below are typical settings for taking a black-and-white Track & Accumulate image (the only difference for a color image would be the setting of the Special Processing command and selecting a file name as was shown in the Basic Imaging section).  Each setting is explained below.

Snapshot Time

This is the duration of each individual exposure.  This should be set to the longest exposure time possible without trailed stars appearing in the image.

Number of Snapshots

Determines how many total exposures will be taken.  If, for example, you wish to take 3 minutes worth of images, and each individual exposure will be 30 seconds long, this parameter would be set to 6.

Frame Size

Usually set to Full to take an unbinned, full-frame image, but can also be set to Medium or Low resolution for smaller images and faster downloads, or if taking binned color exposures for an LRGB image.

Dark Interval

This setting determines how often a dark frame is taken.  Setting this to 1, for example, would cause a dark frame to be taken between each light exposure, while setting it to 3 would take a dark frame after every third exposure.  Most often this is set to Series so a single dark frame is used for the entire sequence of exposures, which saves quite a bit of time.

Track Mode

If you have calibrated the CCD (see previous section) you can use the Relays mode to actually shift the position of the telescope to keep the guide star in place.  Align mode causes the software to register the images by shifting the pixels within the picture.  Setting Track Mode to Both is most common, activating both methods of alignment.

Declination

This compensates for the need to increase RA motions at higher declinations.  To use this you must enter the declination of the object you are imaging.  You must also enter in the Calibration menu the declination at which you calibrate the telescope.  Very few of the top CCD imagers use this function, choosing rather to calibrate before each new object, and this setting is left at 0.  Remember that you must also recalibrate if you rotate the CCD camera relative to the telescope for framing purposes.

Special Processing

Leave this set to None for black-and-white images.  Set to Color Grab to take an automatic sequence of red, green, and blue images.  Track & Accumulate will take the number of exposures set in Number of Snapshots for each color, so setting the Number of Snapshots to 5 and then taking a Color Grab T&A results is 15 total exposures, 5 in each color.  Each color file is saved individually to later be combined using the RGB Combine method.

Selecting a Guide Star

Once the first exposure is taken (along with a dark frame) the image is displayed.  The software asks you to select a guide star from the image.  A guide star is selected simply by clicking on it in the image.  A small white box is drawn around the star showing you the field of view which will be used by CCDOPS to track the star.  Be sure there are no other bright stars in the field that could be confused for the guide star.

Tip:  Select a fairly bright star, especially when taking a color image.  The first exposure will be through the red filter, where the CCD camera is typically most sensitive.  When the blue filter is in place you do not want the star to disappear due to the decreased sensitivity.  Also, since you cannot tell the color of a star from a single exposure, it is possible that you have selected a red-colored star!  In this case, when the blue filter is used, the star's brightness may decrease significantly.  Choosing a brighter star will help avoid these problems.

Above:  An single 60-second image showing a selected guide star.  The star is fairly bright, no other bright stars are nearby, and it is near the center of the field of view where the image is sharpest, resulting in the best image alignment.  Using the settings shown in the screenshot above, 5 more exposures would be taken, aligned using the selected guide star and combined to create a final image.

Above:  Final Track & Accumulate image, a combination of six 60-second exposures.

Track Log

CCDOPS displays a window during Track & Accumulate exposures indicating the tracking status.  After each exposure is downloaded and the telescope is shifted (if necessary), the amount of correction necessary is displayed.  For the most part, if something goes wrong it will be evident from the images, but the Track Log helps identify any problems should you encounter them.  Also displayed is a window showing the guide star with a crosshair on it.  As long as the crosshairs stay on the star, everything should be okay, regardless of what the Track Log reads out.

Color Images

The Track & Accumulate function can also be applied to color images.  In the Track & Accumulate window select Color Grab under Special Processing.  This will take the specified number of exposures in each color.  For example, you could take 5 red, 5 green, and 5 blue exposures which can then be combined to create a color picture.

Next, Self-Guiding CCDs....

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