Dithering

[MarkS]

Nope it doesn't - but when you link PHD to Nebulosity you can pause guiding during image download - I assume that would work the same.

Pause of guiding during download is not dithering.  The reason guiding can be paused during download is to help prevent electrical noise degrading sensitive operations such as reading from the CCD.  When guiding is resumed after download, it goes back to the same guidestar in the original position - i.e. there is no interframe movement between subs.

Dithering is where the guide star is moved a tiny distance across the frame before the next sub is acquired.

In Nebulosity both functions are accessed from the same menu because both functions require the link to PHD.  

Mark

[doug]

     Mark ..... I was in the printing trade for forty nine years.  The only dithering we did was when we had a cuppa and a cheese roll !!!!!
But thanks for the technical explanation. 

    Doug.

[mickw]

Mark
Thanks for that and the Wiki link - very interesting and explains a lot.

Should dithering be applied as a matter of course or only if needed - a case of if it ain't broke don't fix it ?

[MarkS]

Should dithering be applied as a matter of course or only if needed - a case of if it ain't broke don't fix it ?

The problem is that you don't usually know if dithering is required until you process your data and by then it is too late.

Generally speaking, in a non-observatory setup, slight inaccuracies in polar alignment and flexures in the mount/rings/scopes mean that you get slight drift and rotation from image frame to image frame even if the guiding is accurate. This has a very similar effect to dithering.  But is is not guaranteed that this will always happen.  So I use dithering as a matter of course for the Canon.  It may not be so important for dedicated astro-CCDs.

Mark

[Rick]

I think the term was first used in printing - where you wish to create a wider grey scale or a greater number of colours from a limited palette.  Its usage in astro-photography is slightly different - it is something that happens at the acquisition phase.

In printing (including in a lot of computer printers) it relates to the way a screen is applied to a photograph so that it can be printed using ink of one colour (or, for colour, three or more), so each pixel turns into a very fine-scaled pattern of dots (or sometimes dots of varying sizes). Astrophotographers are just doing it backwards...

[mickw]

The problem is that you don't usually know if dithering is required until you process your data and by then it is too late.

That was in the back of my mind 

So in the field (or any iffy obs.setup) it would help to use it as part of the capture process

I think the term was first used in printing - where you wish to create a wider grey scale or a greater number of colours from a limited palette.  Its usage in astro-photography is slightly different - it is something that happens at the acquisition phase.

In printing (including in a lot of computer printers) it relates to the way a screen is applied to a photograph so that it can be printed using ink of one colour (or, for colour, three or more), so each pixel turns into a very fine-scaled pattern of dots (or sometimes dots of varying sizes). Astrophotographers are just doing it backwards...

I think this demonstrates the effect using 3 colours (which could be greyscale) - they are dithered (blended) to produce a different value.
Although this wouldn't be the first time I was wrong 

[RobertM]

Dithering is where the guide star is moved a tiny distance across the frame before the next sub is acquired.

That's not strictly correct.  The guider is used to remove tracking errors so the image remains in the same place on the capture chip.  Dithering is independent of guiding and does not need a guide camera.  If you are using a guide camera then a random offset is applied to the guide star from it's actual position, the guide software treats that as a guiding error and acts to center the guide star again.  The offset positions used in the dither operation are randomised about the original guide star position so acting to reduce noise without loosing any image area (around the edge).  The one disadvantage of this is that if guiding is stopped and restarted the dithering will be about the new guide star position (which was offset by the previous dither operation).  There will also be a short time after the dither operation has started where the guiding system will have to settle down to a steady state again before the next exposure can be taken.

It still has a noticable effect with cooled cameras though the results can be quite subtle.  Also unlike dark and bias frames the technique acts to reduce all captured noise.

I use it by default if at all possible.

Hope that helps
Robert

[MarkS]

Dithering is where the guide star is moved a tiny distance across the frame before the next sub is acquired.

That's not strictly correct. 

Maybe it's not strictly correct but it is exactly what you see when observing the output from the guide camera chip (at least in PHD).  The guide star jumps to a new random position in the guide camera frame calculated by the dither algorithm just before the start of each exposure.  It is equivalent to how I used to do it manually - stop guiding; slew scope randomly by a few pixels; begin guiding at new guide star position.

Mark

[RobertM]

Yes, but my point is that you don't need guider to do it.  Offset of the guide star is the easiest one way to achieve it with a guider.  For example, in MaximDL dithering can be achieved either by direct mount movements or via the guider if connected.

As well as multipixel dithering, dithering at the sub-pixel level can also be used to improve spatial resolution for undersampled images.

[MarkS]

Yes, but my point is that you don't need guider to do it.  

True, but realistically we all do guiding because of periodic error and polar misalignments.   And once we have guiding switched on, the only way to perform dithering is to interface to the guider in some way shape or form.

Mark

[doug]

     The image that Mick has put here with the red,  green and blue colours on the wall .................  where they overlap the colours change to yellow, magenta and cyan.  These are the colours of the inks that printers use for basic colour printing with the addition of black to usually give intensity of shadow areas.  Of course, one can always add other tints to enhance the reproduced picture, such as pink, grey, light greens, etc.  We have even used a transparent white ........

     It`s a long time since I thought about the printing process ........

     Doug.... with ink still on his fingers. 

[Rocket Pooch]

I'm enjoying this, but I would like to point out that under sampling will not get fixed by dithering.

[MarkS]

I'm enjoying this, but I would like to point out that under sampling will not get fixed by dithering.

Actually, if you use dithering during acquisition and drizzle during processing it goes a long way to fixing undersampling.  Add a bit of post-processing deconvolution, then "hey presto!"

Mark

[Rocket Pooch]

Hi,

Bit busy at the moment, but if you have undersampled a small star to less than one pixel, it does not make any difference what you do with it, its still the shape of the pixel. Which is one of the reasons I don't like uba wide field images, they just look like a field of little blocks, not stars.

Chris

P.S. http://www.ccd.com/ccd113.html

[mickw]

Wouldn't dithering round off the blockiness by distorting the square pixel (assuming a star the size of one pixel) ?