Other automation tools have attempted to imitate ACP's adaptive guider startup (some call it predictive autoguiding). But ACP's guiding controls still provide the most reliable and robust guider startup, recovery from guiding loss, and monitoring of guide errors. For this, we must thank guiding wizard Jim McMillan. He is the father of adaptive guider startup and recovery; his ideas have revolutionized robotic auto guiding. Jim's guiding algorithms were developed on ACP first, and are still being refined. We also must thank Mike Rice of New Mexico Skies for helping us get rotated guiding supported, through his knowledge and use of his 24" RC.

ACP's smart hands-off auto-guiding solves the following problems:

• Selecting the exposure interval for the guide star brightness
• Adjusting the exposure interval for filter transmissivity (internal guide-chips)
• Needless use of guider for short exposures
• Guider getting "lost" during a series of repeated exposures
• Guide star that is too close to the edge of the chip
• Jumping between guide stars that are too close together
• Managing a camera rotator (see Rotator Support)

Through intelligent use of MaxIm DL's scriptable guiding features, ACP manages guider usage and guider startup. It also monitors guide errors during repeated sequences of exposures and if the guider gets lost, it attempts to re-start the guider, or if that fails, it stops the current image set. At this point ACP will move on to the next target and try again, etc.

We worked with Jim McMillan to incorporate his ideas. For example, taking his concept of targeting a specific guide-star brightness, we translated that into targeting a specific guide star signal-to-noise ratio. PinPoint's zonal background statistics estimator is the key to being able to do a rigorous job of this.

This makes the adjustment independent of guider imager sensitivity, filter selection (for internal guide chips), and background statistics. We developed an algorithm that achieves this by automatic adjustment of the guiding exposure interval.

Jim has written an excellent paper on autoguiding. We highly recommend that you read through it. Keep in mind that the automatic adjustment of guider exposure interval is part of ACP's standard operation.

To Guide or Not to Guide?

Depending on the tracking capabilities of your mount, there will be a minimum imager exposure interval below which guider usage is not efficient. For example, if your mount will track acceptably for 1 minute, it is a waste of time and possibly risky to start the guider for exposures shorter than one minute. ACP lets you configure a minimum unguided exposure interval. Below this interval, the guider will not be started. You never have to tell ACP whether or not to use the guider, it decides this by itself.

Guider Startup

The objectives of this process are (a) to select a suitable guide star and (b) automatically adjust the guider exposure interval to be as short as possible while still having a guide star that is strong enough to track reliably. This makes it possible to have hands-off reliable guiding even when the guider is behind a filter wheel with filters of widely varying transmissivity. It works in the general automation scenario even when the brightest star is very bright or very faint (within limits of course).

Step 1: Guide Star Selection

Using a configurable starting exposure interval, a full-frame is acquired from the guider. PinPoint is then used to scan the image for star-like objects. This rejects hot pixels. PinPoint measures the total flux relative to the mean background of every star in the guider image. ACP scans PinPoint's list of stars and picks the brightest star that is (a) not too close to the edge of the guide chip, and (b) that does not have another star within 10 pixels.

Step 2: Exposure Calculation

Using the star selected in step 1, ACP calculates the signal to noise ratio of the star. It then calculates a new guiding exposure interval to achieve a configurable target signal to noise ratio (usually about 3). Configurable short and long limits are enforced, preventing guiding rates that are too fast or too slow for your mount. It then loops back to step 1 for another pass. This is repeated up to 4 times. This is a much more robust way of calculating guide star exposure compared to specifying ADU limits.

Guider Monitoring

When doing guided imaging, ACP always checks the guider X and Y errors before starting an imager exposure. It will wait for up to 10 guider cycles for the guide errors to settle below a configurable maximum guiding error in pixels. This prevents guiding problems caused by the guider drifting while the previous image is being downloaded and/or while the shutter is closed (for internal guide chips). ACP does not monitor the guider during an imager exposure.

When using ACP via its browser interface, guider errors are displayed on the web page and periodically updated during an exposure. This allows you to detect guiding problems during a long exposure and abort it in the unlikely event the guider gets lost (usually due to clouds). Of course, when using ACP locally, you can watch the guider by looking at MaxIm DL's guider display.

Guider Cycling

If you are using an internal guider and filters are installed, the guider is always stopped and re-started from one target to another, to allow for transmissivity changes in different filters. If you have an external guider, or if no filters are installed, the guider is allowed to run continuously across sequential co-located targets. This optimizes guider usage in these scenarios.

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