Contents - Index

Guided Operation

CCDAutoPilot provides many options and alternatives for guided operation.  You can set dithering to remove unwanted artifacts, hot and cold pixels and other unwanted artifacts from your combined images, Automatic Guide Star Recovery (AGRS) parameters with or without an audible alarm and guide exposures for different filters.  With CCDSoft, you can also set the size of your guide star box.  



Dithering
Dithering is a process whereby the guide star location on the guider chip is moved between image exposures in either a random or  optimized manner.  When the guider control repositions the guide star to the new location, the image will be slightly displaced on the imager chip.  When the resultant images are aligned and properly combined, hot and cold pixels, cosmic ray hits and other sensor-specific artifacts are removed, much better than any hot/cold pixel routine can ever hope to achieve.  The resultant image is smoother and artifact-free.  See this link for more details on this technique.
Dithering can be done either totally randomly, as determined by a random number generator, or in a controlled manner to maximize separation between each sub-exposures artifacts while minimizing the overall guide star movement.  Both options are provided.  The amount of the dither is user-definable.  Here is a discussion on how to set the amount of dithering.
  • Dither method: Select Enhanced (preferred) or Random.
  • Maximum Dither: This is the peak dither value and can go +/- from the starting direction.  As an example, suppose Enhanced Dithering is selected with a Max Dither of 3 pixels.  The first sub-exposure will leave the guide star position undisturbed.  The second will move the X position of the guide star + 3 pixels.  The third will move the X position of the guide star - 3 pixels from the first exposure.  Thus the total movement between the second and third exposure is 6 pixels but the movement relative to the first sub-exposure is ± 3 pixels.  If dither is set to 0, guiding will not be stopped between exposures except for any specified focusing actions.

    Automatic Guide Star Recovery (AGRS)
    Many times when a guide star fades, the guider drives the telescope off the target in its quest to find the guide star.  CCDAutoPilot has a technique, to prevent this from happening in an attempt to minimize data loss.  If the guide star position is not recovered to the user-specified tolerance in a user-specified number of attempts, AGRS institutes a number of procedures in an attempt to recover the guide star.  If all those attempts fail, the image is allowed to continue unguided to prevent the guide star search from driving the telescope from its intended target.  AGRS is repeated at the start of the next sub-exposure so that, if the passing cloud has passed for example, the guide star is recovered and guiding continues on target.  An optional alarm can be sounded to alert a nearby operator of the failed guide star recovery attempt.  This has proved instrumental for some users in alerting them to an impending bad weather condition, although this should not be relied upon for equipment protection.
  • Max. Error: If the maximum error is specified at 0, AGRS is disabled; if it is any other value, AGRS is enabled.  This entry specifies the value the guide error has to get down to during guider restart before the exposure is permitted to continue.
  • Max. Error Cycles: This specifies how many tries the guider has to get the guide star error below the Maximum Error.  If the guider does not get the guide error below the specified Maxim Error in the specified Maximum Error Cycles. AGRS is activated.
  • AO Center: When an Adaptive Optic corrector is used for guiding, the mirror/glass position could be at some point other than 50% when slewing to a target.  Checking this box centers the mirror/glass at the neutral position.  See also the SBIG/AO notes here.

    Guide Calculator
    This calculator allows you to determine your optimum minimum and maximum move parameters for your system.  Proper setting of these parameters is important for optimal guiding.  When connected to your camera control program, the guide calculator will automatically determine whether DirectGuide (CCDSoft only) or camera relays is in use and calculate minimum and maximum recommended moves automatically.  These parameters can be loaded into your camera control program.  For more information, see the online Guide Calculator.
    Max. Allowable P-P Error: Here we try to determine when we want your mount to actually make a correction. If your guider error is only .05" arc-seconds, there is no need to make an correction. If your min move setting is set to low, then you guider relays will move the mount every guider cycle. This can result in mount oscillations or chasing atmospheric turbulence. My best suggestion in setting this value is about 75% the image scale in which you are imaging. For example, if you are imaging at 1.2" asp, then set this value to 0.9" asp. This way a guider correction is only sent when the centroid of the guide star is 0.45" asp off in either the X or Y axis from the selected guide star position.
    Max. Allowable Movement: This value is not required to achieve great autoguiding, but it does protect us from some extreme situations that could ruin a sub-exposure. For instance, if a cosmic ray hit is sensed on the autoguider CCD, this could cause your autoguiding software to think that the centroid of the star has moved many, many pixels from center. Therefore it is going to attempt to correct the mount for this error. If you have no max move setting, this will result in a very large correction and a ruined exposure. If you have a limit to the size of the correction, i.e. max move, then this effect can be mitigated by not allowing a large correction. Setting this value too low may result in under correction, so its best to set this value about 2X - 4X larger than the above 'Peak to Peak Maximum Allowable Error' setting.
  • Recommended Min. Move: Based on the above entries, this is the recommended minimum move amount for your guider. 
  • Recommended Max. Move: Based on the above entries, this is the recommended maximum move amount for your guider
  • Aggressiveness: With the recommended minimum and maximum move, this is the recommended aggressiveness setting for your guider
  • Apply To Guider:Hitting this button will automatically enter the recommended values into your camera control program.  With CCDSoft, the Guide Calculator will determine whether DirectGuide or camera relays are being used and enter the appropriate values.

    Guide Exposure: Guide exposure is settable on the Light frame page for each series and/or target.

    Guide Box Size (CCDSoft only)
    This setting allows adjusting the size of the guide star box. This is the image that is visible when the guider is operational. The settings must be set before guiding is enabled. You can set either the AO7 guide star box or the conventional guider guide star box. Changing during guiding will not have any effect. Due to Maxim's automation interface limitations, this feature is available only with CCDSoft.
  • Set: Enters the values shown for the guide box size.
  • Reset: Resets the guide box size to the default values of 8 for the AO7 and 32 for conventional guiders.