[Shiraz (Ray)]

First off, I originally headed the thread as Drizzle, when I meant Dither - seniors moment I guess .

Originally Posted by LewisM

Not sure if I am scientifically correct but I dither up to 5 pixels randomly. So far, very little residual noise with the ST8300C and what is left responds to very mild photoshop noise reduction wonderfully (a light Imagenomic pass)

thanks Lewis - whatever works is good . Imagenomic looks to be very capable.

Originally Posted by Atmos

Great post Ray.
When it comes to dithering and how much, it really depends on what you're doing it for. If you are dithering simply to reduce the FPN then you are better off having the values higher rather than lower (greater offset between subs). This stands to reason as even when doing random dithering there is a lower chance of two subs ended up in the same position.

If you are dithering because you are over sampling and wanting to drizzle more detail out, you then want a lower offset so that the pixel is just moving around a small area. An example of this would be a maximum of 0.8 pixel in any direction from the centre. This leads to each pixel mapping out 1.6 pixels in size with some slight potential overlap of surrounding pixels.

In essence, if all you want to do is reduce noise, it doesn't matter just as long as the subs don't double up. If you intend to drizzle out resolution from over sampling, I personally would make a max of 0.8.

Disclaimer: I haven't been able to test this yet, still waiting for new equipment and clear skies!

Very good points Colin - thanks. Despite an original heading indicating that the post was about drizzle, my post was based on dither in terms of imager pixels and the assumption is that dither will be in whole pixels. I will modify the first post. What I was trying to do was provide a justification for broader dither than people may be using by showing why it could possibly help. A user will need to translate the guider dither into imager dither to find any value in the suggestions.

Dither for super-resolution requires sub-pixel misalignment. After registration, 2.3 pixels or 7.3 pixels offset will be functionally equivalent to 0.3 pixels, so I suspect that dither can be arranged to be appropriate for both noise reduction and super-resolution. As Slawomir points out, it is reasonable to assume that any guidance system will produce some degree of mismatch anyway. In a previous life, did some effective super-res experiments with nothing more than linear steps at a scale that was much larger than the main imaging pixels, but not harmonic - it worked fine on much larger than sub-pixel steps.

Originally Posted by Slawomir

Hi Ray,

I probably should have read the other thread before posting my reply...but here we go...

The degree of dithering needed to minimise residual noise most likely depends on the camera you have, so I would suggest creating quality bias and dark frames and taking time to inspect them.

My understanding is that very few systems would be keeping stars exactly in the same place on the CCD, and even with top end mount you will have sub-pixel variations from sub to sub. Also, I always believed that given the same amount of subs, drizzle integration will result in higher noise in the final image as opposed to non-drizzled integration?

Thanks Slawomir. Dither is an extra tool that can either replace or enhance standard calibration. Having high quality calibration data is essential if you rely on conventional calibration, but it can possibly become a burden as much more cal data is needed with deep imaging - dither can reduce the need for massive cal datasets and it deals effectively with any residual noise (eg shot noise from dark current outliers), that will persist even in large cal datasets.

Agree that any guide system will not keep the stars exactly in place, but some come close. The point that I was trying to make is that, even though it seems like a laudable goal, trying to get the guiding so good that stars stay in the same place on the CCD in all of the subs is actually counter-productive.

Dither can definitely help with noise reduction when used with conventional stacking - I stuffed up the heading to the thread and used the word drizzle when I meant dither, sorry about that..