Contents - Index

Tracking & Guiding

CCDAutoPilot provides a number of options for guided and unguided imaging.  You can also image through the meridian using CCDAutoPilot's pioneering meridian flip technology.

Contents
Guiding & Tracking Techniques (Reference)
Guided Operation
Unguided Operation
Crossing the Meridian
Utility Window
Guide Box Size


Guided Operation



After successful initialization, CCDAutoPilot automates the guided operation parameters for optimal guiding.  Regardless of where you are pointed in the sky and at what rotation your camera is, the optimal guide vectors will be developed and applied to your camera control program.  Initial guide star detection is accomplished in CCDAutoPilot.  Its location is determined through three exposures to avoid selecting a cosmic ray hit or other artifact via an outlier rejection routine.  For automatic guide star exposure, three measurements are averaged to more accurately set the guide exposure time.  And if the guide star is lost due to passing clouds, the Automatic Guide Star Recovery routine (see below) is invoked to try to recover the guide star if at all possible.

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.
  • Suggest: this button will enter a Max. Dither value, based on the parameters of your system entered on the settings page.  This can be a starting point from which you can experiment if you desire.

    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 Audible 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.  

    Use Maxim Guide Star Detection: With some guiders, guide star detection may be problematic.  Even though every effort is made by CCDAutoPilot to select an appropriate guide star, in some cases it may be appropriate to use Maxim's guide star selection routine.  Checking this box allows this option.  Note that if this option is checked, Auto Guide Exposure is not possible nor is the ability to ignore saturated guide stars.  (It is generally agreed that best guiding performance is obtained with unsaturated stars.  CCDAutoPilot uses technology to avoid saturated stars for guiding.)

    Automatic Guide Exposure
    When Auto Guide Exposure is checked, CCDAutoPilot will set the guide exposure automatically to achieve the Target Guide Exposure you have entered within the Min. Guide Exposure time and Max. Guide Exposure time you specify.  This automatic setting is done at the star of each series and after a meridian flip.  If it can't get to your target level within the range you specify, it will go to the Min. Guide Exposure time if the Guide ADU is too high or to the Max. Guide Exposure time if the Guide ADU is too low.  

    When using Auto Guide Exposure, the Guide Exposure settings on the Light Frames page are ignored and grayed out as a reminder except in one case.  If the Guide Exposure on the Lights page is 0, this is interpreted as a desire for unguided imaging for the series with a zero value Guide Exposure.  If you intend to use Automatic Guide Exposure for guided imaging, be sure to enter non-zero value(s) for the series you wish to be guided before you select Automatic Guide Exposure, to avoid the series being unguided.  Once you have a non-zero value entered, 1 sec. will suffice, select AutoMatic Guide Exposure and the guide exposure will be in the range you specify, between Min. and Max. Guide Exposure.



    Utility Window



    As a reminder, your guider settings are displayed in the Utility Window.


    Unguided Operation



    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 arc-sec.  The first sub-exposure will leave the guide star position undisturbed.  The second will move the X position of the guide star + 3 arc-sec.  The third will move the X position of the guide star - 3 arc-sec. 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 arc-sec.
  • Suggest: this button will enter a Max. Dither value, based on the parameters of your system entered on the settings page.  This can be a starting point from which you can experiment if you desire.

    Periodic Realignment
    When a long series of unguided exposures are taken, the mount will gradually track off the target.  By using this feature, the telescope is periodically realigned to the target coordinates.  (If any periodic focusing using SkyStar is implemented, target realignment will occur during that process.  If that is used, additional periodic realignment will occur only as long as the time between SkyStar focus runs has exceeded the realignment period.
    Realign scope every x minutes: If 0 is entered, this function is disabled.  If a non-zero value is entered, the telescope will be realigned to the target coordinates via a precision slew after the current exposure completes and after the specified interval has elapsed.


    Crossing the Meridian



    Automatic meridian flipping essentially uses software to turn an equatorial mount into a fork mount functional equivalent.  This is achieved by allowing the telescope to track a target up to the meridian, stop the exposure, move the telescope to the other side of the meridian and then continue tracking.  If a rotator is employed, the camera is rotated by 180°, the guide star is reacquired and guide calibration is appropriately adjusted.  All this is accomplished automatically.  Automatic meridian flipping depends on successful plate solving and initialization.
  • Enable Meridian Flip: Checking this enables the telescope to track through the meridian when going from east of the meridian.  If checked and the target is west of the meridian, it will of course have no effect.  For most users, this option can be left checked.  Uncheck it if you are using a fork mount or an equatorial mount that is capable of tracking through the meridian without damage.
  • Rotate on Flip: If checked and a rotator is connected to CCDAutoPilot, the meridian crossing event will also trigger a rotator movement by 180°.  Thus the images on the west of the meridian will be the same orientation as on the east side of the meridian.
  • Safety Slew: Some mounts, notably the Gemini, will not flip even though the target has crossed the meridian.  Checking Safety Slew will slew the mount one hour past the meridian, which will force the flip, and then back to the target.  This should eliminate the need for the AM stop offset setting and it should be set to 0.
  • Allow PM Re-sync: If this option is checked, an image is taken and plate solved after a meridian flip. The mount is then sync'd to the center of the solved image. The correcting slew is then from this reference. Recommended for portable setups or for setups with poor pointing accuracy. If this option is cleared, there is no sync and the correcting slew is made from the plate solved position. This option should be cleared for permanent setups with good pointing accuracy as may be obtained with a suitable Tpoint model. If checked, any Tpoint models should be disabled as repeated syncing into a Tpoint model is not recommended and will result in pointing inaccuracies.  Use either Allow Re-sync or Tpoint but not both.
  • Auto guide star select: If checked, an automatic detect of the brightest star in the field will be used to guide. If unchecked, the run will pause until you select a guide star and tell it to continue. 
  • Focus of flip: When checked, the focus method chosen on the Focus page will be executed immediately after the meridian flip and before the next exposure. 
  • Flip Delay: This is the number of minutes to delay the meridian flip. Some mounts may require a longer delay than the default 1 minute to determine the target has passed the meridian. You should experimentally determine you particular mount's characteristic. 
  • Meridian Tracking: If the value entered is positive, this indicates the number of minutes your mount can track past the meridian and defers the flip for the your specified number of minutes.  Depending on the above two settings, you may be able to get another exposure in before meridian crossing and eliminate the wait time for the meridian flip.  Observatory users using DDW or ASCOM for observatory control should insure their shutters permit crossing the meridian by the PM Tracking time since dome tracking is disabled 2 minutes before a meridian flip.  AutomaDome has no such limitation since it integrates tightly with TheSky.  If the value entered is negative, the mount backs up 1 hour in RA in the east and the session waits for the specified number of minutes.  This is helpful if you cannot allow your OTA to point to the zenith (Alt. = 90°) due to interference with a camera.  It can also be used if your mount has some ambiguity in slewing to a location whose RA has passed the meridian.  By setting Meridian Tracking to -10 or so, the target RA will be 10 minutes past the meridian before meridian flip is enabled.  Some experimentation may be required for the smallest negative value that will work.

    Tips on efficient meridian usage
    With the various time settings, you can increase your meridian usage efficiency.  AM stop offset is generally not needed and can be left at 0.  Flip delay is the amount of time your mount has to track past the meridian before it (the mount) knows it is time to flip and will do so.  For most mounts, this can be set to 1.  PM Tracking requires a bit more discussion.  Let's assume you have determined your mount can safely track past the meridian for 15 minutes.  Set the PM Tracking to 12 minutes for a safety margin.  Now, assume you are taking 10 minute sub-exposures at the time the mount approaches the meridian.  Assume your meridian crossing is at 22:05.  Here is a schedule of events you might see:

    21:50 Take 10 minute exposure
    22:00 Take 10 minute exposure
    22:05 (Time to flip - deferred)
    22:15 Flip meridian
    22:18 Take 10 minute exposure

    Now, if PM Tracking were set to 0, this would be the result

    21:50 Take 10 minute exposure
    22:00 Wait for meridian flip
    22:05 Flip meridian
    22:09 Take 10 minute exposure

    Here are some excerpts from an actual log with PM Tracking set at 10 minutes:

    >>> Here is the target information
    20:02:28 Target: M1
    20:02:28   Rise: 13:51 6 Feb
    20:02:28   45° E elevation: 17:32 6 Feb
    20:02:28   Transit: 20:50 6 Feb
    20:02:28   45° W elevation: 00:09 7 Feb
    20:02:28   Set: 03:50 7 Feb

    20:05:49 Solved RA: 05 34 30.2, Dec: +21 59 17 
    20:05:49 Target RA: 05 34 30.2, Dec: +21 59 17 , PA: 358.4
    20:05:49 Meridian flip after 20:51 Tue 6 Feb
    >>> The mount would have flipped at 20:51 without the buffer time

    >>> The exposure completed after the meridian flip time but before the 10 minute buffer elapsed.
    20:43:31 Exposing...
    20:54:15 D:\Astronomy\070206_M1\Clear356E_M1_00006.fit

    >>> And then the mount flipped immediately.
    20:54:18 Waiting for meridian flip time...
    20:54:18 Meridian flip starting...

    If your mount is capable of longer tracking past the meridian, the flip can be deferred longer, for example in the case of an Astro-Physics mount which can track for a number of hours past the meridian, depending on telescope, camera, declination, etc.

    Warning: It is up to the user to determine the proper setting for PM Tracking.  In addition to consulting your mount's documentation, you should also experiment   at various declinations, while watching the mount.  If you rotate your camera, you should also verify the PM tracking at various camera rotations.


    Guide Box Size



    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.