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Could some suggest what are the most the common dithering settings. (What difference would it make if I did not dither.) Thanks for reading. |
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Without dithering you will get a walking pattern caused by the hot pixels of your imaging sensor. This effect is even stronger if you use a DSLR (astromodified or not doesn't matter) than a dedicated cooled astro cam. Settings for dithering depend on the FOV of your setup and of course directly with the focus length of your scope/lens. The bigger your FOV the bigger has to be the dithering step (#pixels) and vice versa in order to have the right effect. Concerning the frequency of dithering with a cooled astro cam I would say that dithering every 360" of imaging is sufficient ( this works for my setup with ASI1600+1,5m Newtonian very well), for a DSLR depending on the temperature conditions it could be necessary to dither after every single frame (dither or die). |
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My standard settings are 2pix/dither every 2nd image 747mm focal length/asi2600mm@32F |
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There are 2 reasons why you might dither :- 1. To reduce the effect of walking noise.Walking noise is caused by dark fixed pattern noise (DFPN) of the sensor appearing to move in a consistent direction from frame to frame after registration. This apparent movement is due to drift resulting from imperfect polar alignment.The noise is introduced during registering of image frames. Before registration, the DFPN is in the same place in each frame. Small realignments of the image during registration causes the DFPN to drift from frame to frame. Thats why this iOS known as "correlated" noise. Dithering means the DFPN is *not* in the same place in each frame, so it is averaged out during integration. 2. To compensate for the effects of undersampling. In a nutshell, undersampling is what happens when your pixel size is too large for your telescope. This means that stars are 'under samples' - that is, a small star is represented by 1 or 2 pixels, which means the image cannot resolve the subtle brightness gradient across the stars Airy Disk. The result is block, washed out stars. Dithering means the centre of a star is moved randomly from frame to frame, so the full brightness gradient of stars is captured across a number of frames. This means that the brightness profile is fully captured, but rather than being captured in one frame (as would happen with optimum sampling), it is captured across many frames, so needs to be 'brought together' by recomposing the brightness profile in the final image. This is done using a process called 'Drizzling' - in essence, this means up-sampling each image by a factor (usually 2x or 3x), then integrating the upsampled images. This recreates the profile and gives an image more representative of the true brightness profile. This requires images (ideally every image) to be dithered. The downside is that the upsampled images use a lot of disk space, and integration takes a lot longer. For example, drizzling x 3 means everything is 9x bigger and integration takes correspondingly longer. So - my question is this :- Why do you want to dither ? There really isn't much downside to dithering :- Pros :- - reduces walking noise - essential if you want to drizzle Cons :- - exposures take very slightly longer because your mount needs to settle after each dither - maybe 5 seconds longer for each frame, which can add up to a lot - dithering is an issue with multi-optic setups - because you need to synch cameras to avoid one sequence dithering while the other is exposing So, IMHO, there is no real downside to dithering every frame. I dither on every frame, using a random patter of about 5 pixels. More importantly, I suggest you work out whether your optical setup is undersampling, so you can decide whether dithering will help improve image quality. I hope that helps  |
