• Dithering and Guiding

Dithering and Guiding

The Guiding settings are accessible from
 Settings button in Guide section of Gear tab or with F7 Key.
Related videos:
APT can communicate with various guiding programs and devices in order to monitor and control the guiding. Important purpose of this communication is to provide dithering.
Dithering are gentle random changes of the telescope position between the exposures. By this the celestial objects are placed on slightly different positions on every image while the sensor defects like hot/cold pixels, dust signs or banding noise remains on constant or almost constant places. During the images stacking the stars are detected and all frames are aligned by the found star positions - this means that the objects become constant and while defects become random. All major stacking applications for image post-processing provide methods based on "sigma clipping" / "kappa-sigma clipping" pixel rejection. These methods exclude the random data and preserve the constant signal. As a result in the stacked image there will be much less defects and noise. More images you use in the stack more noise and defects will be removed. In order the sigma clipping to be effective there is need to use at least 6 light frames.
The author's personal opinion is that good amount of images (10 or more) makes dark frames unnecessary. Making darks is quite time consuming and very tricky process, especially for DSLRs where the imaging sensor temperature is not accurately controlled. Also (again personal opinion) darks applying results in loosing the very faint details in the final image.
No matter if you are applying darks or not, dithering is powerful technology is it is highly recommended to use it. The time needed to make dithering depends on the mount and the desired distance, but in most cases it takes less than a minute - quite small price for the benefits it gives.
Usually the dithering is made by your guiding program/device and there is no need to make dedicated connection to the telescope in APT if you don't want to use other features that require mount control.
APT Dithering is special feature made to provide dithering when there is no guiding because your mount is high-end (ASA, 10Micron and etc.) or when you are using short focal length lens or telescope. APT Dithering uses a mount connection to make the needed random position changes between the exposures.
APT Pulse Dithering is other special feature made to provide dithering using pulse guiding commands for self guided mounts like Meade StarLock, self guiding solutions or mounts like SkyWatcher StarAdventure, AstroTrac and etc. In this case the Dithering Distance defines the maximum pulse duration in milliseconds. The value is multiplied by 100. For example Dithering Distance = 5 means up to 500ms pulse.
APT has powerful Dithering System that is able to synchronize exposures and dithering even in multi-camera imaging setups. See Multi-camera Dithering / Synchronization section.
Supported are the following guiding solutions:
Lin_Guider is a Linux based guiding application which can be used on Raspberry Pi or other Linux devices for building small self-guiding solutions.
PHD and PHD2 has common roots, but now are different projects. Everything in the text below that is related to PHD is related to PHD2 too.
When MGen3 connection is made, APT acts like server which can be used by other applications (like virtual handset) to connect to the device in same time.
There are several requirements for the MGen2 usage with APT:
Other interesting feature is named "Auto Cancel Exposure" (supported for PHD, Lin_Guider and MGen2/3). APT monitors the reported distance to the guiding star and if it becomes bigger than a defined limit the running exposure will be stopped before trails to become visible in the image, or to save imaging time for other exposure. Canceled exposure will not be counted as finished in plan execution and another will be started on its place.
Follows detailed settings description.
Dithering and Guiding

Guiding Program / Device

1. Guiding Program / Device
Defines the guiding solution that have to be used, or APT if there is no guiding.
When PHD is used there is need to set it in server mode. Select from menu in PHD 'Tools'->'Enable Server'. For PHD2 this option is enabled by default, but check it for sure.

Auto Dithering

2. Auto Dithering
Enables/Disables the dithering.

Dithering Distance

3. Dithering Distance
Defines how big to be the move. This is the upper limit. By definition dithering is random by direction and by distance move.
For PHD(2) if the distance is too small you can use the Brain button (Advanced Parameters) to enter "Dither scale". For each value, here is how much the dither size would be on the guiding chip (assuming you keep dither scale = 1.0):
For PHD1 the logic is:
Guide Camera Pixels
For PHD2 the distance is in guide camera pixels (Value 1, means 1 pixel of the guiding camera, Value 2, means 2 pixels of the guiding camera, etc). An easy to use calculator what is the best value for your setup can be found here
For APT Dithering the value defines the max distance in pixels of the imaging camera (Value 1, means 1 pixel of the imaging camera, Value 2, means 2 pixels of the imaging camera, etc).
For APT Pulse Dithering the value * 100 gives the max guiding pulse in milliseconds to execute.
Supported by PHD, PHD2, MGen2/3, Lin_Guider, APT Dithering, APT Pulse Dithering.

MGen Mode

4. MGen Mode
Defines the dithering mode for the MGen2/3 device.


5. Stability
Defines the acceptable distance from the guiding start that to be considered as dithering end. For PHD, Lin_Guider the distance is in pixels from 0.00 to 2.55, for PHD2 there is only lower limit 0.00.
PHD and Lin_Guider doesn't report when the action is completed, so APT monitors the distance and when it gets back to normal decides that dithering is completed.
Supported by PHD, PHD2, Lin_Guider

Settle Time

6. Settle Time
Defines how many seconds to wait after dithering completion. This gives time guiding/tracking to settle down.
Supported by PHD, PHD2, MGen2/3, Lin_Guider, APT Dithering, APT Pulse Dithering.


7. Timeout
Defines the maximum time for waiting the dithering process to finish. After that time APT will continue with the next exposure. 0s means that there is no time limit to complete the dithering.
When MetaGuide or AstroArt is used this defines when to resume with next exposure.

Dither Interval

8. Dither Interval
Defines how many images to take before making dithering. Values 0 or 1 means to make dithering after every image.

Dither Delay

9. Dither Delay
Defines how many seconds to wait before sending the Dithering command. Usually there is no need to define such pause.

Auto Cancel Exposure

10. Auto Cancel Exposure
Enables/disables the Auto Canceling. Auto Canceling allows canceling exposure when the distance from the guiding star becomes greater than a defined value.
Supported by PHD, PHD2, MGen2/3, Lin_Guider

Auto Cancel Distance

11. Auto Cancel Distance
Defines the maximum acceptable distance from the guiding star. Distance above this limit will force exposure cancellation. For PHD and Lin_Guider the values are in pixels form 0.00 to 2.55. For PHD2 there are no value limitations.

Control Guiding

12.  Control Guiding
When enabled APT will stop the guiding in PHD, PHD2 or MGen2/3 before making a GoTo, GoTo++ or #GoTo command and will try to resume the guiding after move is completed. This option has to be enabled for Automated Meridian Flip.
For PHD2 when guiding is starting a new guiding star will be auto selected. PHD2 decides itself if there is need to do a calibration. Make sure to apply darks to avoid hot pixel selection.
For PHD when guiding is starting a new guiding star will be auto selected. APT can't get feedback from PHD when the guiding is started, so give some time the guiding to start and settle. Make sure to apply darks to avoid hot pixel selection. We suggest migrating from PHD to PHD2.
For MGEN2/3 a new star is searched using the exposure/gain from the previous guiding. When the guiding star is selected, APT will start a calibration and after its end the actual guiding will be started.

After GoTo delay

13. After GoTo delay
Used when Guiding Control is enabled. Defines how many seconds to wait after mount movement is completed, before sending command for guiding star selection. This delay gives possibility guiding to get image clear of movement trails. Use delay that is 4-5 times the guiding exposure.

Guiding Resume Delay

14. Guiding Resume Delay
Used when Guiding Control is enabled. After mount movement is completed, APT will send command for guiding star selection (defined by "After GoTo delay" ), then will wait N seconds (defined here) in order star selection to complete and tracking to settle before resuming the guiding. Increase this value if the mount needs more time after GoTo command.

Auto Start PHD2

15. Auto Start PHD2
When enabled during the start APT will check if PHD2 is running and if not, will start it automatically.
No matter if this option is enabled or not, when APT makes connection to PHD2 it will send a command to make connection to all devices in the last used profile in PHD2.

Remote IP

16. Remote IP
Defines the IP address of the computer that is used for guiding. This feature is useful if you use separate computer for guiding or self-guiding solution. The default value is which means that same computer is used for imaging and for guiding.
Supported by PHD, PHD2, Lin_Guider

Remote Port

17. Remote Port
Defines which TCP/IP port to be used. This feature is useful if you want to control two or more guiding setups with one computer or self-guiding solution. The default value for PHD1 is 4300, for PHD2 is 4400, for Lin_Guider is 5656.
Supported by PHD, PHD2, Lin_Guider

PHD2 Graph

18. PHD2 Graph
Gives control over the PHD2 graph in Guiding section of Gear tab and Guide graph in Graphs Aid. The PHD2 Graph Metrics option allows selection how of graph to be scaled - in guide camera pixels or arc-seconds. The PHD2 Graph Scale defines the graph scale, for example 1 will set the graph to show the +/- 1 arc-seconds range. Bigger guiding errors will be off the graph.

Multi Camera Dithering

19. Multi Camera Dithering
Defines is the role of this APT instance in server-client architecture. If you use only one camera there is no matter what is selected.

Multi Camera Sync

20. Multi Camera Sync
Defines synchronization mode in server-client architecture. If you use only one camera there is no matter what is selected.

Ping Timeout

21. Ping Timeout
Defines how long the server to wait before decides that a client is stopped. Increase this value if you are loosing connection between the server and a client.