Someone please explain Dithering to me….🤔

[Stuart1971]

I have been imaging for a while now, and getting fairly decent results, but have been told I need to dither, now I never have, I image with a QHY268c with the IMX571 sensor, and it’s really low noise, I have noticed a bit of walking noise, I think it’s called, where it looks like streaks across the images, when stacked with no calibration frames, but not in the final calibrated images out of APP, now people say that darks are not needed with this camera, and other say to use them, I do use them, as well as flats and dark flats, and seems to work well, so my question what is dithering, and do I need to do it..???

here is an example of my latest image of the Heart nebula, with the above calibration frames, 5 hours of 2 min subs, 50 darks, 50 flats and 50 dark flats, processed in APP with a final tweak in affinity Photo, so more data I’d needed for sure as it’s still quite noisy, but will dithering make any difference to this image had I don’t it at the time…??

 

[vlaiv]

Dithering is simply moving scope few pixels between each exposure in random way.

It helps to spread fixed pattern noise around and results in less noisy images for same parameters (may or may not be seen by naked eye but is measurable effect) and is indeed advised thing to do.

You simply tell your guiding software to dither between exposures and communication between guiding and imaging software should do the rest (wait for next exposure until guiding settles down).

[Stuart1971]

2 minutes ago, vlaiv said:

Dithering is simply moving scope few pixels between each exposure in random way.

It helps to spread fixed pattern noise around and results in less noisy images for same parameters (may or may not be seen by naked eye but is measurable effect) and is indeed advised thing to do.

You simply tell your guiding software to dither between exposures and communication between guiding and imaging software should do the rest (wait for next exposure until guiding settles down).

Thanks, so as I guide in one direction only because of DEC backlash and this works very well, I see I can dither  in RA only, is this not as good, or is it at least better than nothing…?

Edited March 21, 2022 by Stuart1971

[vlaiv]

Just now, Stuart1971 said:

Thanks, so as I guide in one direction only because of DEC backlash and this works very well, I see I can either in RA only, is this not as good, or is it at least better than nothing…?

Point is for it to be random so that fixed pattern noise would not create recognizable patterns - like walking noise.

Dithering only in RA is more likely to cause spread in a single line, even if it is random - and that will create streaking artifacts.

Although you guide in one direction only for DEC - you can still use dither. PHD2 should be able to move the mount if you put in backlash compensation and it will take a bit longer for guiding to settle - so that it again uses up all backlash and starts guiding in one side only.

[Stuart1971]

3 minutes ago, vlaiv said:

Point is for it to be random so that fixed pattern noise would not create recognizable patterns - like walking noise.

Dithering only in RA is more likely to cause spread in a single line, even if it is random - and that will create streaking artifacts.

Although you guide in one direction only for DEC - you can still use dither. PHD2 should be able to move the mount if you put in backlash compensation and it will take a bit longer for guiding to settle - so that it again uses up all backlash and starts guiding in one side only.

No, if I have it set on south or North guide only I have to use RA only for dither, well that’s what I have read on the NINA website anyway…

[scotty38]

Now you're guiding and using PHD2 is it worth giving the calibration/guiding assistant a go to see if it can manage the backlash? Or I'll swap you for an EQ6 🙂

[Stuart1971]

1 minute ago, scotty38 said:

Now you're guiding and using PHD2 is it worth giving the calibration/guiding assistant a go to see if it can manage the backlash? Or I'll swap you for an EQ6 🙂

I did, and it told me I have 3200ms of backlash and to guide in one direction only, and this has worked really well…..🤔

[Stuart1971]

10 minutes ago, vlaiv said:

Point is for it to be random so that fixed pattern noise would not create recognizable patterns - like walking noise.

Dithering only in RA is more likely to cause spread in a single line, even if it is random - and that will create streaking artifacts.

Although you guide in one direction only for DEC - you can still use dither. PHD2 should be able to move the mount if you put in backlash compensation and it will take a bit longer for guiding to settle - so that it again uses up all backlash and starts guiding in one side only.

This is what is says on the NINA site re dithering in RA only…

 

[vlaiv]

7 minutes ago, Stuart1971 said:

No, if I have it set on south or North guide only I have to use RA only for dither, well that’s what I have read on the NINA website anyway…

PHD2 docs say that you can still use it but might not want to:

Quote

To set up for uni-directional guiding, you can follow these steps:

1. Move to a field with a good guide star and run the Guiding Assistant for several minutes.  Look at the guiding graph and note whether the guide star is drifting north or south.  Once you see this, reset the Dec guide mode to issue corrections in the appropriate direction.  For example, if the star is drifting north, set the Guide mode to 'south.'
2. Try using the 'LowPass2' guiding algorithm for declination and start with a fairly low aggressiveness factor, say 50%.  If the aggressiveness is too high, the correction may push the star to the "wrong" side of the lock position, where it will remain until the slow drift rate moves it back.  It's better to issue a few consecutive small corrections rather than one larger one in order to minimize this type of over-shoot.
3. Watch the guiding graph to be sure the corrections are being issued in the right direction and the star isn't just steadily drifting off-target.  Over the course of minutes or hours, you may notice the amount of drift is decreasing.  This means you are slowly approaching the point of declination reversal and you should be prepared to change the Dec guide mode accordingly.
4. If you are dithering, you may want to set the dithering parameters to "RA-only" to avoid disrupting the Dec guiding.

 

[scotty38]

3 minutes ago, Stuart1971 said:

I did, and it told me I have 3200ms of backlash and to guide in one direction only, and this has worked really well…..🤔

oh ok fair enough but I'd have expected this class of mount to be, well, pretty decent in this regard...

[Stuart1971]

2 minutes ago, vlaiv said:

PHD2 docs say that you can still use it but might not want to:

 

Yes, that what I said, I can still guide in one direction, and just dither in RA only, and this DOC confirms this….yes…?

[vlaiv]

2 minutes ago, Stuart1971 said:

This is what is says on the NINA site re dithering in RA only…

Point with this is:

- dithering will tell mount to randomly move to new position

- it will then wait for mount to settle with guiding.

Two things can happen if you have high backlash:

- part of random move will be "spent" on clearing backlash if dither is in direction of backlash (this can be handled by dither "size" - you can tell phd2 how much you want to dither)

- once guiding resumes - it might not work properly until any residual backlash clears - this will only impact settling time as mount waits until guiding picks up and then exposure starts again.

 

[Stuart1971]

1 minute ago, scotty38 said:

oh ok fair enough but I'd have expected this class of mount to be, well, pretty decent in this regard...

No, the older EQ8 suffered from terrible DEC backlash due to not absolutely perfect round worm wheel, mine has barely any backlash around half of the worm, and some on the other half, it’s a known issue with them….and one that has had many a thread discussing it, and apparently the later ones are not much better….😮

[vlaiv]

1 minute ago, Stuart1971 said:

Yes, that what I said, I can still guide in one direction, and just dither in RA only, and this DOC confirms this….yes…?

Yes, you can dither only in RA - sure, some mounts like StarAdventurer don't even have powered DEC - and those can only dither in RA, but if you dither in RA only - and guide in DEC - you will put all your offsets on a single line - RA line.

[Stuart1971]

2 minutes ago, vlaiv said:

Point with this is:

- dithering will tell mount to randomly move to new position

- it will then wait for mount to settle with guiding.

Two things can happen if you have high backlash:

- part of random move will be "spent" on clearing backlash if dither is in direction of backlash (this can be handled by dither "size" - you can tell phd2 how much you want to dither)

- once guiding resumes - it might not work properly until any residual backlash clears - this will only impact settling time as mount waits until guiding picks up and then exposure starts again.

 

Sorry confused, not sure what you are saying here, you mean I should not guide in one direction, or I can but I will have to either in RA only, as it will not work in DEC due to the setting I have for the DEC guiding…

[Stuart1971]

2 minutes ago, vlaiv said:

Yes, you can dither only in RA - sure, some mounts like StarAdventurer don't even have powered DEC - and those can only dither in RA, but if you dither in RA only - and guide in DEC - you will put all your offsets on a single line - RA line.

I have no choose but to use RA only, if I continue to guide in only one direction in DEC…correct…? I can’t dither in both directions and guide in just one direction in DEC, it’s one or the other….

[scotty38]

14 minutes ago, Stuart1971 said:

No, the older EQ8 suffered from terrible DEC backlash due to not absolutely perfect round worm wheel, mine has barely any backlash around half of the worm, and some on the other half, it’s a known issue with them….and one that has had many a thread discussing it, and apparently the later ones are not much better….😮

Oh...... rushes to put a line through that option lol.......Mind you after the CEM 120 price rise a dodgy EQ8 is probably still worth it 🙂

[vlaiv]

16 minutes ago, Stuart1971 said:

I have no choose but to use RA only, if I continue to guide in only one direction in DEC…correct…? I can’t dither in both directions and guide in just one direction in DEC, it’s one or the other….

I think you can - but it would be best to try out one night when you don't plan to image but it is still clear - like when the full moon is out or similar.

[Stuart1971]

2 hours ago, scotty38 said:

Oh...... rushes to put a line through that option lol.......Mind you after the CEM 120 price rise a dodgy EQ8 is probably still worth it 🙂

They are still excellent mounts, and I don’t regret buying it at all, I got second hand and I knew about the reputation…

I too have considered the CEM120, not seen the latest price increases…has it gone up a lot then…??

[tomato]

With regard to dithering, I had really bad walking noise when using an Esprit 150/ASI 178 combination which dithering eliminated. However when I moved to a dual rig dithering was a bit more difficult to implement but was possible with NINA, but I still lost some imaging time so I experimented with no dithering and found that walking noise was much less of an issue. I wonder if that is because I am combining a roughly equal number of subs from two optical sources which are not perfectly aligned and have different fixed noise patterns? 

[vlaiv]

2 minutes ago, tomato said:

I wonder if that is because I am combining a roughly equal number of subs from two optical sources which are not perfectly aligned and have different fixed noise patterns? 

That makes sense - it would lessen it, but not remove it completely.

[Stuart1971]

Ok, these are two crops of the same image, the first one was just stacked and no calibration frames at all, and you see the FPN / walking noise, and hot pixels, the second was stacked just using Darks, and the FPN has all but gone, no dithering, so do Darks help with this type of noise, at it seems it does.

the image is very noisy due to height LP and shot under the moon, and very few subs…

 

[scotty38]

13 hours ago, Stuart1971 said:

They are still excellent mounts, and I don’t regret buying it at all, I got second hand and I knew about the reputation…

I too have considered the CEM120, not seen the latest price increases…has it gone up a lot then…??

Well I suppose in the grand scheme of things it's not a big increase but gone from 3998 to 4259 compared with 2999 of the EQ8-R

[vlaiv]

For that sort of money one can get E.fric mount which is in another class (but is somewhat rudimentary / industrial even compared to Chinese mounts).

[gorann]

When I look at your calibrated images I see no walking noise. I expect that dithering would have no noticible effect and I would not worry about it. I have the same camera as you (although the ASI variant) and since I have a quite dark site (Borte 2-3) I have very low noise and never saw anything like walking noise (which was a constant problem during my earlier DSLR era). So I expect that your noise is, as you say, due to light pollution (and dithering would have no effect on that). A drawback with dithering is that you loose imaging time as the mount have to be given time to settle between each frame. Another drawback is of course that it adds one more layer of complexity and one more thing that can go wrong. So, if I was you, I would not worry about dithering. If you do it, make a careful comparison with how it looks with and without dithering, to see if it has any effect at all.