Automatic Guide Exposure

 

This window is to setup the parameters for the Automatic Guide Exposure function in CCD Commander.

 

If this function is enabled, CCD Commander will always automatically try to find a suitable guide star and guide exposure whenever the autoguider is enabled in a Take Images action.

 

The Minimum Guide Exposure and Maximum Guide Exposure entries tell CCD Commander what range of exposure times are valid for the autoguider.  

 

The Maximum Guide Exposure Increment limits the change CCD Commander will make to the guide exposure as it searches for a guide star.  CCD Commander will always start with the Minimum Guide Exposure value.  Each successive increment in the guide exposure will always be less than or equal to the Maximum Guide Exposure Increment.

 

The minimum time is usually dictated by how well your seeing is.  For poor seeing, the minimum time should be longer so that the autoguider does not "chase the seeing".  

 

The maximum time is usually determined by how long your mount can accurately track without receiving a guide correction.  This value should be 2 to 4 times smaller then your maximum unguided exposure time.  If your mount can track well for 4 minutes unguided, you can comfortably set the Maximum Guide Exposure to 60 seconds or more.

 

The Minimum Guide Star Brightness and Maximum Guide Star Brightness values tell CCD Commander what range of brightness values the guide star should fall into.  Note that CCD Commander looks at the maximum brightness of the star recorded by the guide CCD.  This means that, given the same star, the computed exposure time can vary depending on how the star falls on the guide CCD pixels.  

 

The minimum brightness value should be set to give sufficient a signal-to-noise ratio of the star such that the guide algorithm can successfully track the star.  It is important to note that all autoguider centroid calculations work better as the guide star signal-to-noise ratio increases.  This means that brighter guide stars can provide better guiding accuracy than dimmer guide stars.  Keep this in mind when setting your minimum brightness.

 

The maximum brightness value should be set such that the star never saturates the guide detector.

 

The Guide Box X Size and Guide Box Y Size specify what size window CCDSoft uses to track the star.  It is important for CCD Commander to know this because it will check a box this size about a potential guide star for any other stars that might impede CCDSoft's tracking ability.  In addition, CCD Commander will not pick a star that is too close to the edge of the guider since CCDSoft could move the mount significantly to bring the guide star far enough away from the edge for the guide box to fit.

 

You can change the size of the guide box CCDSoft uses.  Software Bisque has documented this procedure here.

 

For MaxIm the default guide box size is 32x32.  Change the value in CCD Commander to match the value you have selected in MaxIm.

 

The Typical Guide Star FWHM indicates the size, in pixels, of the typical guide star.  CCD Commander uses the value help determine where the edge of a star is, and where another star may begin.  The default value is sufficient in most circumstances and should not need to be changed.  But if your guide stars are significantly larger than 4 pixels in any axis, then increase this value.

 

The Ignore 1-pixel Stars checkbox will cause the search algorithm in CCD Commander to disregard stars that occupy only 1 pixel.  This is especially useful for guiders that have hot pixels - but can be a problem for short focal length guide scopes where the guide star FWHM is <1 pixel.  Evaluate your guider images to ensure that the guide star FWHM is routinely greater than 1 pixel before enabling this option.

 

The Maximum Star Movement between exposures entry tells CCD Commander how much a potential guide star can move between exposures and still be considered valid.  In order to rule out cosmic ray hits and other anomalies, CCD Commander takes two images of the Guide CCD in the search for a guide star.  CCD Commander will accept a guide star if its centroid moves less then the amount specified here between the two exposures.  Be cautious increasing this value - only do so if the default value causes CCD Commander to ignore real stars.

 

The Continuous Autoguiding check box tells CCD Commander to never stop the autoguider during an image sequence.  Not until the mount moves will the autoguide be stopped.  This is useful when you have an external guider and want to guide continuously throughout the exposure for optimum speed.  Be aware that when using an SBIG self-guiding camera, the guider will continue uninterrupted trough filter changes.  It is not recommended to do this.  Instead disable Continuous Autoguiding to allow the autoguider exposure to be recomputed for each filter.

 

Disable Automatic Guide Star Recovery will cause CCD Commander to always search for a guide star prior to starting autoguiding.  This is useful if your mount is not able to get the guide star back into the guide box on a return slew.