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  • Dithering and Guiding

Dithering and Guiding

The Guiding settings are accessible from
 Guide button in Gear tab or with F7 shortcut.
 
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 stars position - 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". 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.
 
There are several requirements for the MGen usage with APT:
 
Other interesting feature is named "Auto Cancel Exposure" (supported for PHD, Lin_Guider and MGen). 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.
 
Follows detailed settings description.
 
Dithering and Guiding
 
1

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'.
 
 
2

Auto Dithering

2. Auto Dithering
 
Enables/Disables the dithering.
 
3

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 if the distance is too small you can use the Brain button (Advanced Parameters) to enter "Dither scale".
 
Supported by PHD, MGen, Lin_Guider, APT Dithering, APT Pulse Dithering.
 
4

MGen Mode

4. MGen Mode
 
Defines the dithering mode for the MGen device.
 
5

Stability

5. Stability
 
Defines the acceptable distance from the guiding start that to be considered as dithering end. The distance is in pixels from 0.00 to 2.55.
 
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, Lin_Guider
 
6

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, MGen, Lin_Guider, APT Dithering, APT Pulse Dithering.
 
7

Timeout

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.
 
8

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.
 
9

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.
 
10

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, MGen, Lin_Guider
 
11

Auto Cancel Distance

11. Auto Cancel Distance
 
Defines the maximum acceptable distance from the guiding star. Distance above this limit will force exposure cancellation. The values are in pixels form 0.00 to 2.55.
 
12

Remote IP

12. 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 127.0.0.1 which means that same computer is used for imaging and for guiding.
 
Supported by PHD, Lin_Guider
 
13

Remote Port

13. 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 PHD is 4300, for Lin_Guider is 5656.
 
Supported by PHD, Lin_Guider
 
14

Multi Camera Dithering

14. 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.
 
 
15

Multi Camera Sync

15. Multi Camera Sync
 
Defines synchronization mode in server-client architecture. If you use only one camera there is no matter what is selected.
 
 
16

Ping Timeout

16. 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.
 
17

Control Guiding

17.  Control Guiding
 
When enabled APT will stop the guiding in PHD before making a GoTo, GoTo++ or #GoTo command and will try to resume the guiding after move is completed.
 
It is still experimental feature. Use it with caution and please share feedback.
 
18

After GoTo delay

18. 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.
 
19

Guiding Resume Delay

19. Guiding Resume Delay
 
Used when Guiding Control is enabled. After mount movement is completed, APT will send command for guiding star selection, 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.