Contents - Index

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.
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.  
Choosing a Dither Factor Manually: 
Max. Pixel Dither is used to set the maximum movement that is adjustable from 0 to 10 in arc-sec. for unguided imaging and 0 to 10 in guider pixels for guided imaging.  In order to set the maximum movement most effectively, you need to know some values:
G = Guider image scale in arc-sec./pixel (asp)
I = Imager image scale in arc-sec./pixel (asp)
A good starting point is to dither at least 4 pixels on the imager.  Even though a hot pixel is only one pixel, currently available alignment and combining techniques will smear out that hot pixel over as many as 4 pixels.  In order to effectively eliminate the hot pixel, the dither must at least be that much.
Example: Now, let's assume you are using an ST10XME with a built-in TC-237 autoguider.  In the above example, assume the ST10's 6.8 micron pixel correspond to 0.5 asp.  The guider's image scale can be calculated by:
G = I * Guider pixel size/Imager pixel size = 0.5 * 7.4 / 6.8 = .54 asp
Most bin the guider at 2x2 so the equivalent image scale is 1.08 asp.  Again using current align and combine techniques, you want to move at least 5 pixels on the imager.  For 2x2 binned pixels, you need to move the telescope 5 arc-sec. So, the number of pixels you need to move the guider is given by:
Movement = 5 * (Imager binning * I)  / (guider binning * G)
Assuming the above image scales, your guider is binned at 2x2 and you are taking binned color images, then your movement should be set at:
Movement = 5 * ( 2 * 0.5) / (2 * 0.54) = 4.63.  So use a dither of 5 here as well.
Now, assume you are taking a run that consists solely of unbinned sub-exposures.  Then your movement should be set at:
Movement = 5 * (0.5) / (2 * 0.54) = 2.32.  Here, a dither of 3 could be used.
  • Dither method: Select Enhanced (preferred), Random or None.
  • Max. Pixel 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.
    Choosing a Dither Factor Manually: 
    Max. Pixel Dither is used to set the maximum movement that is adjustable from 0 to 10 in arc-sec. for unguided imaging and 0 to 10 in guider pixels for guided imaging.  In order to set the maximum movement most effectively, you need to know some values:
    G = Guider image scale in arc-sec./pixel (asp)
    I = Imager image scale in arc-sec./pixel (asp)
    A good starting point is to dither at least 4 pixels on the imager.  Even though a hot pixel is only one pixel, currently available alignment and combining techniques will smear out that hot pixel over as many as 4 pixels.  In order to effectively eliminate the hot pixel, the dither must at least be that much.
    Unguided Example:  Assume you are taking LRGB images with the RGB binned at 2x2 and your unbinned image scale, I, is 0.5 asp.  Your 2x2 binned sub-exposures will be 1 asp.  Therefore, you should set your maximum dither at 5 arc-sec., when using version 2.06.
    Guided example: Now, let's assume you are using an ST10XME with a built-in TC-237 autoguider.  In the above example, assume the ST10's 6.8 micron pixel correspond to 0.5 asp.  The guider's image scale can be calculated by:
    G = I * Guider pixel size/Imager pixel size = 0.5 * 7.4 / 6.8 = .54 asp
    Most bin the guider at 2x2 so the equivalent image scale is 1.08 asp.  Again using current align and combine techniques, you want to move at least 5 pixels on the imager.  For 2x2 binned pixels, you need to move the telescope 5 arc-sec. So, the number of pixels you need to move the guider is given by:
    Movement = 5 * (Imager binning * I)  / (guider binning * G)
    Assuming the above image scales, your guider is binned at 2x2 and you are taking binned color images, then your movement should be set at:
    Movement = 5 * ( 2 * 0.5) / (2 * 0.54) = 4.63.  So use a dither of 5 here as well.
    Now, assume you are taking a run that consists solely of unbinned sub-exposures.  Then your movement should be set at:
    Movement = 5 * (0.5) / (2 * 0.54) = 2.32.  Here, a dither of 3 could be used.
  • 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.
  • Dither every <n> Exposures: In most cases, this setting should be left at 1, i.e. dither every sub-exposure. In some applications when taking a large number of exposures, there may be a desire to dither less frequently. This setting allow dithering to be used every <n> sub-exposures. For example, if set to 3, there will be an initial small dither for the first exposure, no dither for the second and third, and then a normal dither for the fourth exposure. This sequence will repeat for the number of exposures in the series.
  • Max. Guide 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.
    When using TheSkyX CAO, it is possible to set a delay between guide exposures. During AGSR, the value specified in TheSkyX CAO guider is saved, temporarily set to 0 until the guide error is within tolerance then the original exposure delay is restored.

    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.
  • Ignore Saturated Pixels: When checked (default) any pixels that exceed approximately 90% of the full scale guider ADU are ignored. This is done to prevent guiding on a saturated pixel, which can lead to inaccurate guiding. With some wide field systems, guiding on a saturate pixel or star will be accurate enough and allow the brightest star in the guider FOV, whether saturated or not, to be used for guiding. The usage of this option is best determined experimentally.
  • 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.)
  • Hot Pixel Filter: When using guiders without a shutter, it is not possible to take a dark-subtracted guide exposure. A hot pixel may be confused for the guide star. CCDAutoPilot's proprietary algorithm provides some rejection of hot pixels that reduces this issue.  The strength of the filter increases with increasing values with 0 fully disabling the filter.  The Hot Pixel Filter is only effective when CCDAutoPilot determines the guide star location.  This filter should not be necessary when using a guider with a shutter that allows dark-subtracted images but one can certainly experiment to see if there is any advantage to using it.  See Tune, below, for how to set the filter strength.
  • Tune: Start with a hot pixel filter strength of 0.1.  Move the telescope as needed to place a guide star in the guider's FOV.  Adjust the guide exposure so that the peak ADU is around 5,000.  Note the guide star position in the camera control program.  Hit the tune button to analyze the guider image.  The detected guide star position will be displayed. If this position does not agree with the position of the guide star determined above, it is likely a hot pixel is being detected as the guide star.  Increase the filter strength and hit Tune again. Repeat until the guide star position and not the hot pixel position is determined as the guide star.  Increase another 0.1 for a safety margin. Try lower guide star ADU values to determine how bright a guide star is needed and still not detect a hot pixel in error.
  • Audible Alarm: When checked, a somewhat annoying sound is played through your computer's speakers to alert a local operator of AGSR activation.  You can replace this sound file, which is located in the program directory with the name alarm.wav with another file.  Just be sure your other file is a .wav format and name it alarm.wav.
  • Guide Through Series Changes: When checked, the guider is not stopped during exposures.  The dithering must be set to 0 as well.  This is useful for multi-filter photometry when disruption for dithering is undesirable. Automatic guide star exposure cannot be used when this option is selected since the guider must be stopped to measure the guide star.  When using this option, be sure to set the Guide Time appropriately for each series on the Session page.
  • 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. You can select None for no centering, Start of Each Series to center before the first exposure of a series or Start of Every Exposure to center the adaptive optic at the start of every exposure. Set to None when not using AO.