Dithering is an important part of the modern image acquisition process. CMOS and CCD sensors suffer from various types of electronic noise, such as fixed-pattern noise, and defects such hot and cold pixels. Photons of light from stellar objects fall onto the sensor and will effectively be lost if the photon falls on a dead or hot pixel. The act of dithering is a commanded move of the mount between successive exposures that very slightly alters, on the pixel scale, where photons fall on the sensor. This means that one exposure can capture light (ie, data) from an object where that same light fell on a dead or hot pixel in the previous exposure. Image alignment issues are easily handled by most astronomy stacking and image post-processing applications.
Because dithering is an operation that must be coordinated with guiding (remember, the mount is being purposefully moved by a few pixels, which guiding would reflexively attempt to counteract), the dithering operation is managed by PHD2 itself. The process is simple. N.I.N.A. suspends imaging and commmands PDH2 to execute a dither operation. PHD2 performs the dither, and then informs N.I.N.A. when the operation is complete. Any required adjustments to guiding are automatically handled by PHD2. N.I.N.A. then resumes commanding normal exposures. Normally, a dither operation takes a few tens of seconds (at most) to complete.
Dithering in N.I.N.A.#
N.I.N.A. offers three different ways to effect dither operations:
- Standard dithering through PHD2
- Synchronized dithering across multiple main cameras on the same mount, also through PHD2
- Built-in dithering using N.I.N.A.'s Direct Guider function
The desired method of dithering is selected in the Equipment > Guider tab.
Standard PHD2 Dithering#
This is the typical scenario for most users. The user has a single main camera, a guide camera, and is using PHD2 for guiding. At the intervals configured in the sequence, N.I.N.A. will pause operations with the main camera and signal PHD2 to begin a dither operation. Photography resumes when the dither operation is completed.
Synchronized Dithering with PHD2#
Multiple imaging telescopes and cameras in addition to a single guide camera has become a common configuration. N.I.N.A. can be used to control these kinds of setups through multiple instances of the application. One instance controls the mount, guiding, and one of the main cameras. Additional instances of N.I.N.A. control each additional main imaging camera present and communicate their actions to the master instance of N.I.N.A. This coordination is automatically set up in the background when multiple instances of N.I.N.A are started. This configuration poses an issue for dithering because, without coordination between the multiple instances of N.I.N.A., a dither operation may be initiated while one of the other main cameras still is busy exposing.
To manage this, a dither operation will be coordinated with PHD2 so that it happens when none of the imaging cameras are exposing. N.I.N.A. developer Stanley Demont describes Synchronized Dithering in his N.I.N.A. 1.8 feature overview video.
There are cases where there is no guiding equipment in use, and thus no PHD2, but where dithering is still desireable. Examples of configuraions like this usually include small, portable setups that have a main camera and telescope or lens, but no guiding. In cases like this, N.I.N.A. can still effect dithering operations, but on its own through its Direct Guider facility. Once activated, dithering operations become available in the Sequence and are effected by N.I.N.A. directly.
N.I.N.A. supports dithering using its direct communication with PHD2 and makes it easy to set up as a part of a sequence. There are a few prerequisites to dither during a sequence:
- A mount needs to be connected
- PHD2 needs to be be running, actively guiding, and also communicating with N.I.N.A.
- The PHD2-related settings in the Equipment Settings need to be correctly set up
- Dithering needs to be enabled in the sequence
In order for N.I.N.A. to communicate with PHD2 and command operations such as dithering and for receiving guiding telemetry, PHD2's internal server must be enabled. To enable PHD2's internal server, go to PHD2's Tools menu and ensure that Enable Server is selected.
Settings for PHD2#
Settings related to guiding and dithering can be found in the Options > Equipment tab. The defaults are suitable for most cases. You will need to specify the full path to
phd2.exe if PHD2 was installed in a non-standard location. This is so N.I.N.A. can automatically start PHD2 as a part of the equipment connection process.
An explanation of the two most important dithering-related settings follows:
- PHD2 Dither Pixels: The amount of pixels (on the guiding camera) that the dithering action will shift. The default works fine for most focal lengths. This value must take into account the focal length of your guiding optical train and the pixel size of your guide camera's sensor, also known as the pixel scale. Obviously, 2 pixels at one focal length and pixel size will cover a different amount of sky than another setup with a different focal length and pixel size.
- Dither RA Only: Turn on if your mount has large amounts of declination backlash. This will cause dithering to happen on the RA axis only and allow the declination axis to continue guiding.
The pixel scale of your guide camera can be calculated using online tools. By inputting the focal length of your guiding optical train and the pixel size of your guide camera's sensor, you will know how many arcseconds of sky is covered by each pixel (arcseconds per pixel). Such a tool is the Astronomy Tools FOV Calculator.
Settings in Sequences#
Regardless of the dither method in use, initiating dithering during the course of a running sequence is simple. Dithering operations can be activated for each step in a sequence, and be initiated every Nth frame in each step. That is, if a step in a sequence specifies that 20 exposures be taken with a dither operation every second exposure, two normal exposures will be taken, a dither operation performed, and then the next two exposures will be taken, etcetera. N.I.N.A. manages these operations itself in conjunction with PHD2 and the process is entirely hands-off.
Dithering operations happen while the previous image is downloading from the camera. If you have a camera with slow download speeds, it might be that the dithering operation is completed in time for the camera to be ready for the next exposure.
If you use a LRGB rotational sequence (See Also: Advanced Sequencing) you might only want to dither on every L frame. If an OSC or DSLR camera is used, it is suggested to dither after every frame.