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How will dithering affect target pointing accuracy [beginner]?


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I think I understand dithering in principle - unwanted artefacts e.g hot pixels are averaged out from multiple light frames, during stacking, since the dither has caused the artefact to move randomly between frames.

I'm going to attempt it via Backyard EOS or Astrophotography Tool during the next session.

  1. However isn't the effort that goes into carefully centring a target by plate solving undone by having the light frames recentreed at a random distribution of co-ordinates? 
  2. Also wont here be considerable cropping to ensure the same stars are overlaid during stacking?

I hope I've expressed myself clearly enough?! Thanks in advance.

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If you dither by 12 pixels which is what I think is recommended for DSLRs and your pixels are 5nm ( probably smaller ) the movement in only 60 nanometers which means cropping off the overlap so not loosing a lot.

Dave

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As Dave says (though pixel size is more like 5um and not 5nm) the movement is fairly small over a nightly session....you do have to crop the edges of the image but you don't lose much. Modern DSLRs are in the region of 5000x4000 so even if dithering goes wrong and it dithers in one direction only you'd lose about 20% real estate for 60 subs....

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11 hours ago, choochoo_baloo said:

I think I understand dithering in principle - unwanted artefacts e.g hot pixels are averaged out from multiple light frames, during stacking, since the dither has caused the artefact to move randomly between frames.

I'm going to attempt it via Backyard EOS or Astrophotography Tool during the next session.

  1. However isn't the effort that goes into carefully centring a target by plate solving undone by having the light frames recentreed at a random distribution of co-ordinates? 
  2. Also wont here be considerable cropping to ensure the same stars are overlaid during stacking?

I hope I've expressed myself clearly enough?! Thanks in advance.

you will lose some of your field of view to dithering and so will have to crop the end image slightly. I tend to monitor the situation as the framing can drift unacceptably over many frames and require that the target is manually re-centered. However the improvement in image quality is well worth it. You can set a spiral dither pattern in PHD that will largely prevent the image randomly moving off in a net direction.

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17 minutes ago, Davey-T said:

Which mount have you got ?

Dave

yes it still works. But you do tend to lose more FOV when only dithering in RA from my experience. Suppose you can compensate by reducing the dither scale.

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Just now, Adam J said:

No in that case you will have a huge pic scale so it would depend on the ratio of the pic we l scale of your guide camera to the imaging camera. You may require quite allot of movement. 

I dont have a guide camera, i will dither it manually between sessions (1 session is like 5images to be able to dither after that) with the slow motion buttons on the SA.

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You can also dither manually in dec with the adjuster on the dovetail bar if you use it.

If you look at your image in live view, move it a bit take another then compare images you could work out  how much you need to move it, then image going round in a spiral pattern.

Dave

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8 minutes ago, Davey-T said:

You can also dither manually in dec with the adjuster on the dovetail bar if you use it.

If you look at your image in live view, move it a bit take another then compare images you could work out  how much you need to move it, then image going round in a spiral pattern.

Dave

Which  on dec you mean? I use the dovetail but with ballhead.

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2 minutes ago, Davey-T said:

Yes, it's what I do.

It helps if the camera is aligned with dec / ra.

Dave

Okay thanks

But 1 question...

I use the buttons it moves left and right and with the one you showed it will move left right again so no point or I am wron?

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3 minutes ago, serbiadarksky said:

I use the buttons it moves left and right and with the one you showed it will move left right again so no point or I am wron?

They move at right angles to each other.

Dave

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