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Dithering and fast telescopes

Sky-Watcher Explorer 150P Acquisition Dithering Deep Sky Walking Noise
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Alejandro Navarro avatar
Alejandro Navarro avatar
Hello there.
I did a little research, and I didn't found what I needed.
So I wanted to ask to the gurus out there.
What's the perfect o recommended dither for a fast telescope?
I have a skywatcher 150p quattro, working at f3.5
In one of my pictures, I think I had walking noise. 
I'm working with 5 pixels and a 5 frames interval.
I used to dither every 3 subs, and I think it was better.
Should be 1 even better? 
What do you think?
Best regards from Chile
V avatar
V avatar
Ah, common misconception with dithering, you need to dither every frame for the best results. 5 pixels is far too small aswell, dithering when your pixel scale is larger than your guider's pixel scale needs to be ~10-30 pixels.
Concise
John Hayes avatar
John Hayes avatar
Dithering is how you spatially filter fixed signals that come from the sensor so the amount of dithering  that you need depends on the characteristics of the sensor–not the telescope.  With the IMX 455 sensor on both of my  telescopes, I dither randomly up to 9 pixels and I dither every frame to get good results.  Dithering every frame improves the statistics that the stack filter works with.  Dithering is trivial and there is no reason not to dither every frame.  With a large sensor like the IMX 455 or the IMX 461, you could certainly dither further–say up to 50 pixels and it wouldn't hurt anything.  

The optimum distance can be determined mathematically by taking the autocovariance of a master dark frame to find the correlation distance, which is the the distance over which two points on the image are statistically unrelated to one another; but, that's more of an academic nuance than practical advice.

John
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Huib Wouters avatar
Huib Wouters avatar
Five up to 50 pixels dithering. So far I've always used the standard settings in KStars/Ekos, which I think is dithering every frame by 2 pixels (IMX571 sensor, slow optics).

I'll try the autocovariance trick, probably not going to improve my pictures, just math for the fun of it.
Alejandro Navarro avatar
Alejandro Navarro avatar
Woow!!! Mind blowing
I'll try what you said guys.
thank you both!!